GSEB Class 10 Science Solutions Chapter 2 Acids, Bases and Salts

 

Question 1. You have been provided with three test tubes, one of them contains distilled water and the other two contain an acidic solution and a basic solution respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer: Add a few drops of solution from all three test tubes onto the red litmus paper separately. The solution that changes red litmus to blue contains the basic solution. Use this newly made blue litmus paper to test the solutions in the other two test tubes. The solution from the test tube that changes blue litmus paper to red will be the acidic solution. The solution from the test tube that does not change either red or blue litmus paper contains water.
In simple words: First, put a drop from each tube on red litmus paper. The one that turns it blue is the basic solution. Then, use that blue paper to test the other two tubes. The one that turns it red is the acidic solution. The last tube, which doesn't change any paper, has distilled water.

Exam Tip: Remember the color changes: acids turn blue litmus red, and bases turn red litmus blue. Distilled water causes no change in either color.

 

Question 2. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and sour substances contain acids which can react with copper vessels and brass to form toxic compounds.
In simple words: Curd and sour foods have acids. These acids can react with copper and brass pots to make harmful substances.

Exam Tip: Always recall that acidic foods should be stored in inert containers like glass or stainless steel to prevent harmful reactions with metals.

 

Question 3. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer: When an acid reacts with a metal, it releases hydrogen gas. Example: \( \text{Zn(s)} + \text{2HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} \) To test for the presence of \( \text{H}_2 \) gas, bring a burning matchstick near the mouth of the test tube where \( \text{H}_2 \) gas is released. The matchstick burns with a distinctive pop sound.
In simple words: When a metal and an acid mix, they make hydrogen gas. To check for it, hold a lit match near the gas – it will make a 'pop' sound.

Exam Tip: The 'pop' sound is a key identifier for hydrogen gas and is essential to mention in your answer for full marks.

 

Question 4. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer: Metal compound A is \( \text{CaCO}_3 \). The gas evolved is \( \text{CO}_2 \). Balanced equation: \( \text{CaCO}_3\text{(s)} + \text{2HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \)
In simple words: The metal compound is calcium carbonate. When it reacts with acid, it produces carbon dioxide gas, which puts out flames.

Exam Tip: Remember that carbon dioxide is the only common gas that extinguishes a burning flame, distinguishing it from hydrogen (pop sound) or oxygen (relights glowing splint).

 

Question 5. Why do HCl, HNO3 etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer: Solutions like \( \text{HCl} \), \( \text{HNO}_3 \) etc., get ionized in aqueous solutions. Due to the presence of \( \text{H}^+ \) ions, they show acidic characters. In contrast, solutions of compounds like alcohol and glucose do not form any such ions, so they do not show acidic characters.
In simple words: Acids like \( \text{HCl} \) and \( \text{HNO}_3 \) become ions in water and release \( \text{H}^+ \) ions, which makes them acidic. Alcohol and glucose don't make these ions in water, so they are not acidic.

Exam Tip: The key differentiator for acidic behavior in aqueous solutions is the release of \( \text{H}^+ \) (or hydronium, \( \text{H}_3\text{O}^+ \)) ions. Compounds that do not ionize to release these ions will not exhibit acidic properties.

 

Question 6. Why does an aqueous solution of an acid conduct electricity?
Answer: When an acid forms a solution in water, it gets ionized. Due to the presence of these ions, electricity is conducted through it.
In simple words: Acids in water create charged particles called ions. These ions move and allow electricity to flow through the solution.

Exam Tip: Emphasize the formation of ions as the primary reason for electrical conductivity in acid solutions. Without free-moving ions, electricity cannot be carried.

 

Question 7. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: Dry \( \text{HCl} \) gas does not release \( \text{H}^+ \) ions; hence, the acidic property of the gas is not imparted.
In simple words: Dry \( \text{HCl} \) gas cannot give off \( \text{H}^+ \) ions. Because it doesn't have these ions, it can't act like an acid or change the litmus paper color.

Exam Tip: The presence of water is crucial for acids to dissociate and release \( \text{H}^+ \) ions, which are responsible for acidic properties. Without water, dry \( \text{HCl} \) behaves as a covalent compound, not an acid.

 

Question 8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: When acid and water mix, the reaction is highly exothermic. The acid may splash, cause burns, and even the bottle/container can break. To avoid this and allow the heat evolved to be absorbed by water slowly, acid is added to water for diluting it.
In simple words: Mixing acid and water makes a lot of heat. If you add water to acid, it can splash and cause burns or break the container. Adding acid slowly to water helps the water absorb the heat safely.

Exam Tip: This is a crucial safety procedure in chemistry. Always remember to add acid to water slowly with constant stirring, not the other way around, to control the exothermic reaction.

 

Question 9. How is the concentration of hydronium ions \( (\text{H}_3\text{O}^+) \) affected when a solution of an acid is diluted?
Answer: When the solution of acid is diluted, the concentration of hydronium ion \( (\text{H}_3\text{O}^+) \) per unit volume decreases.
In simple words: When you add more water to an acid solution, the number of hydronium ions in each small amount of liquid goes down.

Exam Tip: Dilution reduces the number of solute particles (ions) per unit volume, thereby decreasing the concentration, which is why acidic strength decreases upon dilution.

 

Question 10. How is the concentration of hydroxide ions \( (\text{OH}^-) \) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: When base is mixed with water, there is a decrease in the concentration of \( \text{OH}^- \) ions per unit volume.
In simple words: When a base is mixed with water, the amount of hydroxide ions in a specific volume becomes less.

Exam Tip: Similar to acids, diluting a base also lowers the concentration of its characteristic ions, in this case, hydroxide ions, per unit volume.

 

Question 11. You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Answer: A with \( \text{pH} = 6 \) is acidic. B with \( \text{pH} = 8 \) is basic. 'A' has more hydrogen ion concentration.
In simple words: Solution A, with a pH of 6, is acidic. Solution B, with a pH of 8, is basic. Solution A has a higher amount of hydrogen ions.

Exam Tip: Remember that a lower pH value indicates higher hydrogen ion concentration and greater acidity, while a higher pH indicates lower hydrogen ion concentration and greater alkalinity.

 

Question 12. What effect does the concentration of \( \text{H}^+ \text{(aq)} \) ions, have on the nature of the solution?
Answer: If a solution has a higher concentration of \( \text{H}^+ \) ions, it is more acidic in nature.
In simple words: A solution with more hydrogen ions is more acidic.

Exam Tip: Directly link the concentration of \( \text{H}^+ \) ions to the solution's acidity—more \( \text{H}^+ \) means stronger acid, less \( \text{H}^+ \) means weaker acid or basic solution.

 

Question 13. Do basic solutions also have \( \text{H}^+ \text{(aq)} \) ions? If yes, then why are these basic?
Answer: Yes, basic solutions also have \( \text{H}^+ \) ions, but they are basic in nature due to a greater number of \( \text{OH}^- \) ions.
In simple words: Basic solutions do contain hydrogen ions, but they are basic because they have many more hydroxide ions than hydrogen ions.

Exam Tip: All aqueous solutions contain both \( \text{H}^+ \) and \( \text{OH}^- \) ions. The relative concentrations determine whether the solution is acidic (more \( \text{H}^+ \)), basic (more \( \text{OH}^- \)), or neutral (equal concentrations).

 

Question 14. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Answer: When the soil is acidic in nature, the farmer would add quicklime \( (\text{CaO}) \) or slaked lime \( (\text{Ca(OH)}_2) \) or chalk \( (\text{CaCO}_3) \) to make it neutral.
In simple words: A farmer would add quicklime, slaked lime, or chalk to soil if it is too acidic, to bring its pH back to a neutral level.

Exam Tip: These compounds are bases and are used to neutralize acidic soil, thus improving conditions for crop growth. Remember the chemical names and formulas for these common basic substances.

 

Question 15. What is the common name of the compound \( \text{CaOCl}_2 \)?
Answer: The common name of \( \text{CaOCl}_2 \) is bleaching powder, and its chemical name is calcium oxychloride.
In simple words: The everyday name for \( \text{CaOCl}_2 \) is bleaching powder, and its chemical name is calcium oxychloride.

Exam Tip: Be sure to know both the common name and the chemical name for important compounds like bleaching powder, as both can be asked in exams.

 

Question 16. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: Calcium hydroxide \( (\text{Ca(OH)}_2) \) when treated with chlorine yields bleaching powder.
\( \text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O} \)
In simple words: When calcium hydroxide is mixed with chlorine, it makes bleaching powder.

Exam Tip: Memorize this reaction as it's a direct method for the industrial preparation of bleaching powder. Understand the reactants and products involved.

 

Question 17. Name the sodium compound which is used for softening hard water.
Answer: Sodium carbonate.
In simple words: Sodium carbonate is the compound utilized to make hard water soft.

Exam Tip: Sodium carbonate (washing soda) is a common chemical used to remove the hardness of water by precipitating metal ions like calcium and magnesium.

 

Question 18. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Answer: When sodium hydrocarbonate is heated, sodium carbonate, water, and carbon dioxide gas are obtained.
\( \text{2NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \)
In simple words: Heating sodium bicarbonate creates sodium carbonate, water, and carbon dioxide gas.

Exam Tip: This is a common thermal decomposition reaction. Know the products and ensure the chemical equation is correctly balanced, including the heat symbol over the arrow.

 

Question 19. Write an equation to show the reaction between Plaster of Paris and water.
Answer:
\( \text{CaSO}_4\text{.} \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4\text{.2H}_2\text{O} \)
Plaster of Paris (white powder) + water \( \rightarrow \) Gypsum (solid mass)
In simple words: Plaster of Paris combines with water to form gypsum.

Exam Tip: Understand that Plaster of Paris is a hemihydrate of calcium sulfate, and it rehydrates to form gypsum, a dihydrate, in the presence of water. This reaction is responsible for its setting properties.

In-Text Activities Solved

Activity 2.1

Answer:

In simple words: This table shows how different solutions react with various indicators like litmus paper, phenolphthalein, and methyl orange, helping to classify them as acids or bases based on color changes.

Exam Tip: For activities involving indicators, meticulously record and remember the specific color changes for each indicator in both acidic and basic solutions. This knowledge is fundamental for identifying substances.

Activity 2.2

Answer: Cloth strips, when kept in a bag with finely chopped onions and left overnight in the fridge, would smell of onion.
Cloth strip + dilute \( \text{HCl} \) Solution \( \rightarrow \) We can smell onion, red color of cloth strip changes to pale red.
Cloth strip + dilute \( \text{NaOH} \) solution \( \rightarrow \) Red color of cloth strip changes to green.
Odour test:
On adding vanilla essence \( \rightarrow \) In dilute \( \text{NaOH} \rightarrow \) no smell. In dilute \( \text{HCl} \rightarrow \) Smell of vanilla essence exists.
On adding clove oil \( \rightarrow \) In dilute \( \text{HCl} \rightarrow \) smell of clove exists.
In dilute \( \text{NaOH} \rightarrow \) smell of clove does not exist.
In simple words: Onion-soaked cloth smells like onion. With acid, its red color fades, and with base, it turns green. Vanilla and clove scents vanish in base but stay in acid.

Exam Tip: Olfactory indicators change smell in acidic or basic media. This activity demonstrates how substances like onion, vanilla essence, and clove oil can be used to distinguish between acids and bases by their distinct smell changes.

Activity 2.3

Answer: When 5 \( \text{mL} \) of dilute sulphuric acid and a zinc granule are mixed, bubbles of hydrogen gas are formed on the surface of the zinc granules. On passing the evolved hydrogen gas through a soap solution, bubbles are formed due to the low surface tension of the soap solution. When a burning candle is brought near these gas-filled bubbles, the gas burns with a pop sound. All other acids like \( \text{HCl} \), \( \text{HNO}_3 \), and \( \text{CH}_3\text{COOH} \) show the same observation.
In simple words: When zinc reacts with sulfuric acid, hydrogen gas bubbles form. Passing this gas through soap solution makes more bubbles. If you light a candle near these bubbles, they burn with a pop. Other acids act the same way.

Exam Tip: This activity clearly illustrates the reaction of metals with acids to produce hydrogen gas, and the 'pop' sound test is a definitive method for identifying hydrogen. Remember that this is a common characteristic reaction for many active metals with acids.

Exam Tip: Be able to draw and label diagrams for experimental setups like this one, as they often appear in exams to test your understanding of practical procedures.

Activity 2.4

Answer: 2 \( \text{mL} \) of \( \text{NaOH} \) solution + granulated zinc.
Reaction: \( \text{2NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \)
(Sodium zincate)
\( \rightarrow \) Bubbles of hydrogen gas are formed (same as in activity 2.3).
In simple words: When sodium hydroxide solution mixes with zinc, it creates sodium zincate and hydrogen gas bubbles, just like in the previous activity.

Exam Tip: This reaction demonstrates that some metals can react with strong bases to produce hydrogen gas. Remember the equation for the formation of sodium zincate.

Activity 2.5

Answer: Test tube A: \( \text{Na}_2\text{CO}_3\text{(s)} + \text{2HCl(aq)} \rightarrow \text{2NaCl(aq)} + \text{H}_2\text{O(Z)} + \text{CO}_2\text{(g)} \)
Test tube B: \( \text{NaHCO}_3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \)
On passing the \( \text{CO}_2 \) gas through lime water, the lime water turns milky because insoluble white precipitate of \( \text{CaCO}_3 \) is formed as shown below.
\( \text{Ca(OH)}_2 \text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} \)
(lime water) (white precipitate)
On passing excess gas through lime water, it becomes colorless.
\( \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2 \text{(aq)} \)
soluble in water
In simple words: Sodium carbonate and bicarbonate react with hydrochloric acid to make carbon dioxide. This gas makes limewater milky at first, then clear again if too much is added.

Exam Tip: Pay close attention to the two stages of the lime water test for carbon dioxide: initial milkiness due to calcium carbonate formation, followed by clarification if excess \( \text{CO}_2 \) forms soluble calcium bicarbonate. Ensure equations are balanced and states are correct.

Activity 2.6

Answer: \( \text{NaOH} \) solution + 2 drops of phenolphthalein solution \( \rightarrow \) Color of the solution is pink.
On adding dilute \( \text{HCl} \) solution drop by drop to the above solution \( \rightarrow \) The reaction mixture changes to colorless.
Phenolphthalein color changes from pink to colorless as neutralization of \( \text{HCl} \) and \( \text{NaOH} \) takes place.
On adding a few drops of \( \text{NaOH} \) to the above mixture, pink color reappears because initially the effect of base was nullified by an acid, so the color changed to colorless. However, upon the addition of some more base, the pink color of phenolphthalein reappeared.
In simple words: Adding phenolphthalein to \( \text{NaOH} \) makes it pink. Adding \( \text{HCl} \) turns it clear as it neutralizes. If more \( \text{NaOH} \) is added, the pink color returns.

Exam Tip: This activity beautifully demonstrates the concept of neutralization and how indicators like phenolphthalein change color at different pH levels, making them useful for titration experiments.

Activity 2.7

Answer: When dilute hydrochloric acid is added to copper oxide solution, the color of the solution becomes blue-green, and the copper oxide dissolves. The blue-green color is due to the formation of copper \( \text{(II)} \) chloride.
\( \text{CuO} + \text{2HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} \)
(blue-green color)
In simple words: When copper oxide mixes with dilute hydrochloric acid, the solution turns blue-green because copper (II) chloride is formed, and the copper oxide dissolves.

Exam Tip: This reaction is an example of a metal oxide reacting with an acid to form a salt and water, with a characteristic color change indicating the formation of a copper salt. Remember the color associated with copper (II) compounds.

Activity 2.8

Answer: In the given setup, when different solutions are poured into the beaker separately, the following observations were recorded:

In simple words: The diagram shows that solutions of acids and bases conduct electricity and make the bulb glow because they release ions. Glucose and alcohol solutions do not conduct electricity because they do not release ions.

Exam Tip: This experiment is key to understanding the electrolytic nature of acids and bases versus non-electrolytes. The presence of mobile ions (cations and anions) in solution is essential for electrical conductivity.

Activity 2.9

Answer: On adding concentrated sulphuric acid to the test tube containing \( \text{NaCl} \), the reaction takes place and dry \( \text{HCl} \) gas is produced. On testing this gas with litmus paper, the following observations were recorded:
\( \text{HCl} \) gas \( \rightarrow \) Dry litmus paper \( \rightarrow \) No change of color of blue litmus paper.
\( \text{HCl} \) gas \( \rightarrow \) Wet litmus paper \( \rightarrow \) blue litmus paper changes to red.
This activity infers that only \( \text{HCl} \) solution releases \( \text{H}^+ \) ions, and the acidic property exists due to \( \text{H}^+ \) ions.
In simple words: Concentrated sulfuric acid reacts with salt to make dry \( \text{HCl} \) gas. This gas doesn't change dry litmus paper. But with wet litmus paper, it turns blue paper red, showing it's acidic only when water is present to release \( \text{H}^+ \) ions.

Exam Tip: This activity reinforces the critical role of water for acids to exhibit their characteristic properties. Dry acids do not dissociate to form \( \text{H}^+ \) ions, hence they do not show acidic behavior.

Activity 2.10

Answer: On adding a few drops of concentrated \( \text{H}_2\text{SO}_4 \) to the water in a beaker, it becomes hot as the reaction is highly exothermic. On adding \( \text{NaOH} \) pellets to water, the beaker becomes hot, and the reaction is exothermic.
In simple words: When concentrated sulfuric acid or \( \text{NaOH} \) pellets are added to water, the mixture gets hot, meaning both reactions release a lot of heat.

Exam Tip: Be aware that both acid dilution and the dissolution of strong bases in water are highly exothermic processes. Always handle these substances with care and follow safety precautions.

Activity 2.11

Answer:

In simple words: This table displays different solutions, their pH paper color changes, approximate pH values, and whether they are acidic, basic, or neutral substances.

Exam Tip: Understanding the pH scale and the corresponding color changes of universal indicator paper is essential. Practice associating common substances with their approximate pH values and nature (acidic, basic, or neutral).

Activity 2.12

Answer: To test the pH of soil, take 2g soil in a test tube, add 5 \( \text{mL} \) water and shake it. Filter the contents, and collect the filtrate to check the pH.
If pH of soil is 7: It is neutral - plants grow well.
If pH is less than 7: It is acidic - plants are affected.
If pH is more than 7: It is basic - plants are affected.
In simple words: To find the soil's pH, mix 2g of soil with 5 \( \text{mL} \) of water, shake, and filter it. Check the liquid's pH. If pH is 7, plants grow well. Below 7 (acidic), plants are harmed. Above 7 (basic), plants are also harmed.

Exam Tip: Soil pH is crucial for agriculture as it impacts nutrient availability and plant health. Know the process for testing soil pH and the general effects of acidic and basic soil conditions on plant growth.

Activity 2.13

Answer: Formulae of the salts:

In simple words: This table provides the chemical formula for various salts, along with the specific acid and base from which each salt can be formed.

Exam Tip: Understanding how different acids and bases combine to form various salts is fundamental. Practice recalling the parent acid and base for common salts and their respective chemical formulas.

Activity 2.14

Answer:

In simple words: This table lists various salt solutions, their acid/basic/neutral nature, and the acids and bases used to form them.

Exam Tip: Classifying salts as acidic, basic, or neutral based on their parent acid and base (strong/weak) is an important skill. Neutral salts come from strong acid/strong base, acidic salts from strong acid/weak base, and basic salts from weak acid/strong base.

Activity 2.15

Answer: On heating blue crystals of copper sulphate, it becomes colorless, and a few drops of water are seen on the test tube. On adding a few drops of water to the heated (anhydrous) copper sulphate, the blue color of copper sulphate reappears.
In simple words: Blue copper sulfate crystals turn colorless when heated, and water droplets appear. If you add water back to the now colorless copper sulfate, it turns blue again.

Exam Tip: This activity demonstrates that water of crystallization is a chemical component of hydrated salts. Its removal changes the compound's physical properties (like color), and its reintroduction reverses these changes.

Gujarat Board Class 10 Science Acids, Bases and Salts Textbook Questions and Answers

 

Question 1. A solution turns red litmus blue, its pH is likely to be -
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10
In simple words: A solution that changes red litmus to blue is basic, meaning its pH will be higher than 7. Out of the given choices, only 10 is a basic pH.

Exam Tip: Recall the pH scale: pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is basic. Basic solutions turn red litmus blue.

 

Question 2. A solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains:
(a) \( \text{NaCl} \)
(b) \( \text{HCl} \)
(c) \( \text{LiCl} \)
(d) \( \text{KCl} \)
Answer: (b) HCl
In simple words: Eggshells are calcium carbonate. Only acids react with calcium carbonate to produce carbon dioxide, which turns limewater milky. Among the options, \( \text{HCl} \) is an acid.

Exam Tip: Eggshells are primarily calcium carbonate. Remember that carbonates react with acids to produce carbon dioxide gas, which then causes lime water to turn milky (due to calcium carbonate precipitation).

 

Question 3. 10 \( \text{mL} \) of a solution of \( \text{NaOH} \) is found to be completely neutralised by 8 \( \text{mL} \) of a given solution of \( \text{HCl} \). If we take 20 \( \text{mL} \) of the same solution of \( \text{NaOH} \), the amount \( \text{HCl} \) solution (the same solution as before) required to neutralise it will be
(a) 4 \( \text{mL} \)
(b) 8 \( \text{mL} \)
(c) 12 \( \text{mL} \)
(d) 16 \( \text{mL} \)
Answer: (d) 16 \( \text{mL} \)
In simple words: If 10 \( \text{mL} \) of \( \text{NaOH} \) needs 8 \( \text{mL} \) of \( \text{HCl} \), then doubling the \( \text{NaOH} \) to 20 \( \text{mL} \) will also double the required \( \text{HCl} \) to 16 \( \text{mL} \).

Exam Tip: This is a direct proportionality problem related to neutralization. If you double the volume of one reactant, you must double the volume of the other reactant (assuming concentrations remain constant) to achieve neutralization.

 

Question 4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: (c) Antacid
In simple words: Antacids are medicines that help calm an upset stomach by neutralizing too much acid.

Exam Tip: Understand the function of each type of medicine. Antacids specifically neutralize excess stomach acid, which is the cause of indigestion.

 

Question 5. Write word equations and then balance equations for the reaction taking place when:
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Answer:
(a) Zinc granule + dil. Hydrochloric acid \( \rightarrow \) Zinc chloride + Hydrogen gas
\( \text{Zn(s)} + \text{2HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} \)
(b) Magnesium + dil. Hydrochloric acid \( \rightarrow \) Magnesium chloride + Hydrogen gas
\( \text{Mg(s)} + \text{2HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} \)
(c) Aluminium + dil. Sulphuric acid \( \rightarrow \) Aluminium sulphate + Hydrogen gas
\( \text{2Al(s)} + \text{3H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Al}_2\text{(SO}_4)_3\text{(aq)} + \text{3H}_2\text{(g)} \)
In simple words: (a) Zinc and hydrochloric acid make zinc chloride and hydrogen gas. (b) Magnesium and hydrochloric acid produce magnesium chloride and hydrogen gas. (c) Aluminium and sulfuric acid form aluminium sulfate and hydrogen gas.

Exam Tip: For reactions involving metals and acids, remember the general pattern: Metal + Acid \( \rightarrow \) Salt + Hydrogen gas. Practice balancing these equations, ensuring all atoms are accounted for on both sides.

 

Question 5. Write word equations and then balance equations for the reaction taking place when:
(d) dilute hydrochloric acid reacts with iron filings.
Answer:
(d) Iron + dil. Hydrochloric acid \( \rightarrow \) Iron chloride + Hydrogen
\( \text{2Fe(s)} + \text{6HCl(aq)} \rightarrow \text{2FeCl}_3\text{(aq)} + \text{3H}_2\text{(g)} \)
In simple words: Iron and dilute hydrochloric acid create iron chloride and hydrogen gas.

Exam Tip: Remember to consider the oxidation state of the metal. Iron can form both \( \text{Fe(II)} \) and \( \text{Fe(III)} \) chlorides; here, it forms iron (III) chloride \( (\text{FeCl}_3) \). Always ensure the equation is balanced correctly.

 

Question 6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Answer: 1. Take a cork with two nails fixed on it.
2. Keep this cork inside the beaker.
3. Connect the nails to the battery, bulb, and key as shown in the figure.
4. Now add ethanol in it and record your observation. Repeat the same setup for glucose and record your observations.
Observation: The bulb will not glow because charge is not flowing through it.
Conclusion: The experiment shows that glucose and ethanol do not ionize (hydrogen ions are not released); therefore, they are not categorized as acids.
In simple words: Set up an electrical circuit with a bulb, battery, and two nails in a beaker. Fill the beaker with ethanol, then glucose. The bulb won't light up for either. This means they don't release \( \text{H}^+ \) ions and aren't acids, even though they contain hydrogen.

Exam Tip: This activity is crucial for distinguishing between substances containing hydrogen that are acidic (like \( \text{HCl} \)) and those that are not (like glucose and alcohol). The key is the ability to ionize and release \( \text{H}^+ \) ions in an aqueous solution.

Exam Tip: Be able to replicate this diagram to demonstrate conductivity differences. The key takeaway is that conductivity depends on the presence of mobile ions, not just the presence of hydrogen in the chemical formula.

 

Question 7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water is pure water and it does not form ions. Rain water, however, contains impurities like acid, which contain ions and release them when dissolved in water. Hence, there are no ions in distilled water, so electricity is not conducted, but there are ions in rain water, so electricity is conducted.
In simple words: Pure distilled water doesn't have ions, so it can't carry electricity. Rainwater has impurities, like dissolved acids, which form ions, allowing it to conduct electricity.

Exam Tip: The presence of dissolved ions is crucial for electrical conductivity in water. Distilled water lacks these ions, making it a poor conductor, while rainwater's impurities make it a good conductor.

 

Question 8. Why do acids not show acidic behaviour in the absence of water?
Answer: Acids cannot release \( \text{H}^+ \) ions in the absence of water; only upon dissolving in water do acids release \( \text{H}^+ \) ions. Acids show acidic behavior only due to \( \text{H}^+ \) ions released in the presence of water.
In simple words: Acids only act like acids and give off \( \text{H}^+ \) ions when they are mixed in water. Without water, they don't show their acidic properties.

Exam Tip: This question emphasizes the crucial role of water in the ionization of acids. Without water, acids exist as molecules and do not furnish the \( \text{H}^+ \) ions responsible for their acidic properties.

 

Question 9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen ion concentration.
Answer:
(i)
(a) 'D' with \( \text{pH} = 7 \) is neutral.
(b) 'C' with \( \text{pH} = 11 \) is strongly alkaline.
(c) 'B' with \( \text{pH} = 1 \) is strongly acidic.
(d) 'A' with \( \text{pH} = 4 \) is weakly acidic.
(e) 'E' with \( \text{pH} = 9 \) is weakly alkaline.
(ii) pH in increasing order of hydrogen ion concentration:
\( 11 < 9 < 7 < 4 < 1 \)
\( \text{C} < \text{E} < \text{D} < \text{A} < \text{B} \)
In simple words: Based on their pH values: Solution D (pH 7) is neutral. Solution C (pH 11) is strongly basic. Solution B (pH 1) is strongly acidic. Solution A (pH 4) is weakly acidic. Solution E (pH 9) is weakly basic. For hydrogen ion concentration, the order from lowest to highest is C, E, D, A, B.

Exam Tip: Remember the pH scale: pH 7 is neutral, lower pH values mean stronger acidity (higher \( \text{H}^+ \) concentration), and higher pH values mean stronger alkalinity (lower \( \text{H}^+ \) concentration). Strong acids are typically pH 1-2, strong bases pH 13-14.

 

Question 10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid \( (\text{HCl}) \) is added to test tube A, while acetic acid \( (\text{CH}_3\text{COOH}) \) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Answer: In test tube A, hydrochloric acid is present, which is a strong acid compared to acetic acid present in test tube B. The fizzing occurs more vigorously in test tube A as \( \text{HCl} \) is strong and dissociates completely into \( \text{H}^+ \) and \( \text{Cl}^- \) ions for the reaction.
In simple words: The fizzing will be stronger in test tube A because it has hydrochloric acid. Hydrochloric acid is a strong acid that fully breaks apart into ions, making it react faster than acetic acid, which is a weak acid.

Exam Tip: The vigor of fizzing (rate of reaction) is directly related to the strength of the acid. Strong acids ionize completely, releasing a high concentration of \( \text{H}^+ \) ions, which leads to a faster reaction with metals compared to weak acids.

 

Question 11. Fresh milk has pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer: The pH of milk will decrease when it turns into curd due to the formation of lactic acid.
In simple words: As milk turns into curd, its pH will go down because lactic acid is formed, making it more acidic.

Exam Tip: Remember that curdling involves fermentation by lactic acid bacteria, which convert lactose sugar into lactic acid. The increase in lactic acid concentration lowers the pH, making the curd acidic.

 

Question 12. A milkman adds a very small amount of baking soda to fresh milk.
1. Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
2. Why does this milk take a long time to set as curd?
Answer:
1. pH is shifted to alkaline so that milk does not spoil by becoming sour by releasing lactic acid.
2. Milk is made alkaline by adding baking soda so the lactic acid formed will get neutralized and therefore will take a little more time to set and become acidic.
In simple words: 1. The milkman adds baking soda to make milk slightly basic so it doesn't spoil quickly from lactic acid. 2. Because the milk is basic, the lactic acid forming during curdling is neutralized, so it takes longer for the milk to become acidic enough to set into curd.

Exam Tip: Baking soda (sodium bicarbonate) is a mild base. Adding it to milk increases the pH, slowing down the natural souring process caused by lactic acid formation. This delay prolongs the milk's freshness and extends the time needed for curd formation.

 

Question 13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: Plaster of Paris reacts with moisture/water to form gypsum.
In simple words: Plaster of Paris must be kept in a dry container because it reacts with water to turn into hard gypsum.

Exam Tip: This question tests your knowledge of the chemical properties of Plaster of Paris. The setting of Plaster of Paris is a rehydration reaction, so it must be protected from any moisture to prevent premature hardening.

 

Question 14. What is a neutralisation reaction? Give two examples.
Answer: The reaction in which an acid reacts with a base (loses its property) to form salt and water is called a neutralization reaction.
Example:
(i) \( \text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
Acid + Base \( \rightarrow \) Salt + Water
(ii) \( \text{HCl(aq)} + \text{KOH(aq)} \rightarrow \text{KCl} + \text{H}_2\text{O} \)
In simple words: A neutralization reaction happens when an acid and a base mix, losing their original properties to form salt and water. For example, hydrochloric acid reacting with sodium hydroxide or potassium hydroxide.

Exam Tip: Define neutralization precisely: the reaction between an acid and a base to produce a salt and water. Provide balanced chemical equations for two different examples, clearly identifying the acid, base, salt, and water.

 

Question 15. Give two important uses of washing soda and baking soda.
Answer:
Washing soda:
- It helps in glass, soap, and paper production.
- It is also used to remove permanent hardness from water.
Baking soda:
- It works as an antacid to relieve indigestion.
- It is also used in soda-acid fire extinguishers to put out fires.
In simple words: Washing soda helps make glass and soap, and removes tough water hardness. Baking soda acts as an antacid for upset stomachs and is used in fire extinguishers.

Exam Tip: Remember specific applications for common chemical compounds. For washing soda, focus on industrial uses and water treatment. For baking soda, highlight its use as an antacid and in fire safety.

Short Answer Type Questions

 

Question 1. What happens when
(a) Baking soda is heated
(b) Blue coloured copper sulphate crystals are heated
(c) Water is added to lime?
Answer:
(a) When baking soda is warmed, it produces sodium carbonate, water, and gives off carbon dioxide gas.
\( 2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \)
(b) Blue tinted copper sulphate, when warmed, turns colourless because it gives up its water of crystal structure.
\( \text{CuSO}_4.5\text{H}_2\text{O} \xrightarrow{\text{heat}} \text{CuSO}_4 + 5\text{H}_2\text{O} \)
(c) On mixing water with lime, heat is produced, and slaked lime is created.
\( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{heat} \)
In simple words: Heating baking soda creates sodium carbonate, water, and carbon dioxide. When blue copper sulphate is heated, it becomes clear because it loses its water. Mixing water with lime produces heat and slaked lime.

Exam Tip: For reactions involving heating or mixing, ensure you remember the key products and state changes. Always include balanced chemical equations.

 

Question 2. A white powder which sets hard on adding water is also used in hospitals. Name this powder. How is it prepared? Write the chemical reaction involved in its preparation.
Answer: The white powder mentioned is Plaster of Paris \( (\text{CaSO}_4.\frac{1}{2} \text{H}_2\text{O}) \). It is made by warming gypsum at 393 K.
\( \text{CaSO}_4.2\text{H}_2\text{O} \xrightarrow{373\text{K}} \text{CaSO}_4.\frac{1}{2} \text{H}_2\text{O} + \frac{3}{2} \text{H}_2\text{O} \)
In simple words: The powder is Plaster of Paris. It's made by heating gypsum to a specific temperature.

Exam Tip: Remember the common name, chemical formula, and preparation method for key compounds. Pay attention to specific conditions like temperature.

 

Question 3. Write balanced chemical equations for the following:
1. Calcium carbonate reacts with hydrochloric acid
2. Dilute sulphuric acid reacts with zinc granules
3. Calcium oxychloride reacts with hydrochloric acid.
Answer: The balanced chemical equations for these reactions are:
1. \( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow \)
2. \( \text{H}_2\text{SO}_4 + \text{Zn} \rightarrow \text{ZnSO}_4 + \text{H}_2 \)
3. \( \text{CaOCl}_2 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{Cl}_2 \)
In simple words: These are the correct chemical equations that show how the substances react and balance out.

Exam Tip: Always ensure chemical equations are balanced for both atoms and charge. Indicate the state symbols if required, and recognize common reaction products like carbon dioxide or hydrogen gas.

 

Question 4. Name the ions present in the following salts. Name the acid and base from which they can be obtained. magnesium sulphate, sodium carbonate, potassium chloride.
Answer: The specific ions found in these compounds are as follows:

In simple words: This table shows which parts (ions) make up each salt, and which acid and base come together to create them.

Exam Tip: To identify the parent acid and base of a salt, separate the cation (from base) and anion (from acid). Remember common polyatomic ions and their charges.

 

Question 5. Give three ways in which salts can be prepared.
Answer: Salts can be gotten in these ways:
(i) Acid combines with base to yield salt and water.
\( \text{HCl} + \text{KOH} \rightarrow \text{KCl} + \text{H}_2\text{O} \)
(ii) Metals combine with acids to create salt and produce hydrogen gas.
\( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \)
(iii) Metallic oxides combine with acids to create salt and water.
\( \text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} \)
In simple words: Salts can be made in a few ways: by mixing an acid and a base, by a metal reacting with an acid, or by a metal oxide reacting with an acid.

Exam Tip: When asked for preparation methods, always include the general principle and a specific chemical equation as an example for each method.

 

Question 6. Give one example for each of the following acids salts – chloride salts, carbonate salts and sulphate salts.
Answer:
1. Chloride salts: Magnesium chloride, Calcium chloride
2. Carbonate salts: Sodium carbonate, Potassium carbonate
3. Sulphate salts: Calcium sulphate, Magnesium sulphate
In simple words: Here are some examples of salts, grouped by the type of non-metal part they contain: chloride, carbonate, and sulphate.

Exam Tip: Remember common examples of salts for different acid types. Familiarity with everyday chemicals helps.

 

Question 7. Name the acid present in the following: Vinegar, Lemon, Tomato, Tamarind, Orange, Curd.
Answer: The acids found in these items are:
- Vinegar: Acetic acid
- Lemon: Citric acid
- Tomato: Oxalic acid
- Tamarind: Tartaric acid
- Orange: Citric acid
- Curd: Lactic acid
In simple words: Each of these food items gets its sour taste from a specific acid, like acetic acid in vinegar or citric acid in lemons.

Exam Tip: Memorize the common acids associated with everyday substances, especially fruits and food products, as these are frequently asked.

 

Question 8. Name the properties responsible for the following uses of baking powder:
1. Baking industry
2. As an antacid
3. As soda-acid fire extinguisher
Answer: The properties of baking powder that make it useful for the given purposes are:
1. Baking industry: When it warms up, baking powder gives off carbon dioxide gas, which helps dough rise and makes baked goods fluffy.
2. As an antacid: It is alkaline, which means it helps neutralize too much acid in the stomach, easing indigestion.
3. As soda-acid fire extinguisher: When it combines with an acid, it gives off carbon dioxide gas, which can put out fires.
In simple words: Baking powder is used in baking because it releases gas when heated, as an antacid because it's alkaline, and in fire extinguishers because it produces CO2 with acid.

Exam Tip: When linking properties to uses, clearly state the property first and then explain how it enables the specific application. Focus on chemical reactions or physical characteristics.

 

Question 9. Give the properties and uses of bleaching powder.
Answer: Bleaching powder has a strong oxidising property, which gives it several important uses:
1. Bleaching: It is used for making cotton, wood pulp, and clothes whiter.
2. Oxidising agent: It serves as an oxidising agent in various chemical industries.
3. Disinfectant: It helps to eliminate germs and purify drinking water.
In simple words: Bleaching powder is good at oxidizing things. This property makes it useful for making things white, in factories, and for cleaning water to kill germs.

Exam Tip: For compounds like bleaching powder, remember its key property (oxidizing agent) and how this property applies to its various uses, such as bleaching and disinfection.

 

Question 10. Acid when reacts with metal release hydrogen gas but there is one acid which when reacts with metal does not release hydrogen except for two metals. Prove this statement.
Answer: Generally, when an acid combines with a metal, it produces salt and releases hydrogen gas. Here are some examples:
\( 2\text{HCl} + 2\text{Na} \rightarrow 2\text{NaCl} + \text{H}_2 \)
\( \text{H}_2\text{SO}_4 + 2\text{Na} \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2 \)
However, nitric acid \( (\text{HNO}_3) \) does not typically release hydrogen gas when it reacts with metals. This is because nitric acid functions as a powerful oxidising agent.
\( \text{HNO}_3 + \text{Na} \rightarrow \text{No hydrogen} \)
Nitric acid only produces hydrogen gas when it reacts with magnesium and manganese, making these two metals exceptions.
\( \text{Mg} + 2\text{HNO}_3 \rightarrow \text{Mg(NO}_3)_2 + \text{H}_2\uparrow \)
\( \text{Mn} + 2\text{HNO}_3 \rightarrow \text{Mn(NO}_3)_2 + \text{H}_2\uparrow \)
In simple words: Most acids reacting with metals make hydrogen gas. But nitric acid doesn't, because it's a strong oxidizer. The only exceptions are magnesium and manganese, which do make hydrogen with nitric acid.

Exam Tip: Remember the general rule for acid-metal reactions (salt + hydrogen) and the important exception for nitric acid, noting that it produces hydrogen only with magnesium and manganese.

 

Question 11. Give six uses of acids.
Answer: Acids are used for various purposes, including:
- In storage batteries, such as with \( \text{H}_2\text{SO}_4 \).
- As a food preservative, using substances like acetic acid.
- In making baking powder, specifically tartaric acid.
- In manufacturing fertilizers, where nitric acid is used.
- In producing PVC (Polyvinyl chloride), utilizing hydrochloric acid.
- As a component in bathroom cleaners, typically hydrochloric acid.
In simple words: Acids have many uses, from car batteries and food preservation to making baking powder, fertilizers, plastics, and cleaners.

Exam Tip: When listing uses, provide specific examples or chemical names (like \( \text{H}_2\text{SO}_4 \) for storage batteries) to show a deeper understanding.

 

Question 12. Give six uses of bases.
Answer: Bases serve several functions, such as:
- In creating soap, often using \( \text{NaOH} \).
- As an antacid, like \( \text{Mg(OH)}_2 \).
- In producing bleaching powder, involving \( \text{Ca(OH)}_2 \).
- For decreasing soil acidity, using \( \text{Ca(OH)}_2 \).
- In white washing, with \( \text{Ca(OH)}_2 \).
- In making fertilizers, using \( \text{NH}_4\text{OH} \).
In simple words: Bases are used to make soap, as antacids, in bleaching powder, to reduce soil acidity, for whitewashing, and to create fertilizers.

Exam Tip: For each use of a base, try to recall the specific chemical involved (e.g., \( \text{NaOH} \) for soap) to enhance your answer.

 

Question 13. Give six uses of salts.
Answer: Salts have many uses, including:
- Adding flavor to food, such as common salt \( (\text{NaCl}) \).
- In making freezing mixtures, also using common salt \( (\text{NaCl}) \).
- As washing soda for cleaning clothes \( (\text{Na}_2\text{CO}_3) \).
- For preserving food items through salting.
- In manufacturing processes for items like glass and paper, using compounds like \( \text{Na}_2\text{CO}_3 \).
- As a de-icing agent to melt ice on roads during winter.
In simple words: Salts are used for flavoring food, creating freezing mixtures, washing clothes, preserving food, in making glass, and melting ice on roads.

Exam Tip: Think of common household and industrial applications for various salts. Provide specific examples where possible to illustrate each use.

 

Question 14. Four samples A, B, C and D were given to test their nature. A student found the change in pH paper as follows:
A → green colour C → blue colour
B → orange colour D → pink colour
Find the nature of given sample.
Answer: Based on the observed color changes in pH paper and their corresponding pH values, the nature of each sample is determined as follows:

In simple words: By looking at the pH paper color and pH number, we can tell if each sample is neutral, weakly acidic, basic, or strongly acidic.

Exam Tip: Remember the pH scale and its correlation with universal indicator colors. pH 7 is neutral, below 7 is acidic (lower pH means stronger acid), and above 7 is basic (higher pH means stronger base).

Long Answer Type Questions

 

Question 1. What are acids? Give three properties of acids. Give the test for acids.
Answer: Acids are compounds that give off hydrogen ions \( (\text{H}^+) \) when put in water. Key features of acids include:
- Acids taste sour.
- Acids carry electricity when put in water.
- Acids combine with metals to make salt and hydrogen gas.
To test for acids:
- Blue litmus paper turns red.
- Methyl orange indicator turns red in the presence of acid.
In simple words: Acids are compounds that release H+ ions in water, taste sour, conduct electricity, and react with metals. They turn blue litmus red and methyl orange red.

Exam Tip: For definitions, include the key chemical characteristic (H+ ion release). When listing properties, give both physical (taste) and chemical (reactions, conductivity) aspects. For tests, mention specific color changes with indicators.

 

Question 2. Design an activity to prove that acids show acidic behaviour only when dissolved in water.
Answer: To demonstrate that acids show acidic properties only when dissolved in water, an experiment can be carried out using dry HCl gas.
1. Put solid NaCl (common salt) into a clean test tube. After that, add a few drops of strong sulphuric acid.
2. Seal the test tube's opening with a cork holding a delivery tube. The reaction will begin, and hydrogen chloride gas will exit through the tube.
3. Initially, hold a piece of dry blue litmus paper close to the delivery tube's exit. You will observe no change in the paper's hue.
4. Following this, moisten a piece of blue litmus paper and bring it near the delivery tube. The moist blue litmus paper will then turn red.
This experiment reveals that dry HCl gas does not give off hydrogen ions \( (\text{H}^+) \) and, as a result, does not display acidic characteristics. Nevertheless, when the HCl gas meets the water on the wet litmus paper, it dissolves, releases \( \text{H}^+ \) ions, and shows its acidic nature by turning the litmus paper red.
In simple words: To prove acids act acidic only in water, use dry HCl gas. Dry litmus paper won't change color. But wet litmus paper turns red, showing HCl needs water to become an acid.

Exam Tip: When describing an activity, clearly outline the materials, procedure, observations, and conclusion. Emphasize the role of water in acid dissociation and color change with indicators.

 

Question 3. How do acids and bases react with each other? Explain with an activity.
Answer: Use a test tube containing 2 mL of a base, like \( \text{NaOH} \) solution. Include 2 drops of phenolphthalein solution as an indicator. The solution becomes pink.
Next, add \( \text{HCl} \) solution drop by drop to this test tube; the pink color starts fading. At this moment, we state that the acid combines with the base to become neutral. If you add a few drops of \( \text{NaOH} \) to the previously created neutral solution, the pink color of phenolphthalein shows up again. Adding a few more drops of \( \text{HCl} \) makes the color vanish. This demonstrates that acids combine with bases to create (neutral) salt and water.
In simple words: Acids and bases react in a process called neutralization to form salt and water. This can be seen by adding an acid to a pink basic solution with phenolphthalein; the pink disappears when neutral and reappears with more base.

Exam Tip: When explaining neutralization, clearly describe the indicator's role and color changes. Emphasize that adding acid/base reverses the color, demonstrating the point of neutrality.

 

Question 4. What is bleaching powder? How is bleaching powder produced? Give its chemical . equation and write its three uses.
Answer: Bleaching powder is chemically known as calcium oxychloride, \( \text{CaOCl}_2 \).
Preparation: By bubbling chlorine gas through a test tube holding dry slaked lime \( \text{Ca(OH)}_2 \), bleaching powder is formed.
\( \text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O} \)
Its applications include:
- It helps as a bleaching agent in textile industries and paper mills.
- It serves as an oxidising agent in various chemical industries.
- It acts as a disinfectant for purifying drinking water.
In simple words: Bleaching powder is \( \text{CaOCl}_2 \), made by reacting chlorine gas with slaked lime. It's used to bleach, as an oxidizer in factories, and to disinfect water.

Exam Tip: For important compounds, remember its chemical name, formula, preparation method (with equation), and at least three distinct uses. Understand the underlying property (e.g., oxidizing) that drives these uses.

 

Question 5. What is baking soda and baking powder chemically? What would happen if we add baking soda in making cakes instead of baking powder?
Answer: Baking soda is \( \text{NaHCO}_3 \), also known as sodium hydrogen carbonate.
Baking powder is a mixture of \( \text{NaHCO}_3 \) and tartaric acid.
If we include only baking soda in making cakes and then heat them, it will make sodium carbonate, which gives a bitter flavor to the cake. To prevent this, tartaric acid is combined with baking soda; this forms sodium salt of the acid, which does not alter the cake's taste.
\( 2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \)
\( \text{NaHCO}_3 + \text{H}^+ \xrightarrow{\text{heat}} \text{CO}_2 + \text{H}_2\text{O} + \text{sodium salt of acid} \)
In simple words: Baking soda is sodium hydrogen carbonate, while baking powder adds tartaric acid. Using only baking soda in cakes makes them bitter due to sodium carbonate. Tartaric acid in baking powder stops this, ensuring a good taste.

Exam Tip: Differentiate between baking soda and baking powder by their chemical composition. Explain the chemical reason for the bitter taste (sodium carbonate) and how tartaric acid mitigates this issue.

 

Question 6. Crystals of salt are formed by holding some water molecules. How can you show that crystals of salt are not really dry?
Answer: To prove that salt crystals are not completely dry:
Use a dry test tube, place some blue copper sulphate crystals inside. Secure the test tube in a stand and warm it over a flame. The blue color of the copper sulphate turns colourless, and drops of water are observed on the inner walls of the test tube.
In simple words: To show salt crystals aren't dry, heat blue copper sulphate in a test tube. It turns colorless and water drops appear, proving the crystals held water.

Exam Tip: When describing an experiment, ensure all steps are clear and concise. Highlight the observable changes (color change, water formation) that lead to the conclusion about water of crystallization.

Practical Based Questions (Solved)

 

Question 1. A student wants to test the pH of human saliva at different times. Suggest the right time to collect the pH of it for wide variety and why?
Answer: The student needs to gather saliva samples first thing in the morning without cleaning teeth. Subsequently, samples should be taken after cleaning teeth, prior to eating any meal, and post-eating. The pH of the mouth alters when we consume food and also when we clean teeth.
In simple words: To get a full range of saliva pH, collect samples first thing in the morning (before brushing), after brushing, before eating, and after eating. This shows how brushing and food change mouth pH.

Exam Tip: For experiments involving biological samples, consider all factors that might influence the reading (e.g., food, brushing). Collect samples at various stages to get a comprehensive understanding.

 

Question 2. In the lab a test tube rack is placed with test tubes containing some acids in it. How will you classify these acids?
Answer: To sort the acids, we need to check their pH values and categorize them into strong or weak acids.
In simple words: To sort acids, we check their pH. Acids with a low pH (e.g., 1-2) are strong, while those with a higher pH (e.g., 4-6) are weak.

Exam Tip: Classification of acids primarily relies on their pH values. Lower pH indicates a stronger acid, while higher pH (still below 7) indicates a weaker acid.

 

Question 3. What is the pH of water? Does it change on heating the water? Explain.
Answer: The pH of pure water is typically 7 at 25 degrees Celsius, showing it is neutral.
When water is warmed, its pH will go down. This occurs because the autoionization of water \( (\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-) \) goes up at higher temperatures, meaning more \( \text{H}^+ \) and \( \text{OH}^- \) ions are made. Even though the amounts of \( \text{H}^+ \) and \( \text{OH}^- \) ions stay balanced (keeping the solution neutral), the actual pH number itself gets smaller because the \( \text{H}^+ \) amount has increased compared to what it would be at a cooler temperature for the identical pH figure. So, while it appears 'less neutral' in terms of the pH number, it stays chemically neutral because \( [\text{H}^+] = [\text{OH}^-] \).
In simple words: Pure water has a pH of 7 at 25°C. When heated, its pH drops because water creates more H+ and OH- ions, but it stays chemically neutral since the H+ and OH- ions remain equal.

Exam Tip: Be precise about the definition of neutral pH (7 at 25°C). When explaining the effect of temperature, focus on the increased autoionization of water and how this impacts the numerical pH while maintaining chemical neutrality.

 

Question 4. A test was conducted in the lab to find the pH of different cold drinks. Draw the observation table to collect the data for this experiment and predict the result for the same.
Answer: Observation table to gather data:

Prediction: The pH of dark-colored cold drinks (like cola) will likely be quite low (acidic), usually around pH 2.5-3.5. This occurs because they frequently contain phosphoric acid and carbonic acid (from the carbonation process), making them very acidic. Cold drinks that give off more bubbles, showing strong carbonation, will also have a lower pH due to the creation of carbonic acid.
In simple words: Dark soft drinks are usually very acidic (low pH) because of added acids and carbonation. Drinks with lots of bubbles are also acidic due to carbonic acid.

Exam Tip: When predicting pH, consider the ingredients. Carbonated drinks are acidic due to carbonic acid. Many dark soft drinks contain additional acids, leading to very low pH values.

 

Question 5. A student wants to test the pH of few Salts or ER Rening is highly acidic of dr aching powder and eon salt. How can he find the pH of dry salts? Explain
Answer: The pH of dry salts cannot be determined. So, all these salts must be mixed in distilled water to find their pH, and the samples can then be checked with pH paper.
In simple words: You can't test the pH of dry salts. You must dissolve them in distilled water first, then use pH paper to check the solution's pH.

Exam Tip: Remember that pH is a measure of ion concentration in a solution. Dry salts do not have free ions in sufficient quantity to measure pH directly; they must be dissolved in a solvent like distilled water first.

 

Question 6. Four juices were taken in different test tubes to find their pH, the colour changes on the pH paper were recorded as pink, orange, yellow and green. What conclusion can you draw from this data.
Answer: The sample that showed a pink color has a reduced pH, meaning it is more strongly acidic than the others. The samples that showed orange and yellow colors have an elevated pH, indicating they are not as acidic as the pink one. The sample with a green color change is balanced.
In simple words: Pink pH paper means very acidic (low pH), orange/yellow means less acidic (higher pH but still acidic), and green means neutral. So, pink is strongest acid, green is neutral.

Exam Tip: Relate pH paper color changes directly to the pH scale. Pink/red indicates strong acid, orange/yellow indicates weaker acid, green indicates neutral, and blue/violet indicates base.

 

Question 7. In one of the projects at school students were told to make natural indicators to test the samples for their nature. Suggest two natural indicators students can make easily.
Answer: Students could prepare natural indicators from red cabbage or hibiscus flowers. By mashing the red cabbage or hibiscus and extracting the juice, it can be applied as a natural indicator because these substances alter their hue and show varied outcomes in acids and bases.
In simple words: Students can make natural indicators from red cabbage or hibiscus flowers by mashing them and extracting their juice. These juices change color differently in acids and bases.

Exam Tip: Natural indicators like red cabbage and hibiscus are effective. Remember to mention that the key is extracting their juice, which then shows different colors in acidic and basic solutions.