Samacheer Kalvi Class 9 Science Solutions Chapter 14 Acids, Bases and Salts

I. Choose The Correct Answer :

Question 1. \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \dots \uparrow \)
(a) \( \text{H}_2 \)
(b) \( \text{O}_2 \)
(c) \( \text{CO}_2 \)
Answer: (a) \( \text{H}_2 \)
In simple words: When zinc reacts with hydrochloric acid, it forms zinc chloride and releases hydrogen gas. This is a common single displacement reaction where a more reactive metal displaces hydrogen from an acid.

🎯 Exam Tip: Remember the activity series of metals to predict displacement reactions and the common gases produced when metals react with acids (usually hydrogen).

Question 2. Apple contains malic acid. Orange contains ................
(a) citric acid
(b) ascorbic acid
Answer: (b) ascorbic acid
In simple words: Apples have malic acid, and oranges have ascorbic acid, which is also known as Vitamin C. Citrus fruits are well-known for their high vitamin C content, which is vital for health.

🎯 Exam Tip: Knowing the common acids found in different fruits and foods is important for general science knowledge.

Question 3. Acids in plants and animals are organic acids. Whereas Acids in rocks and minerals are ................
(a) Inorganic acids
(b) Weak acids
Answer: (a) Inorganic acids
In simple words: Acids that come from living things like plants and animals are called organic acids. Acids found in non-living things like rocks and minerals are known as inorganic acids, also sometimes called mineral acids.

🎯 Exam Tip: Differentiating between organic and inorganic acids based on their source (living vs. non-living) is a key concept.

Question 4. Acids turn blue litmus paper to ................
(a) green
(b) red
(c) orange
Answer: (b) red
In simple words: Acids are known for turning blue litmus paper into a red color. This color change is a classic way to test if a substance is acidic.

🎯 Exam Tip: Always remember the color changes for litmus paper: Acids turn blue litmus red, while bases turn red litmus blue.

Question 5. Since metal carbonate and metal bicarbonate are basic, they react with acids to give salt and water with the liberation of ................
(a) \( \text{NO}_2 \)
(b) \( \text{SO}_2 \)
(c) \( \text{CO}_2 \)
Answer: (c) \( \text{CO}_2 \)
In simple words: When acids react with basic metal carbonates or bicarbonates, they produce salt and water, along with releasing carbon dioxide gas. This reaction is why you see bubbles when you mix vinegar (an acid) with baking soda (a bicarbonate).

🎯 Exam Tip: The liberation of carbon dioxide gas is a characteristic test for the reaction of acids with carbonates and bicarbonates. Always remember the products of such reactions: salt, water, and carbon dioxide.

Question 6. The hydrated salt of copper sulphate has ................colour.
(a) red
(b) white
(c) blue
Answer: (c) blue
In simple words: Copper sulfate that has water molecules attached to it (hydrated copper sulfate) is typically blue. These water molecules are part of its crystal structure and give it its distinct color.

🎯 Exam Tip: Be aware that the color of hydrated salts can change when they lose their water of crystallization, for example, hydrated copper sulfate turns white when heated.

II. Answer In Briefly :

Question 1. Classify the various types of Acids based on their sources.
Answer: Acids are categorized based on where they come from. There are two main types:
1. Organic acids: These are acids that are found naturally in plants and animals.
2. Inorganic acids: These are acids that are made from rocks and minerals.
In simple words: Acids are split into organic (from living things) and inorganic (from non-living things) groups, depending on their original source. For instance, citric acid is organic, while hydrochloric acid is inorganic.

🎯 Exam Tip: Provide clear definitions and examples for both organic and inorganic acids to score full marks.

Question 2. Write any four uses of acids.
Answer: Here are some important uses of acids:
- Sulfuric acid is known as the 'King of Chemicals' because it is used to make many other compounds and is also found in car batteries.
- Hydrochloric acid helps clean surfaces and is used as a cleaning agent in toilets.
- Citric acid is used in making fizzy (effervescent) salts and as a food preservative to keep food fresh.
- Nitric acid is used in making fertilizers, dyes, paints, and medicines.
- Oxalic acid helps clean iron and manganese stains from quartz crystals and is also used to bleach wood and remove dark stains.
- Carbonic acid is found in aerated drinks, giving them their fizziness.
- Tartaric acid is an important part of baking powder, which helps baked goods rise.
In simple words: Acids are used for many things like cleaning, making medicines, preserving food, and even helping cakes rise. Different acids have different jobs based on their properties.

🎯 Exam Tip: When listing uses, always include specific examples of acids for better clarity and impact on the examiner.

Question 3. Give the significance of pH of soil in agriculture.
Answer: The pH level of soil is very important in farming because it affects how well plants grow. Different crops need different soil pH levels to thrive. For example, citrus fruits like oranges prefer soil that is slightly basic or alkaline, while rice plants need acidic soil. Sugarcane, on the other hand, grows best in soil that has a neutral pH. The correct pH ensures that nutrients are available to plants.
In simple words: Soil pH matters a lot in agriculture because each crop grows best at a certain pH level. Farmers need to check soil pH to choose the right crops and ensure they get enough nutrients.

🎯 Exam Tip: Mentioning specific examples of crops and their preferred pH ranges demonstrates a deeper understanding of the concept.

Question 4. What are the various uses of Aquaregia?
Answer: Aqua regia has specific and powerful uses:
- It is mainly used to dissolve noble metals like gold and platinum, which most other acids cannot do alone.
- It is also used for cleaning and refining gold, ensuring its purity.
In simple words: Aqua regia is a strong acid mixture used to dissolve precious metals like gold and platinum and to clean them. This is because these metals don't react with other common acids.

🎯 Exam Tip: Highlight that aqua regia is unique in its ability to dissolve noble metals, which is its primary application.

Question 5. What are the uses of Plaster of Paris?
Answer: Plaster of Paris has several important uses:
- It is used for plastering bones, especially when they are broken, to hold them in place so they can heal properly.
- It is also used for making casts for statues and other decorative items due to its ability to set quickly and harden.
In simple words: Plaster of Paris is used to fix broken bones and to create molds or casts for statues because it hardens fast. Its quick-setting property makes it very useful in both medicine and art.

🎯 Exam Tip: Focus on its quick-setting and hardening properties as the main reasons for its applications.

Question 6. Two acids 'A' and 'B' are given. Acid A gives one hydrogen ion per molecule of the acid in solution. Acid B gives two hydrogen ions per molecule of the acid in solution.
(i) Find out acid A and acid B.
(ii) Which acid is called the King of Chemicals?
Answer:
(i) Acid A – HCl – Hydrochloric acid.
Acid B – \( \text{H}_2\text{SO}_4 \) – Sulfuric acid.
(ii) Sulfuric acid – \( \text{H}_2\text{SO}_4 \). This acid is widely used in various industries, from fertilizers to detergents, earning it the title.
In simple words: Acid A releases one hydrogen ion, so it's hydrochloric acid. Acid B releases two hydrogen ions, making it sulfuric acid. Sulfuric acid is called the 'King of Chemicals' because it's used so much in different industries.

🎯 Exam Tip: The number of replaceable hydrogen ions determines the basicity of an acid (mono-, di-, or tri-basic). Sulfuric acid's wide industrial use is why it's called 'King of Chemicals'.

Question 7. Define aquaregia.
Answer: Aqua regia is a strong mixture of hydrochloric acid and nitric acid. These two acids are mixed in a specific ratio of 3 parts hydrochloric acid to 1 part nitric acid. This special mixture is known for its ability to dissolve noble metals like gold and platinum.
In simple words: Aqua regia is a powerful acid mix made from three parts hydrochloric acid and one part nitric acid. It's special because it can dissolve gold and platinum, which most acids cannot.

🎯 Exam Tip: Always remember the specific molar ratio (3:1 HCl to HNO3) when defining aqua regia, as this is crucial for its unique properties.

Question 8. Correct the mistakes :
(a) Washing soda is used for making cakes and bread soft, spongy.
(b) Calcium sulphate hemihydrate is used in textile industry.
Answer:
(a) Baking soda (Sodium bicarbonate – \( \text{NaHCO}_3 \)) is used for making cakes and bread soft and spongy.
(b) Calcium sulfate hemihydrate (\( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} \)) is used for plastering bones.
In simple words: Baking soda makes cakes soft, not washing soda. Plaster of Paris (calcium sulfate hemihydrate) is used to fix bones, not in the textile industry. Each chemical has its correct use based on its properties.

🎯 Exam Tip: Pay close attention to the specific chemical compounds and their primary uses to avoid common misconceptions.

Question 9. What is neutralization reaction? Give an example.
Answer: A neutralization reaction happens when an acid and a base react together. This reaction creates a salt and water. During this process, the hydrogen ions (\( \text{H}^+ \)) from the acid combine with the hydroxide ions (\( \text{OH}^- \)) from the base to form water. If a strong acid and a strong base react, the resulting solution will have a neutral pH of 7. For example:
\( \text{HCl} \) (Hydrochloric Acid) \( + \text{NaOH} \) (Sodium hydroxide) \( \rightarrow \text{NaCl} \) (Sodium Chloride) \( + \text{H}_2\text{O} \) (Water)
In simple words: Neutralization is when an acid and a base mix to make salt and water. It's like they cancel each other out, often resulting in a neutral solution.

🎯 Exam Tip: Clearly define neutralization as the reaction between an acid and a base forming salt and water, and always provide a balanced chemical equation as an example.

III. Answer In Detail :

Question 1. Differentiate hydrate and anhydrous salts with examples.
Answer:

🎯 Exam Tip: When differentiating, ensure you highlight the presence or absence of water molecules in the crystal structure and provide clear, distinct examples for each type.

Question 2. Give the tests to identify Acids and Bases.
Answer: There are several common tests using indicators to identify acids and bases:
- **Litmus Paper Test**: Acids will turn blue litmus paper red, while bases will turn red litmus paper blue. This is a very basic and widely used test.
- **Phenolphthalein Test**: In acidic solutions, phenolphthalein remains colorless. In basic solutions, it turns a pink color.
- **Methyl Orange Test**: In acidic solutions, methyl orange turns pink. In basic solutions, methyl orange turns yellow. The color change of indicators happens due to the interaction of the indicator dye with hydrogen or hydroxide ions.
In simple words: We use special papers or liquids called indicators to tell acids and bases apart. Litmus paper changes color (blue to red for acids, red to blue for bases), phenolphthalein turns pink in bases, and methyl orange turns pink in acids and yellow in bases.

🎯 Exam Tip: Clearly state the initial color of the indicator and its final color in both acidic and basic environments for each test.

Question 3. Write any four uses of bases.
Answer: Bases have many practical applications:
- Sodium hydroxide is extensively used in making soap and detergents, as well as in the paper industry. It is a very strong base.
- Calcium hydroxide is important for whitewashing buildings and treating acidic soils in agriculture.
- Magnesium hydroxide is commonly used as a medicine for stomach disorders, especially to relieve heartburn or indigestion.
- Ammonium hydroxide is used to remove grease stains from clothes, acting as a cleaning agent.
In simple words: Bases are used to make soap, whitewash buildings, treat stomach acid, and clean grease from clothes. They are important in many household and industrial products.

🎯 Exam Tip: Give specific examples of bases and their corresponding uses rather than just generic applications of bases.

Question 4. Write any five uses of salts.
Answer: Salts are incredibly versatile and have numerous uses:
- **Common Salt (\( \text{NaCl} \))**: It is a staple in our daily food as a seasoning and is also used as a preservative to cure meats and fish. This helps prevent spoilage.
- **Washing Soda (\( \text{Na}_2\text{CO}_3 \))**: Used to soften hard water, which helps in better cleaning. It is also used in the manufacturing of glass, soap, and paper, playing a key role in industrial processes.
- **Baking Soda (Sodium bicarbonate – \( \text{NaHCO}_3 \))**: It's a component of baking powder, which helps dough rise and makes cakes and bread soft and spongy. It is also used in soda-acid fire extinguishers and can neutralize excess stomach acid for indigestion relief.
- **Bleaching Powder (Calcium Oxychloride – \( \text{CaOCl}_2 \))**: Used as a disinfectant to purify water and sanitize surfaces. It also plays a role in the textile industry for bleaching cotton and linen, helping to remove colors.
- **Plaster of Paris (Calcium Sulfate Hemihydrate – \( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} \))**: Essential for plastering broken bones to aid healing. It's also used to make casts for statues and various decorative items due to its ability to set quickly.
In simple words: Salts like common salt are for food, washing soda cleans, baking soda helps bake and fights fire, bleaching powder disinfects, and Plaster of Paris fixes bones and makes statues. They are very useful in everyday life.

🎯 Exam Tip: When listing uses of salts, group them by specific salt name and chemical formula for a more organized and comprehensive answer.

Question 5. Sulphuric acid is called King of Chemicals. Why is it called so?
Answer: Sulfuric acid is known as the 'King of Chemicals' because it is vital in the production of many other chemicals and compounds. Its extensive use across various industries, including fertilizers, detergents, dyes, and paints, makes it one of the most important industrial chemicals. This wide applicability underlines its 'kingly' status in chemistry.
In simple words: Sulfuric acid is called the 'King of Chemicals' because it is used to make so many other important products in factories. It's like the main building block for a lot of useful substances.

🎯 Exam Tip: Emphasize the broad range of industrial applications and its role in synthesizing other chemicals as the key reasons for its title.

Intext Activities

ACTIVITY - 2
Take solutions of hydrochloric acid or sulphuric acid. Fix two nails on a cork and place the cork in a 100 ml beaker. Connect the nails to the two terminals of a 6 V battery through a bulb and a switch as shown in Figure. Now pour some dilute HCl in the beaker and switch on the current. Repeat the activity with dilute sulphuric acid, glucose and alcohol solutions. What do you observe now? Does the bulb glow in all cases?
Answer:
- The bulb glows when hydrochloric acid and sulfuric acid solutions are used. This happens because acids produce ions in water, allowing electricity to pass through.
- The bulb does not glow when the activity is done with alcohol and glucose solutions. This is because alcohol and glucose are not electrolytes and do not form ions in solution.
In simple words: The bulb lights up with acid solutions like hydrochloric or sulfuric acid because they have charged particles (ions) that carry electricity. But it won't light up with alcohol or glucose because they don't make these charged particles.

🎯 Exam Tip: Explain that the conductivity of solutions depends on the presence of free ions. Acids ionize to produce ions, while alcohol and glucose do not, hence the difference in conductivity.

ACTIVITY - 3
Collect the following samples from the science laboratory – Hydrochloric acid, Sulphuric acid and Nitric acid, Sodium hydroxide, Potassium hydroxide. Take 2 ml of each solution in a test tube and test with a litmus paper and indicators phenolphthalein and Methyl orange. Tabulate your observations.
Answer:

🎯 Exam Tip: Memorize the color changes for each indicator (litmus, phenolphthalein, methyl orange) in both acidic and basic solutions, as this is fundamental for identifying substances.

ACTIVITY-4
Fill in the blanks in the following table based on the concept of water of crystallisation.
Answer:

🎯 Exam Tip: Accurately writing chemical formulas for both anhydrous and hydrated salts, including the correct number of water molecules, is essential.

ACTIVITY -5
Boil about 100 ml of groundwater in a vessel to dryness. After all the water get evaporated observe the inner wall of the vessel. Can you observe any deposits? This is the deposit of dissolved salts present in water.
Answer: Yes, there will be deposits observed on the inner wall of the vessel. These deposits are the dissolved salts that were present in the groundwater. Water often contains various minerals and salts, which are left behind when the water evaporates completely. This experiment demonstrates the presence of impurities in natural water sources.
In simple words: Yes, you will see a deposit on the vessel. This is because groundwater has many dissolved salts and minerals, which are left behind after the water evaporates completely.

🎯 Exam Tip: Relate the observed deposits to the concept of dissolved solids or impurities in water, and mention that this is why hard water leaves residue.

9th Science Guide Acids, Bases and Salts Additional Important Questions and Answers

I. Choose The Correct Answer :

Question 1. Acid secreted in our stomach is ................
(a) hydrochloric acid
(b) sulphuric acid
(c) nitric acid
(d) carbonic acid
Answer: (a) hydrochloric acid
In simple words: Our stomach makes hydrochloric acid to help break down food and kill harmful bacteria. This strong acid is important for digestion.

🎯 Exam Tip: Remember that hydrochloric acid is the primary acid in the stomach and plays a crucial role in digestion.

Question 2. Hydrochloric Acid reacts with metal bicarbonates to give ................
(a) metal chloride
(b) water
(c) carbon di – oxide
(d) all the above
Answer: (d) all the above
In simple words: When hydrochloric acid reacts with metal bicarbonates, it produces three things: a metal chloride (salt), water, and carbon dioxide gas. All these products are formed in the reaction.

🎯 Exam Tip: Always recall the general rule for acid-bicarbonate reactions: Acid + Bicarbonate \( \rightarrow \) Salt + Water + Carbon Dioxide.

Question 3. ................ metals do not react with \( \text{HCl} \) or \( \text{HNO}_3 \).
(a) Gold & Magnesium
(b) Silver & Magnesium
(c) Gold & Silver
(d) Zinc & Silver
Answer: (c) Gold & Silver
In simple words: Gold and silver are known as noble metals because they don't easily react with common acids like hydrochloric acid or nitric acid. This is why they are often used for jewelry.

🎯 Exam Tip: Remember the reactivity series of metals; metals below hydrogen in the series (like gold and silver) do not displace hydrogen from acids and thus do not react with them easily.

Question 4. The molar ratio of hydrochloric acid and nitric acid in aquaregia is ................
(a) 1:3
(b) 6:3
(c) 2:3
(d) 3:1
Answer: (d) 3:1
In simple words: Aqua regia is a special mixture made by combining hydrochloric acid and nitric acid in a specific ratio: three parts hydrochloric acid to one part nitric acid. This ratio is important for its strong dissolving power.

🎯 Exam Tip: Precisely recall the 3:1 molar ratio of \( \text{HCl} \) to \( \text{HNO}_3 \) for aqua regia, as this is a frequently tested fact.

Question 5. Bases ionise in water to form ................ ions.
(a) \( \text{H}^+ \)
(b) \( \text{H}_3\text{O}^+ \)
(c) \( \text{OH}^- \)
(d) \( \text{O}^{2-} \)
Answer: (c) \( \text{OH}^- \)
In simple words: When bases dissolve in water, they break apart to release hydroxide ions (\( \text{OH}^- \)). These hydroxide ions are what give bases their unique properties.

🎯 Exam Tip: Understand the Arrhenius definition of bases, which states that bases produce hydroxide ions (\( \text{OH}^- \)) in aqueous solutions.

Question 6. Which of the following pairs are weak base?
(a) \( \text{NH}_4\text{OH} \) & \( \text{NaOH} \)
(b) \( \text{Ca(OH)}_2 \) & \( \text{KOH} \)
(c) \( \text{NH}_4\text{OH} \) & \( \text{Ca(OH)}_2 \)
(d) \( \text{NaOH} \) & \( \text{KOH} \)
Answer: (c) \( \text{NH}_4\text{OH} \) & \( \text{Ca(OH)}_2 \)
In simple words: A weak base is one that does not fully break apart into ions when dissolved in water. Ammonium hydroxide and calcium hydroxide are examples of weak bases because they only partially ionize.

🎯 Exam Tip: Differentiate between strong and weak bases by their degree of ionization in water. Strong bases ionize completely, while weak bases ionize partially.

Question 7. \( \text{NaOH} \) & \( \text{KOH} \) are ................
(a) strong bases
(b) metal Oxides
(c) weak bases
(d) diacidic bases
Answer: (a) strong bases
In simple words: Both sodium hydroxide (\( \text{NaOH} \)) and potassium hydroxide (\( \text{KOH} \)) are very strong bases. This means they completely break apart into ions when put in water, making them highly reactive.

🎯 Exam Tip: Recognize common strong bases like \( \text{NaOH} \) and \( \text{KOH} \), which are often used in industrial and laboratory settings for their high reactivity.

Question 8. Which of the following solution is soapy to touch?
(a) Acidic
(b) Basic
(c) Salt
(d) Aquaregia
Answer: (b) Basic
In simple words: Basic solutions, also called alkaline solutions, feel slippery or soapy when touched. This is a common physical property of bases, unlike acids or salts.

🎯 Exam Tip: Recall the characteristic properties of bases, such as bitter taste and soapy feel, to identify them.

Question 9. Which of the following solutions do not conduct electricity?
(a) alcohol
(b) glucose
(c) sulphuric acid
(d) both a & b
Answer: (d) both a & b
In simple words: Alcohol and glucose solutions do not carry electricity because they don't break down into charged particles (ions) in water. Sulfuric acid, however, does form ions and conducts electricity.

🎯 Exam Tip: Remember that electrical conductivity in solutions requires the presence of free-moving ions. Substances that do not ionize in water, like glucose and alcohol, will not conduct electricity.

Question 10. The pH value of neutral solution is ................
(a) = 7
(b) <7
(c) none of the options
(d) -7
Answer: (a) = 7
In simple words: A neutral solution has a pH value of exactly 7. This means it is neither acidic nor basic, like pure water.

🎯 Exam Tip: Clearly remember the pH scale: pH < 7 is acidic, pH > 7 is basic, and pH = 7 is neutral.

Question 11. The pH of stomach fluid is ................
(a) 4
(b) 2
(c) 6
(d) 7
Answer: (b) 2
In simple words: The fluid in our stomach is very acidic, with a pH of about 2. This low pH is due to hydrochloric acid and is necessary for digesting food.

🎯 Exam Tip: Knowing the approximate pH values of common biological fluids (like stomach acid, blood, saliva) can be useful.

Question 12. A salt which is formed by complete neutralization of an acid and a base is called ................ salt.
(a) basic
(b) acid
(c) double
(d) normal
Answer: (d) normal
In simple words: When an acid and a base completely react with each other, they form a normal salt. This kind of salt does not contain any replaceable hydrogen or hydroxide ions.

🎯 Exam Tip: Understand that a normal salt results from the complete replacement of acidic hydrogen atoms by a metal or ammonium ion, and hydroxide ions of a base by an acid radical.

Question 13. The number of water molecules present in one molecule of copper sulphate is
(a) 2
(b) 3
(c) 4
(d) 5
Answer: (d) 5
In simple words: When copper sulphate forms a crystal, it combines with five water molecules, making it a hydrated salt. This specific form, with five water molecules, is also known as blue vitriol.

🎯 Exam Tip: Remember that the number of water molecules in hydrated salts (like copper sulphate pentahydrate) is fixed and essential for their properties.

Question 14. The molecular formula of copper sulphate pentahydrate is
(a) \( \text{CuSO}_4 \cdot \text{H}_2\text{O} \)
(b) \( \text{CuSO}_4 \cdot \text{2H}_2\text{O} \)
(c) \( \text{CuSO}_4 \cdot \text{5H}_2\text{O} \)
(d) \( \text{CuSO}_4 \cdot \text{3H}_2\text{O} \)
Answer: (c) CuSO4.5H2O
In simple words: Copper sulphate pentahydrate means one copper sulphate unit joins with five water units. Its formula shows this with '5H2O' at the end. The term 'pentahydrate' literally means 'five waters', which is reflected in the formula.

🎯 Exam Tip: The prefix 'penta-' in 'pentahydrate' indicates five water molecules. Learn common prefixes for numbers in chemical nomenclature.

Question 15. ..................is the gas produced when HCl is added with carbonate salt.
(a) \( \text{H}_2 \)
(b) \( \text{N}_2 \)
(c) \( \text{CO}_2 \)
(d) \( \text{O}_2 \)
Answer: (c) CO2
In simple words: When you mix hydrochloric acid with a carbonate salt, it makes carbon dioxide gas, which you can see as bubbles. This reaction is a common way to test for the presence of carbonate salts, as the carbon dioxide gas causes fizzing.

🎯 Exam Tip: Carbonate reactions with acids always produce carbon dioxide. This is a key characteristic of carbonates.

Question 16. The formula of bleaching powder is ..................
(a) \( \text{CaCl}_2 \)
(b) \( \text{CaOCl}_2 \)
(c) \( \text{Ca(OH)}_2 \)
(d) \( \text{CaO} \)
Answer: (b) CaOCl2
In simple words: Bleaching powder, also called calcium oxychloride, has the chemical formula \( \text{CaOCl}_2 \). Bleaching powder is also known as calcium oxychloride and is widely used for disinfection and bleaching.

🎯 Exam Tip: Memorize the common name and chemical formula for important compounds like bleaching powder.

Question 17. The chemical name of plaster of paris is ..................
(a) Calcium sulphate hemihydrate
(b) Calcium sulphate monohydrate
(c) Calcium sulphate dihydrate
(d) Calcium sulphate trihydrate
Answer: (a) Calcium sulphate hemihydrate
In simple words: Plaster of Paris is really calcium sulphate hemihydrate, which means it has half a water molecule attached to each calcium sulphate unit. Plaster of Paris is known for its quick-setting properties when mixed with water, forming gypsum.

🎯 Exam Tip: The prefix 'hemi-' means half, referring to the half molecule of water in Plaster of Paris.

Question 18. Which of the following metal does not react with sodium hydroxides?
(a) Cu
(b) Ag
(c) Cr
(d) All of the options
Answer: (d) All of the options
In simple words: Copper, silver, and chromium do not react with sodium hydroxide. So, all these metals behave similarly in this reaction. Most metals are amphoteric or noble, and do not readily react with strong bases like sodium hydroxide.

🎯 Exam Tip: Remember that not all metals react with bases; typically, amphoteric metals (like aluminum, zinc, lead) or very reactive ones show significant reactions. Copper, silver, and chromium are less reactive with bases.

Question 19. Curd contains ................acid.
(a) malic
(b) formic
(c) lactic
(d) ascorbic
Answer: (c) lactic
In simple words: Curd has lactic acid in it. This acid gives curd its sour taste. Lactic acid is produced by the fermentation of lactose (milk sugar) by bacteria, giving curd its characteristic sour taste.

🎯 Exam Tip: Associate specific foods with the acids they contain, like curd with lactic acid or lemon with citric acid.

Question 20. Which one of the following acids undergoes complete ionisation?
(a) \( \text{HCl} \)
(b) \( \text{CH}_3\text{COOH} \)
(c) \( \text{H}_2\text{SO}_4 \)
(d) All of the options
Answer: (a) HCl
In simple words: Hydrochloric acid (\( \text{HCl} \)) fully breaks apart into ions when put in water. This means it's a strong acid. Hydrochloric acid is a strong acid that completely dissociates into ions in water, making it a good conductor of electricity.

🎯 Exam Tip: Strong acids like \( \text{HCl} \), \( \text{H}_2\text{SO}_4 \), and \( \text{HNO}_3 \) ionize completely, while weak acids like \( \text{CH}_3\text{COOH} \) ionize partially.

II. Fill in the Blanks:

Question 1. Acid reacts with base to form a neutral product called ..................
Answer: Salt
In simple words: When an acid and a base mix, they create a neutral substance called salt. This fundamental reaction is known as neutralization, where the acid and base properties cancel each other out.

🎯 Exam Tip: Understand the concept of neutralization, where acids and bases combine to form salt and water, which are typically neutral.

Question 2. The taste of acid is ..................
Answer: Sour
In simple words: Acids have a sour taste. Many fruits like lemons and oranges taste sour because of the acids they contain.

🎯 Exam Tip: While acids taste sour, it's unsafe to taste unknown chemicals in a lab. Indicators are used for identification.

Question 3. .................. contain one or more replaceable hydrogen atoms.
Answer: Acids
In simple words: Acids are substances that have one or more hydrogen atoms that can be easily replaced in a reaction. The number of these replaceable hydrogen atoms determines the basicity of the acid.

🎯 Exam Tip: Focus on the definition of an acid: a substance that can donate hydrogen ions (protons) in a reaction.

Question 4. .................. acids have a relatively smaller amount of acids dissolved in a solvent.
Answer: Dilute
In simple words: Dilute acids are those where only a small amount of the acid is mixed into a lot of liquid. Dilute acids are generally safer to handle than concentrated acids due to their lower concentration of active acid molecules.

🎯 Exam Tip: Distinguish between dilute and concentrated solutions by the amount of solute present in the solvent.

Question 5. Acids react with metallic oxides to produce ..................
Answer: salt and water
In simple words: When acids combine with metal oxides, they make salt and water. This reaction is similar to the neutralization reaction between an acid and a base, as metallic oxides are often basic in nature.

🎯 Exam Tip: Remember that metallic oxides are usually basic, so their reaction with acids is a form of neutralization.

Question 6. .................. acid is used in aerated drinks.
Answer: Carbonic acid
In simple words: The acid used in fizzy drinks like sodas is carbonic acid. Carbonic acid is formed when carbon dioxide gas dissolves in water under pressure, giving soda its fizz.

🎯 Exam Tip: Carbonic acid is responsible for the fizzy sensation in carbonated beverages and is an example of a weak acid.

Question 7. Chemical formula of aquaregia is ..................
Answer: \( \text{3HCl} + \text{HNO}_3 \)
In simple words: The special mix called aqua regia is made from three parts hydrochloric acid and one part nitric acid. Aqua regia is unique because it can dissolve noble metals like gold and platinum, which single acids cannot.

🎯 Exam Tip: Aqua regia is a highly corrosive mixture, typically used to dissolve noble metals like gold and platinum. The specific ratio of 3:1 (HCl:HNO3) is crucial.

Question 8. Water soluble bases are called ..................
Answer: alkali
In simple words: Bases that can dissolve in water are called alkalis. Alkalis feel soapy to the touch and can be corrosive, so they should be handled with care.

🎯 Exam Tip: All alkalis are bases, but not all bases are alkalis. Alkalis are specifically water-soluble bases.

Question 9. Non-metallic oxides are .................. in nature.
Answer: acid
In simple words: Oxides from non-metals are usually acidic. When non-metallic oxides dissolve in water, they often form acids, such as sulfur dioxide forming sulfurous acid.

🎯 Exam Tip: Non-metallic oxides are typically acidic (e.g., \( \text{CO}_2 \), \( \text{SO}_2 \)), while metallic oxides are basic.

Question 10. .................. are bitter in taste.
Answer: Bases
In simple words: Bases taste bitter. Many cleaning products contain bases, and their bitter taste is a common characteristic.

🎯 Exam Tip: Like acids, bases should not be tasted in a laboratory setting due to their corrosive nature.

Question 11. .................. alkali has a relatively high percentage of alkali in its aqueous solution.
Answer: Concentrated
In simple words: A concentrated alkali has a lot of alkali mixed into its water solution. Handling concentrated alkalis requires strict safety precautions because they are highly corrosive.

🎯 Exam Tip: Concentrated solutions contain a large amount of solute compared to the solvent, making them more reactive and potentially hazardous.

Question 12. Acids turn blue litmus to ..................
Answer: red
In simple words: Acids change blue litmus paper to red. Litmus paper is a natural indicator that changes color depending on whether it's in an acidic or basic solution.

🎯 Exam Tip: Remember the color changes for litmus paper: blue to red for acids, red to blue for bases.

Question 13. Phenolphthalein and methyl Orange are ..................
Answer: indicators
In simple words: Phenolphthalein and methyl orange are special substances used to show if something is an acid or a base by changing color. These indicators are crucial tools in titrations to determine the endpoint of a reaction.

🎯 Exam Tip: Indicators are substances that show different colors in acidic and basic mediums, helping to identify the nature of a solution.

Question 14. \( \text{pH} \) stands for ............ in a solution.
Answer: power of hydrogen ion concentration
In simple words: \( \text{pH} \) tells us how much hydrogen ion is in a solution, which shows how acidic or basic it is. The pH scale ranges from 0 to 14, indicating the acidity or alkalinity of a solution.

🎯 Exam Tip: Understand that pH is a measure of the hydrogen ion concentration, which determines the acidity or alkalinity of a solution.

Question 15. The \( \text{pH} \) value of acids are .................. than 7.
Answer: lesser
In simple words: Acids have a \( \text{pH} \) value that is less than 7. Solutions with a pH less than 7 are considered acidic, while those above 7 are basic.

🎯 Exam Tip: The pH scale defines acids as having pH < 7, bases as pH > 7, and neutral solutions as pH = 7.

Question 16. White enamel coating of our teeth is ..................
Answer: Calcium phosphate
In simple words: The white outer layer of our teeth is made of calcium phosphate. This makes tooth enamel the hardest substance in the human body, protecting teeth from decay.

🎯 Exam Tip: Calcium phosphate is a strong mineral, but it can be eroded by acids produced by bacteria in the mouth.

Question 17. Salt is .................. in nature.
Answer: hygroscopic
In simple words: Salt likes to pull moisture from the air, which means it is hygroscopic. Hygroscopic substances like salt tend to absorb moisture from the air, which is why salt can clump in humid conditions.

🎯 Exam Tip: Hygroscopic substances absorb water from the atmosphere. This property is important in many chemical and biological contexts.

Question 18. Salt which is formed by the partial replacement of hydrogen ions of an acid by a metal is called ..................
Answer: acid salt
In simple words: When a metal replaces only some of the hydrogen ions in an acid, the salt formed is called an acid salt. These salts still contain replaceable hydrogen atoms and can react further as acids.

🎯 Exam Tip: Acid salts contain replaceable hydrogen ions, meaning they can still act as acids in certain reactions.

Question 19. Salts containing water of crystallisation are called .................. salts.
Answer: hydrated
In simple words: Salts that have water built into their crystal structure are called hydrated salts. This water is chemically bound within the crystal structure and gives the salt its specific crystalline form and often its color.

🎯 Exam Tip: Water of crystallization gives hydrated salts their characteristic shapes and often their colors; removing it usually changes their appearance.

Question 20. Salts that do not contain water of crystallisation is called ..................
Answer: anhydrous salt
In simple words: Salts that do not have water in their crystal structure are called anhydrous salts. Anhydrous salts are often used as drying agents due to their ability to absorb water.

🎯 Exam Tip: 'Anhydrous' literally means 'without water,' which helps remember that these salts do not contain water of crystallization.

Question 21. \( \text{pH} \) value of human blood is ................. (7.0, 7.4, 7.6).
Answer: 7.4
In simple words: The \( \text{pH} \) of human blood is normally around 7.4, which is slightly basic. Maintaining a stable blood pH is vital for proper bodily functions, and the body has various buffer systems to achieve this.

🎯 Exam Tip: Recognize that the pH of human blood is maintained within a very narrow, slightly alkaline range for health.

Question 22. The nature of the toothpaste commonly used is .................. in nature (acidic, basic, neutral)
Answer: basic
In simple words: Toothpaste is usually basic. Toothpastes are generally basic because they are designed to neutralize the excess acid produced by bacteria in our mouths.

🎯 Exam Tip: Toothpaste is typically basic to neutralize the acids in the mouth, which helps prevent tooth decay.

Question 23. You are given pure water to test the \( \text{pH} \) value using \( \text{pH} \) paper. It shows colour (White, black, green)
Answer: green
In simple words: When you test pure water with \( \text{pH} \) paper, it turns green. A green color on pH paper typically indicates a neutral solution, like pure water, with a pH of around 7.

🎯 Exam Tip: Pure water is neutral (pH 7), and pH indicator papers typically show a green color for neutral solutions.

III. To Match:

Question 1. Match the following:

🎯 Exam Tip: Knowing the common acids found in various fruits and food items is important for general science knowledge.

Question 2. Complete the following table:

🎯 Exam Tip: Familiarize yourself with common polyatomic ions and their names, as they are crucial in understanding chemical formulas.

Question 3. Complete the following table:

🎯 Exam Tip: Understanding the different classifications of salts (normal, acid, basic, double) helps in predicting their properties and reactions.

Question 4. Complete the following table:

🎯 Exam Tip: Flame tests are qualitative tests used to identify certain metal ions based on the characteristic color they impart to a flame.

IV. Complete the Following Equations.

Question 1. \( \text{HCl} + \text{H}_2\text{O} \rightarrow \text{? + ?} \)
Answer: \( \text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- \)
In simple words: When hydrochloric acid mixes with water, it creates hydronium ions and chloride ions. This equation shows how hydrochloric acid ionizes in water to form hydronium ions and chloride ions.

🎯 Exam Tip: Remember that acids like \( \text{HCl} \) donate a proton to water, forming \( \text{H}_3\text{O}^+ \) (hydronium ion), which represents the acidic nature of the solution.

Question 2. \( \text{H}^+ + \text{H}_2\text{O} \rightarrow \text{?} \)
Answer: \( \text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ \)
In simple words: A hydrogen ion combines with a water molecule to form a hydronium ion. Free hydrogen ions \( \text{H}^+ \) are highly reactive and always exist as hydronium ions \( \text{H}_3\text{O}^+ \) in aqueous solutions.

🎯 Exam Tip: In aqueous solutions, \( \text{H}^+ \) ions are always associated with water molecules, forming \( \text{H}_3\text{O}^+ \).

Question 3. \( \text{Mg} + \text{?} \rightarrow \text{?} + \text{H}_2 \)
Answer: \( \text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2\uparrow \)
In simple words: Magnesium reacts with sulfuric acid to make magnesium sulfate and hydrogen gas. This is a single displacement reaction where the more reactive magnesium displaces hydrogen from the sulfuric acid.

🎯 Exam Tip: Metals generally react with acids to produce salt and hydrogen gas, provided the metal is more reactive than hydrogen.

Question 4. \( \text{Na}_2\text{CO}_3 + \text{2HCl} \rightarrow \text{? + ? + CO}_2\uparrow \)
Answer: \( \text{Na}_2\text{CO}_3 + \text{2HCl} \rightarrow \text{2NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow \)
In simple words: Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide gas. This reaction is characteristic of carbonates reacting with acids, always producing carbon dioxide, water, and a salt.

🎯 Exam Tip: Carbonates reacting with acids always yield carbon dioxide gas, which can be observed as effervescence (fizzing).

Question 5. \( \text{ZnO} + \text{2HCl} \rightarrow \text{? + ?} \)
Answer: \( \text{ZnO} + \text{2HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}\uparrow \)
In simple words: Zinc oxide reacts with hydrochloric acid to create zinc chloride and water. Zinc oxide is an amphoteric oxide, meaning it can react with both acids and strong bases.

🎯 Exam Tip: Metallic oxides (like ZnO) are typically basic and react with acids to form salt and water, which is a neutralization reaction.

Question 6. \( \text{Zn} + \text{?} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \uparrow \)
Answer: \( \text{Zn} + \text{2NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\uparrow \)
In simple words: Zinc reacts with sodium hydroxide to make sodium zincate and hydrogen gas. This reaction shows zinc behaving as an amphoteric metal, reacting with a strong base to produce a salt and hydrogen gas.

🎯 Exam Tip: Metals like zinc and aluminum are amphoteric and can react with strong bases to produce hydrogen gas.

Question 7. \( \text{CaO} + \text{H}_2\text{SO}_4 \rightarrow \text{? + H}_2\text{O} \)
Answer: \( \text{CaO} + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} \)
In simple words: Calcium oxide mixes with sulfuric acid to form calcium sulfate and water. Calcium oxide, being a basic oxide, readily reacts with sulfuric acid in a neutralization reaction.

🎯 Exam Tip: Remember that basic oxides react with acids to produce salt and water, a classic neutralization pattern.

Question 8. \( \text{HCl} + \text{?} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
Answer: \( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
In simple words: Hydrochloric acid reacts with sodium hydroxide to make sodium chloride (common salt) and water. This is a classic example of a strong acid reacting with a strong base to form a neutral salt and water.

🎯 Exam Tip: This is the most common example of a neutralization reaction between a strong acid (HCl) and a strong base (NaOH).

Question 9. \( \text{Ca(OH)}_2 + \text{?} \rightarrow \text{?} + \text{H}_2\text{O} \)
Answer: \( \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \)
In simple words: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water. This reaction is often used to test for carbon dioxide, as the calcium carbonate precipitate makes the limewater cloudy.

🎯 Exam Tip: Limewater (\( \text{Ca(OH)}_2 \)) is often used to detect carbon dioxide, as the gas turns it milky by forming insoluble \( \text{CaCO}_3 \).

Question 10. Complete the following reaction scheme:
\( \text{NaOH} \xrightarrow{\quad \text{A} \quad} \text{NaCl} + \text{H}_2\text{O} + \text{B} \)
\( \text{NaOH} \xrightarrow{\quad \text{C} \quad} \text{NaCl} + \text{H}_2\text{O} \)
\( \text{NaOH} \xrightarrow{\quad \text{H}_2\text{SO}_4 \quad} \text{D} + \text{E} \)
Answer:
\( \text{A} = \text{NH}_4\text{Cl} \)
\( \text{B} = \text{NH}_3\uparrow \)
\( \text{C} = \text{HCl} \)
\( \text{D} = \text{NaHSO}_4 \)
\( \text{E} = \text{H}_2\text{O} \)
In simple words: Here, we see how sodium hydroxide reacts with different things. A and B are what you get when it reacts with ammonium chloride. C is the acid that makes sodium chloride and water. D and E are the products when it reacts with sulfuric acid. These reactions demonstrate the diverse ways sodium hydroxide, a strong base, reacts with different types of acids and ammonium salts.

🎯 Exam Tip: Understanding the products of reactions between strong bases (like NaOH) and different types of acids (HCl, H2SO4) or ammonium salts (NH4Cl) is fundamental to chemistry.

V. Answer in Briefly:

Question 1. What are organic acids? Given examples.
Answer: Organic acids are types of acids found naturally in plants and animals. They contain carbon atoms. For example, formic acid (\( \text{HCOOH} \)) is found in ant stings, and acetic acid (\( \text{CH}_3\text{COOH} \)) is found in vinegar. These acids often have a distinct smell and are weaker than mineral acids.
In simple words: Organic acids come from living things like plants and animals. Examples are formic acid and acetic acid.

🎯 Exam Tip: Remember that organic acids are characterized by the presence of carbon in their chemical structure and are generally weaker than inorganic (mineral) acids.

Question 2. How are acids classified based on ionisation? Give examples.
Answer: Acids are classified based on how much they ionize (break apart into ions) in water.
- Strong Acids: These acids ionize completely in water, meaning all their molecules turn into ions. An example is hydrochloric acid (\( \text{HCl} \)).
- Weak Acids: These acids ionize only partially in water, meaning only some of their molecules turn into ions. An example is acetic acid (\( \text{CH}_3\text{COOH} \)). The extent of ionization affects an acid's strength and its ability to conduct electricity in solution.
In simple words: Acids are called strong if they fully break into ions in water, like \( \text{HCl} \). They are called weak if they only partly break into ions, like \( \text{CH}_3\text{COOH} \).

🎯 Exam Tip: The degree of ionization is key to distinguishing between strong and weak acids, which impacts their reactivity and pH.

Question 3. What is Aquaregia? Mention its uses.
Answer: Aqua regia is a strong mixture of hydrochloric acid and nitric acid, prepared in a specific ratio of 3:1 (hydrochloric acid to nitric acid). It looks like a yellow-orange fuming liquid and is very corrosive. It is powerful enough to dissolve noble metals like gold and platinum, which ordinary acids cannot. The synergistic action of the two acids allows aqua regia to dissolve gold, an otherwise unreactive metal.
**Uses:**
- It is used to dissolve noble metals such as gold, platinum, and palladium.
- It is used for cleaning and refining gold.
In simple words: Aqua regia is a very strong mix of hydrochloric acid and nitric acid (3 parts to 1 part). It can dissolve precious metals like gold and platinum and is used to clean and purify them.

🎯 Exam Tip: The unique property of aqua regia is its ability to dissolve noble metals, which is due to the combined action of its constituent acids, not just one.

Question 4. What does the acidity of base mean?
Answer: The acidity of a base refers to the number of replaceable hydroxyl (\( \text{OH}^- \)) groups present in one molecule of that base. For example, a base with one hydroxyl group is monoacidic, while a base with two is diacidic. This concept is similar to the basicity of an acid, which refers to the number of replaceable hydrogen ions.
In simple words: The acidity of a base tells us how many hydroxyl (\( \text{OH}^- \)) groups a base has that can be replaced in a chemical reaction.

🎯 Exam Tip: Do not confuse the acidity of a base with the acidity of a solution. The acidity of a base refers to its capacity to react with an acid based on its \( \text{OH}^- \) groups.

Question 5. What is Potash alum? Write its formula.
Answer: Potash alum is a double salt that is a mixture of potassium sulfate and aluminum sulfate. Its chemical formula is \( \text{KAl(SO}_4\text{)}_2 \cdot \text{12H}_2\text{O} \). Potash alum is commonly used in water purification as a coagulant and in leather tanning.
In simple words: Potash alum is a special salt made from potassium sulfate and aluminum sulfate, and it also contains 12 water molecules. Its formula is \( \text{KAl(SO}_4\text{)}_2 \cdot \text{12H}_2\text{O} \).

🎯 Exam Tip: Potash alum is an important example of a double salt and is recognized by its complex formula including water of crystallization.

Question 6. What are double salts? Give an example?
Answer: Double salts are salts that are formed when two simple salts combine in a fixed molar ratio from their saturated solutions, and then crystallize together. They lose their identity in solution and dissociate into their individual ions. Unlike complex salts, double salts completely dissociate into simple ions when dissolved in water.
**Example:** Potash alum (\( \text{KAl(SO}_4\text{)}_2 \cdot \text{12H}_2\text{O} \)) is a common example of a double salt.
In simple words: Double salts are made by mixing two simple salts in water and letting them crystallize. In water, they break back into their original separate ions. Potash alum is one such example.

🎯 Exam Tip: Key characteristics of double salts include forming from two simple salts, having a fixed molar ratio, and dissociating completely into constituent ions in solution.

Question 7. What are basic salts? Give suitable reaction for this.
Answer: Basic salts are formed when a diacidic or triacidic base partially reacts with an acid, meaning not all the hydroxyl (\( \text{OH}^- \)) ions are replaced by the acid radical. This leaves some \( \text{OH}^- \) groups in the salt. Basic salts can exhibit basic properties due to the presence of unreacted hydroxyl groups.
**Reaction Example:**
\( \text{Pb(OH)}_2 + \text{HCl} \rightarrow \text{Pb(OH)Cl} + \text{H}_2\text{O} \)
In simple words: Basic salts are made when a base with two or three \( \text{OH} \) groups reacts with an acid but doesn't fully swap all its \( \text{OH} \) groups for the acid's part. So, the salt still has some \( \text{OH} \) left. For example, lead hydroxide reacts with hydrochloric acid to make basic lead chloride and water.

🎯 Exam Tip: Basic salts are formed due to incomplete neutralization of a polyacidic base, retaining hydroxyl groups in their structure.

Question 8. Why are tooth pastes basic?
Answer: Toothpastes are generally basic because they are designed to neutralize the excess acid produced by bacteria in our mouths. These acids can erode tooth enamel and lead to tooth decay. By being basic, toothpaste helps to bring the mouth's pH back to a healthy, less acidic level, preventing damage to teeth. This neutralization helps protect the calcium phosphate enamel, the hardest substance in the body, from acidic attack.
In simple words: Toothpaste is basic because it helps stop the acid made by bacteria in our mouths. Too much acid can harm our teeth, so basic toothpaste makes the mouth less acidic and helps prevent cavities.

🎯 Exam Tip: The basic nature of toothpaste is a crucial factor in maintaining oral hygiene by counteracting the detrimental effects of bacterial acids.

Question 9. What is water of crystallisation?
Answer: Water of crystallization refers to the specific number of water molecules that are chemically bound within the crystal structure of some salts. These water molecules are part of the crystal lattice and give the hydrated salt its characteristic geometric shape and often its color. When this water is removed, the crystal structure often collapses, and the salt's appearance may change significantly.
In simple words: Water of crystallization is the water molecules that are fixed inside the crystal structure of certain salts. This water gives the crystals their shape and color.

🎯 Exam Tip: Water of crystallization is distinct from absorbed moisture; it is an integral part of the crystal structure and is present in a fixed stoichiometric ratio.

Question 10. Why do blue colour copper sulphate becomes white on heating?
Answer: Blue-colored copper sulfate is actually copper sulfate pentahydrate (\( \text{CuSO}_4 \cdot \text{5H}_2\text{O} \)), meaning it contains five molecules of water of crystallization. When this blue salt is heated, it loses these water molecules. As the water is driven off, the crystal structure changes, and the anhydrous (water-free) copper sulfate that remains is white. This color change is a classic chemical test for the presence of water.
In simple words: Blue copper sulfate has water inside its crystals. When you heat it, the water goes away, and the copper sulfate turns white.

🎯 Exam Tip: The color change of copper sulfate from blue to white upon heating is a practical demonstration of losing water of crystallization and a reversible reaction.

Question 11. Acidic or basic solutions are good conductors of electricity. Justify your answer.
Answer: Acidic and basic solutions are good conductors of electricity because they produce ions when dissolved in water. Acids release hydrogen ions (\( \text{H}^+ \) or \( \text{H}_3\text{O}^+ \)), while bases release hydroxide ions (\( \text{OH}^- \)). These free-moving ions carry the electric charge through the solution, allowing electricity to flow. Pure water itself is a poor conductor because it has very few free ions, highlighting the role of dissolved acids or bases.
In simple words: Both acid and basic solutions can conduct electricity well. This is because when they dissolve in water, they create free-moving charged particles called ions, which carry the electric current.

🎯 Exam Tip: Electrical conductivity in solutions is directly proportional to the concentration of free ions present; strong acids and bases produce more ions than weak ones.

Question 12. What are hygroscopic substances?
Answer: Hygroscopic substances are materials that readily absorb and hold water molecules from their surroundings, usually from the air, through either absorption or adsorption. This often results in a change in their physical state, such as becoming damp or dissolving. Common examples include sugar, salt, and concentrated sulfuric acid, which can even dehydrate other substances.
In simple words: Hygroscopic substances are materials that easily pull water out of the air or their surroundings and hold onto it.

🎯 Exam Tip: Hygroscopic substances are often used as drying agents (desiccants) in laboratories and industries because of their strong affinity for water.

Question 13. Define indicator. Give examples
Answer: Indicators are special chemical substances. We use them to find out if a given liquid solution is an acid or a base. They often show this by changing color. For example, phenolphthalein and methyl orange are common indicators. These help scientists safely test substances.
In simple words: Indicators are chemicals that change color to show if something is an acid or a base. Phenolphthalein and methyl orange are two examples.

🎯 Exam Tip: Remember to list at least two examples when asked to define indicators, as it shows a broader understanding.

Question 14. Define ionisation.
Answer: Ionisation is when a substance breaks apart into smaller charged particles called ions. This process can happen when a substance is dissolved in water, or when exposed to heat, radiation, or electrical discharge. For instance, when salt dissolves in water, it splits into positive and negative ions.
In simple words: Ionisation is when a substance breaks into tiny charged pieces called ions. This can happen in water or with heat.

🎯 Exam Tip: When defining ionisation, ensure you mention both the breaking apart into ions and the conditions that cause it (e.g., in solution, heat, radiation).

Question 15. How is normal salt obtained? Give a suitable reaction.
Answer: A normal salt is created when an acid and a base completely neutralise each other. This means all the hydrogen ions from the acid react with all the hydroxide ions from the base, forming water and a salt. This reaction forms a neutral product with a pH of 7.
\( \text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
In simple words: A normal salt forms when an acid and a base mix and cancel each other out completely. Like when hydrochloric acid and sodium hydroxide react to make common salt and water.

🎯 Exam Tip: Always include a balanced chemical equation when asked for a suitable reaction to illustrate the formation of a normal salt.

Question 16. How are bases classified based on ionisation?
Answer: Bases are sorted into groups based on how much they ionise, or break apart into ions, when in water.
(a) Strong Bases: These bases completely break apart into ions in water. This means almost all their molecules turn into ions when dissolved.
Example: NaOH, KOH
(b) Weak Bases: These bases only partially break apart into ions when in water. Only some of their molecules form ions, while others stay as whole molecules.
Example: \( \text{NH}_4\text{OH} \), \( \text{Ca(OH)}_2 \). Understanding this helps predict how reactive a base will be.
In simple words: Bases are sorted by how much they break into ions in water. Strong bases break completely, like NaOH. Weak bases only break a little bit, like \( \text{NH}_4\text{OH} \).

🎯 Exam Tip: For classifications, clearly state the definition of each type and provide at least one correct example for full marks.

VI. To Interpret:

Question 1. CH4 and NH3, are not acids.
Answer: \( \text{CH}_4 \) (methane) and \( \text{NH}_3 \) (ammonia) are not acids because they do not produce hydrogen ions (\( \text{H}^+ \)) when dissolved in water. Acids are defined by their ability to donate \( \text{H}^+ \) ions in a solution. These substances do not show that acidic property.
In simple words: \( \text{CH}_4 \) and \( \text{NH}_3 \) are not acids because they do not release hydrogen ions in water. Acids need to release these ions.

🎯 Exam Tip: The key characteristic of an acid is the production of hydrogen ions (\( \text{H}^+ \)) in an aqueous solution; always refer to this definition when explaining why something is or isn't an acid.

Question 2. Acetic acid (CH3COOH) is a mono basic acid.
Answer: Acetic acid (\( \text{CH}_3\text{COOH} \)) is indeed a monobasic acid. Even though it contains four hydrogen atoms, only one of them can be replaced or released as a hydrogen ion (\( \text{H}^+ \)) in its water solution. This unique structure makes it a monobasic acid, which means it has only one acidic hydrogen atom.
\( \text{CH}_3\text{COOH} \rightarrow \text{H}^+ + \text{CH}_3\text{COO}^- \)
In simple words: Acetic acid is a monobasic acid because it only releases one hydrogen ion when it dissolves in water, even though it has more hydrogen atoms.

🎯 Exam Tip: For organic acids, remember that not all hydrogen atoms are acidic; only those attached to highly electronegative atoms (like oxygen in a carboxyl group) are typically ionisable. Always show the correct ionisation equation.

Question 3. Al(OH)3 & Zn(OH)2 are not alkalis.
Answer: \( \text{Al(OH)}_3 \) (aluminium hydroxide) and \( \text{Zn(OH)}_2 \) (zinc hydroxide) are not alkalis because they are water-insoluble bases. Alkalis are a specific type of base that can dissolve in water. Since these two bases do not dissolve well in water, they are not considered alkalis, although they are still bases. Their limited solubility is a key differentiating factor.
In simple words: \( \text{Al(OH)}_3 \) and \( \text{Zn(OH)}_2 \) are not alkalis because alkalis are bases that dissolve in water, and these two do not.

🎯 Exam Tip: Clearly distinguish between a 'base' and an 'alkali': all alkalis are bases, but not all bases are alkalis. Solubility in water is the defining characteristic for an alkali.

Question 4. NaOH & KOH are strong bases.
Answer: \( \text{NaOH} \) (sodium hydroxide) and \( \text{KOH} \) (potassium hydroxide) are indeed strong bases. This is because they completely ionise, or break apart, in a water solution. They release all their hydroxide ions (\( \text{OH}^- \)) into the solution, making them very effective at neutralising acids. This complete ionisation is a hallmark of strong bases.
In simple words: \( \text{NaOH} \) and \( \text{KOH} \) are strong bases because they totally break apart into ions when mixed with water.

🎯 Exam Tip: When identifying strong bases, look for complete ionisation in water, which results in a high concentration of hydroxide ions.

Question 5. NaHSO4 is an acid salt.
Answer: \( \text{NaHSO}_4 \) (sodium bisulphate) is an acid salt. It forms when sulphuric acid (\( \text{H}_2\text{SO}_4 \)) is only partially neutralised by a metal like sodium from sodium hydroxide (\( \text{NaOH} \)). In this process, only one of the two hydrogen ions in sulphuric acid is replaced, leaving one acidic hydrogen atom still present in the salt. This makes the salt behave like an acid.
\( \text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{NaHSO}_4 + \text{H}_2\text{O} \)
In simple words: \( \text{NaHSO}_4 \) is an acid salt because it forms when only part of sulphuric acid's hydrogen is replaced, leaving some acidic character.

🎯 Exam Tip: Acid salts contain a replaceable hydrogen atom, allowing them to act as an acid in solution. Always remember to check for remaining acidic hydrogens.

Question 6. Non-metallic oxides are acidic is nature.
Answer: Non-metallic oxides are indeed acidic in nature. This is because when they react with bases, they form salt and water, which is similar to how an acid reacts with a base. For example, carbon dioxide (\( \text{CO}_2 \)), a non-metallic oxide, reacts with calcium hydroxide (\( \text{Ca(OH)}_2 \)), a base, to form calcium carbonate (a salt) and water. This reaction clearly shows their acidic behaviour.
\( \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \)
In simple words: Non-metallic oxides act like acids because they react with bases to make salt and water. Carbon dioxide is an example, it reacts with a base just like an acid would.

🎯 Exam Tip: To demonstrate the acidic nature of non-metallic oxides, always provide a reaction showing them reacting with a base to form salt and water.

VII. Complete the following table:

Question 1.

🎯 Exam Tip: When completing tables, ensure each cell is accurately filled. Pay close attention to subscripts, superscripts, and charges in chemical formulas and ions.

Question 2.

🎯 Exam Tip: Pay close attention to the valency of the metal ion and the number of hydroxide ions formed to correctly fill the table for bases.

Question 3.
1) Hydrochloric acid
2) Citric acid
3) Nitric acid
4) Oxalic acid
5) Sulphuric acid
a) Fertilizer
b) Baking powder
c) Car batteries
d) Cleansing agent in toilets
e) Removing black stains
f) Effervescent salts
Answer:
1. - d, 2. - f, 3. - a, 4. - e, 5. - c
In simple words: We match each acid with its main use. Hydrochloric acid cleans toilets. Citric acid is in fizzy salts. Nitric acid is for fertilizers. Oxalic acid removes black stains. Sulphuric acid is used in car batteries.

🎯 Exam Tip: When matching, read all options first and eliminate obvious mismatches. Focus on the most common or significant use for each substance.

Question 4.
1) Hardest substance in our body
2) Citrus fruits
3) Rice plant
4) Double salts
5) Silver chloride
a) Aluminium phosphate
b) Potash alum
c) Insoluble in water
d) Acidic soil
e) Calcium phosphate
f) Alkaline soil
Answer:
1. - e, 2. - f, 3. - d, 4. - b, 5. - c
In simple words: Let's match these items. The hardest body substance is calcium phosphate. Citrus fruits like alkaline soil. Rice plants grow best in acidic soil. Potash alum is an example of double salts. Silver chloride does not dissolve in water.

🎯 Exam Tip: For matching questions, a good strategy is to first match the pairs you are most confident about, then use elimination for the remaining options.

VIII. Find odd one out & give reason :

Question 1. HCI, HNO3, HCOOH, H3PO4
Answer:
Odd one out: \( \text{H}_3\text{PO}_4 \)
Reason: \( \text{H}_3\text{PO}_4 \) (phosphoric acid) is a tribasic acid, meaning it can release three hydrogen ions. The other three acids- \( \text{HCl} \) (hydrochloric acid), \( \text{HNO}_3 \) (nitric acid), and \( \text{HCOOH} \) (formic acid)- are all monobasic acids, releasing only one hydrogen ion each. This makes phosphoric acid unique in its basicity among the group.
In simple words: \( \text{H}_3\text{PO}_4 \) is the odd one out because it's a tribasic acid (gives 3 H+ ions), while the others (HCl, HNO3, HCOOH) are monobasic (give only 1 H+ ion).

🎯 Exam Tip: When finding the odd one out, clearly state the identifying characteristic that groups the other items together, and how the odd one differs.

Question 2. Acetic acid, formic acid, tartaric acid, sulphuric acid.
Answer:
Odd one out: Sulphuric acid
Reason: Sulphuric acid is a mineral acid, also known as an inorganic acid. It is derived from mineral sources. Acetic acid, formic acid, and tartaric acid are all organic acids, which means they are derived from living organisms or organic compounds. This difference in origin sets sulphuric acid apart.
In simple words: Sulphuric acid is the odd one out because it's a mineral acid, while acetic, formic, and tartaric acids are all organic acids.

🎯 Exam Tip: Remember that mineral acids (like sulphuric, hydrochloric, nitric) are inorganic, while organic acids contain carbon and are typically found in living things or derived from them.

Question 3. CaO, Na2O, ZnO, NaOH
Answer:
Odd one out: \( \text{NaOH} \)
Reason: \( \text{NaOH} \) (sodium hydroxide) is a base that contains a replaceable hydroxyl ion (\( \text{OH}^- \)). The other three substances- \( \text{CaO} \) (calcium oxide), \( \text{Na}_2\text{O} \) (sodium oxide), and \( \text{ZnO} \) (zinc oxide)- are all metallic oxides. These oxides react with water to form bases, but they are not bases themselves in the same way \( \text{NaOH} \) is. \( \text{NaOH} \) already has the hydroxyl group.
In simple words: \( \text{NaOH} \) is different because it's a base with a hydroxyl ion. The others are metal oxides that become bases when they react with water.

🎯 Exam Tip: Be careful to distinguish between a hydroxide (which directly contains OH- ions) and a metallic oxide (which forms OH- ions upon reacting with water).

Question 4. Bitter taste, soupy to touch, turns red litmus to blue, produce pink colour with methyl orange.
Answer:
Odd one out: Produce pink colour with methyl orange.
Reason: The characteristic "produce pink colour with methyl orange" is a property of acids. When methyl orange indicator is added to an acid, it turns pink. The other three characteristics- bitter taste, soupy to touch, and turning red litmus blue- are all properties of bases. Bases feel slippery and turn red litmus paper blue. This makes the pink color with methyl orange the outlier.
In simple words: "Produce pink colour with methyl orange" is the odd one out because acids do this. The other things, like bitter taste and turning red litmus blue, are properties of bases.

🎯 Exam Tip: Memorise the characteristic properties and indicator color changes for both acids and bases to easily identify the odd one out in such questions.

Question 5. Litmus paper, phenolphthalein, methyl orange, Aquaregia.
Answer:
Odd one out: Aquaregia
Reason: Aquaregia is a mixture of two strong acids, hydrochloric acid and nitric acid, famous for its ability to dissolve noble metals like gold and platinum. Litmus paper, phenolphthalein, and methyl orange, on the other hand, are all indicators. Indicators are used to identify the nature of a solution (whether it's acidic or basic) by changing color. Thus, Aquaregia is functionally different from the rest.
In simple words: Aquaregia is the odd one out because it's a strong acid mix. The others (litmus paper, phenolphthalein, methyl orange) are all indicators used to test for acids or bases.

🎯 Exam Tip: Group items by their function. Indicators change color to test solutions, while Aquaregia is a powerful solvent used in specific chemical reactions.

IX. Spot the error / Correct the wrong statement given below :

Question 1. An acid is the compound which are capable of forming hydroxyl ions (OH¯) in aqueous solution. .
Answer: Correct statement:
An acid is a compound capable of forming hydrogen ions (\( \text{H}^+ \)) in an aqueous solution. OR A base is a compound capable of forming hydroxyl ions (\( \text{OH}^- \)) in an aqueous solution.
The original statement incorrectly describes an acid by using the properties of a base. Acids release hydrogen ions, while bases release hydroxyl ions. This is a fundamental difference in their chemical behaviour.
In simple words: The mistake is saying acids form hydroxyl ions. Acids actually form hydrogen ions. Bases form hydroxyl ions.

🎯 Exam Tip: Always remember the definitions: acids produce \( \text{H}^+ \) ions, and bases produce \( \text{OH}^- \) ions in aqueous solutions.

Question 2. Nitric Acid is a constituent of baking powder.
Answer: Correct statement:
Tartaric acid is a constituent of baking powder.
The original statement incorrectly identifies nitric acid as a component of baking powder. Nitric acid is a strong mineral acid. Tartaric acid, a mild organic acid, is commonly used in baking powder along with baking soda to create the leavening effect (producing carbon dioxide gas).
In simple words: The mistake is saying nitric acid is in baking powder. It should be tartaric acid, which is a key part of baking powder.

🎯 Exam Tip: Be aware of the specific ingredients in common household products, as these are often used in "spot the error" questions.

Question 3. The pH value of the base in lesser than 7.
Answer: Correct statement:
The pH value of a base is greater than 7. OR The pH value of an acid is less than 7.
The original statement incorrectly describes the pH of a base. Basic solutions have a pH value greater than 7, indicating their alkalinity. Solutions with a pH less than 7 are acidic. Neutral solutions have a pH of exactly 7. This is a core concept of the pH scale.
In simple words: The mistake is saying bases have a pH less than 7. Bases have a pH greater than 7. Acids have a pH less than 7.

🎯 Exam Tip: Understand the pH scale thoroughly: pH 7 is neutral, pH less than 7 is acidic, and pH greater than 7 is basic/alkaline.

Question 4. Ca(OH)2 is a triacidic base.
Answer: Correct statement:
\( \text{Ca(OH)}_2 \) is a diacidic base. OR \( \text{Al(OH)}_3 \) is a triacidic base.
The original statement incorrectly classifies \( \text{Ca(OH)}_2 \) (calcium hydroxide) as a triacidic base. Calcium hydroxide has two hydroxyl groups, making it a diacidic base, capable of neutralising two molecules of a monobasic acid. A triacidic base would have three hydroxyl groups, like \( \text{Al(OH)}_3 \) (aluminium hydroxide).
In simple words: The mistake is calling \( \text{Ca(OH)}_2 \) a triacidic base. It's actually a diacidic base because it has two hydroxyl groups. A triacidic base would have three.

🎯 Exam Tip: The acidity of a base is determined by the number of replaceable hydroxyl groups it contains; for example, \( \text{Ca(OH)}_2 \) has two, making it diacidic.

Question 5. Magnesium hydroxide is used in whitewashing of buildings.
Answer: Correct statement:
Magnesium hydroxide is used as an antacid. OR Calcium hydroxide is used in whitewashing of buildings.
The original statement incorrectly attributes the use of magnesium hydroxide. Magnesium hydroxide is well-known as an antacid to neutralise stomach acid. Calcium hydroxide (slaked lime), not magnesium hydroxide, is traditionally used for whitewashing buildings due to its properties. This distinction is important for practical applications.
In simple words: The mistake is saying magnesium hydroxide is used for whitewashing. It is actually used as an antacid. Calcium hydroxide is used for whitewashing buildings.

🎯 Exam Tip: Familiarise yourself with the common uses of different chemical compounds, as questions often test practical applications.

X. Answer in detail :

Question 1. Explain the classification of acids based on their basicity.
Answer: Acids are categorised based on their basicity, which means the number of hydrogen ions (\( \text{H}^+ \)) a molecule of the acid can give up when dissolved in a solution. This classification helps understand their reactivity.
Monobasic Acid: These acids contain only one replaceable hydrogen atom per molecule. They release one hydrogen ion in solution.
Example: \( \text{HCl} \), \( \text{HNO}_3 \)
Dibasic Acid: These acids provide two hydrogen ions per molecule in a solution.
Example: \( \text{H}_2\text{SO}_4 \), \( \text{H}_2\text{CO}_3 \)
Tribasic Acid: These acids release three hydrogen ions per molecule in solution.
Example: \( \text{H}_3\text{PO}_4 \)
In simple words: Acids are classified by how many hydrogen ions they can release. Monobasic acids give one (like HCl), dibasic acids give two (like H2SO4), and tribasic acids give three (like H3PO4).

🎯 Exam Tip: When explaining basicity, clearly state the number of replaceable hydrogen ions for each type and provide accurate chemical examples for each category.

Question 2. Write notes on the properties of acids.
Answer: Acids have several distinct properties:
(a) Taste: They have a sour taste. Think of lemon juice.
(b) Electrical Conductivity: Their water solutions conduct electricity because they contain ions.
(c) Litmus Test: Acids turn blue litmus paper red. This is a common test to identify them.
(d) Reaction with Active Metals: Acids react with active metals to produce hydrogen gas.
\( \text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2\uparrow \)
\( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\uparrow \)
(e) Reaction with Metal Carbonates and Metal Hydrogen Carbonates: Acids react with these compounds to give off carbon dioxide gas.
\( \text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow \)
\( \text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow \)
(f) Reaction with Metallic Oxides: Acids react with metallic oxides to form salt and water.
\( \text{CaO} + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} \)
(g) Reaction with Bases: Acids react with bases to form salt and water. This is called a neutralisation reaction.
\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
All these reactions define the characteristic behaviour of acids.
In simple words: Acids taste sour, conduct electricity in water, and turn blue litmus red. They react with metals to make hydrogen gas, with carbonates to make carbon dioxide, and with metal oxides and bases to make salt and water.

🎯 Exam Tip: When listing properties, try to include both physical characteristics (like taste, conductivity, litmus test) and chemical reactions (with metals, carbonates, oxides, and bases). Include relevant chemical equations.

Question 3. Write notes on the properties of bases. .
Answer: Bases possess several key properties:
(a) Taste: They generally have a bitter taste.
(b) Feel: Their water solutions feel soapy or slippery to the touch.
(c) Litmus Test: Bases turn red litmus paper blue, which is a common way to identify them.
(d) Electrical Conductivity: Like acids, their water solutions conduct electricity because they form ions.
(e) Reaction with Metals: Some bases react with metals to form salt and liberate hydrogen gas.
\( \text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\uparrow \)
(f) Reaction with Non-metallic Oxides: Bases react with non-metallic oxides to produce salt and water. This is similar to a neutralisation reaction, showing that non-metallic oxides are acidic.
\( \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \)
(g) Reaction with Acids: Bases react with acids to form salt and water. This is a fundamental neutralisation reaction.
\( \text{KOH} + \text{HCl} \rightarrow \text{KCl} + \text{H}_2\text{O} \)
(h) Reaction with Ammonium Salts: When heated with ammonium salts, bases release ammonia gas.
\( \text{NaOH} + \text{NH}_4\text{Cl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3\uparrow \)
These properties help us understand how bases behave chemically.
In simple words: Bases taste bitter, feel soapy, and turn red litmus blue. They conduct electricity and react with certain metals to make hydrogen gas. They also react with non-metallic oxides and acids to make salt and water.

🎯 Exam Tip: Provide a balanced mix of physical and chemical properties. For reactions, always include clear chemical equations to demonstrate the process.

Question 4. Describe the classification of bases based on their acidity.
Answer: Bases are classified based on their acidity, which refers to the number of hydroxyl groups (\( \text{OH}^- \)) that a base molecule can provide per molecule during neutralisation. This helps determine how many acid molecules they can neutralise.
(a) Monoacidic Base: A monoacidic base is one that ionises in water to give only one hydroxide ion per molecule. This type of base can neutralise one molecule of a monobasic acid.
Example: \( \text{NaOH} \), \( \text{KOH} \)
(b) Diacidic Base: A diacidic base ionises in water to give two hydroxide ions per molecule. It can neutralise two molecules of a monobasic acid.
Example: \( \text{Ca(OH)}_2 \), \( \text{Mg(OH)}_2 \)
(c) Triacidic Base: A triacidic base ionises in water to give three hydroxide ions per molecule. It can neutralise three molecules of a monobasic acid.
Example: \( \text{Al(OH)}_3 \), \( \text{Fe(OH)}_3 \)
This classification helps predict the stoichiometry of neutralisation reactions.
In simple words: Bases are sorted by how many hydroxide ions they can release. Monoacidic bases give one \( \text{OH}^- \) (like NaOH). Diacidic bases give two \( \text{OH}^- \) (like \( \text{Ca(OH)}_2 \)). Triacidic bases give three \( \text{OH}^- \) (like \( \text{Al(OH)}_3 \)).

🎯 Exam Tip: Clearly define acidity for bases as the number of replaceable hydroxyl groups. Provide examples with the correct number of \( \text{OH} \) groups to illustrate each category.

Question 5. Write notes on importance of pH in everyday life.
Answer: The pH level is very important in many aspects of our daily lives, influencing health, agriculture, and the environment.
pH in our digestive system: The stomach produces hydrochloric acid with a pH of about 2.0. This acid helps digest food without harming the stomach lining. If there's too much acid due to indigestion, it can cause pain and irritation. Antacids, which are bases, are then used to neutralise this excess acid.
pH changes and tooth decay: The hard white enamel of our teeth is made of calcium phosphate. When the pH in our mouth drops below 5.5, the enamel starts to dissolve, leading to tooth decay. Toothpastes are basic and help neutralise this excess acid, preventing tooth decay. This highlights the importance of oral hygiene.
pH of soil: For agriculture, the pH of soil is crucial for plant growth. Citrus fruits, for example, need slightly alkaline soil, while rice thrives in acidic soil, and sugarcane prefers neutral soil. Farmers adjust soil pH to suit different crops.
pH of rain water: Normal rainwater has a pH of approximately 7 (neutral), signifying its purity. However, when air pollution introduces oxides of sulphur and nitrogen, these dissolve in rainwater, making its pH less than 7. This is known as acid rain. Acid rain can lower the pH of rivers, making it difficult for aquatic life to survive. This shows how human activities impact the environment.
In simple words: pH is vital in daily life. Our stomach acid helps digestion. Tooth decay happens if mouth pH is too low, so we use basic toothpaste. Plants need specific soil pH to grow. Acid rain, caused by pollution, harms rivers and aquatic life.

🎯 Exam Tip: When discussing the importance of pH, provide diverse examples from biology (digestion, teeth), agriculture (soil), and environmental science (acid rain) to show a comprehensive understanding.

Question 6. List the properties of salts.
Answer: Salts possess several characteristic properties:

• Salts are mostly solids. They typically have high melting and boiling points, meaning they need a lot of heat to melt or boil.
• Most salts dissolve well in water. For instance, common table salt (sodium chloride), potassium chloride, and sodium chloride are all water-soluble. However, some salts, like silver chloride, are insoluble in water.
• They are generally odourless. Most salts appear as white, cubic crystals or crystalline powders. They have a distinct salty taste.
• Salts can be hygroscopic, meaning they absorb moisture from the air. This property is seen in common salt that clumps together on humid days.

🎯 Exam Tip: When listing properties, include examples to illustrate solubility (e.g., soluble NaCl vs. insoluble AgCl) and other key characteristics like physical state, melting/boiling points, and hygroscopy.