CBSE Class 10 Science Acids bases and salts Notes

CBSE Class 10 Science Acids bases and salts Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

CHAPTER - 2

ACIDS, BASES AND SALTS

ACIDS

An acid is a substance which furnishes H⁺ ions or H₃O⁺ ions when dissolved in water. Acids have one or more replaceable hydrogen atoms. The word acid is derived from the Latin word 'acidus', meaning sour. Substances with a sour taste are acids. Lemon juice, vinegar and grape juice taste sour, so they are acidic. Acids change blue litmus to red. They are colourless with phenolphthalein and pink with methyl orange. Many foods contain acids and taste sour, such as curd, tamarind and lemon.

CLASSIFICATION OF ACIDS

1. Based on their sources: Acids are classified as organic acids and inorganic acids.
Organic acids are present in plants and animals (living beings), for example HCOOH and CH₃COOH. They are weak acids.
Inorganic acids are obtained from rocks and minerals, and are also called mineral acids, for example HCl, HNO₃ and H₂SO₄. They are strong acids.

2. Based on basicity:
Monobasic acid: gives one hydrogen ion per molecule in solution, for example HCl and HNO₃.
Dibasic acid: gives two hydrogen ions per molecule in solution, for example H₂SO₄ and H₂CO₃.
Tribasic acid: gives three hydrogen ions per molecule in solution, for example H₃PO₄.

3. Based on ionisation:
Strong acids ionise completely in water, for example HCl.
Weak acids ionise partially in water, for example CH₃COOH.

4. Based on concentration: Acids are classified as concentrated acids and dilute acids, depending on the amount of acid dissolved in water.
A concentrated acid has a relatively high percentage of acid in its aqueous solution.
A dilute acid has a relatively low percentage of acid in its aqueous solution.

INTEXT QUESTIONS PAGE NO. 18

Question 1: You have three test tubes. One contains distilled water and the other two contain an acidic solution and a basic solution. If you are given only red litmus paper, how will you identify the contents of each test tube?

Answer:
Mark the test tubes A, B and C. Put a drop of each solution on red litmus paper. The solution that turns the paper blue is the basic solution. Take a drop of this basic solution and add it to each of the other two solutions, then test each mixture with red litmus paper. If the mixture turns red litmus blue, the second solution was neutral and the base was still in excess. If there is no change in colour, the base has been neutralised, so the second solution was acidic. This way all three solutions are identified.

CHEMICAL PROPERTIES OF ACIDS

Reaction of acids with metals: Acids react with metals to give a salt and hydrogen gas.
Metal + Acid → Salt + Hydrogen

Examples:
Zn + 2HCl → ZnCl₂ + H₂ (hydrochloric acid and zinc give zinc chloride and hydrogen)
2Na + 2HCl → 2NaCl + H₂ (hydrochloric acid and sodium give sodium chloride and hydrogen)
Fe + 2HCl → FeCl₂ + H₂ (hydrochloric acid and iron give iron(II) chloride and hydrogen)
Zn + H₂SO₄ → ZnSO₄ + H₂ (sulphuric acid and zinc give zinc sulphate and hydrogen)

Reaction of acids with metal carbonates: Acids react with metal carbonates to give a salt, carbon dioxide and water.
Metal carbonate + Acid → Salt + Carbon dioxide + Water

Examples:
CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
MgCO₃ + 2HCl → MgCl₂ + CO₂ + H₂O
Na₂CO₃ + 2HNO₃ → 2NaNO₃ + CO₂ + H₂O

Reaction of acids with metal hydrogen carbonates (bicarbonates): Acids react with metal hydrogen carbonates to give a salt, carbon dioxide and water.
Acid + Metal hydrogen carbonate → Salt + Carbon dioxide + Water

Examples:
NaHCO₃ + HCl → NaCl + CO₂ + H₂O
2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2CO₂ + 2H₂O

Sodium hydrogen carbonate (NaHCO₃) is also known as baking soda, baking powder, bread soda, cooking soda and bicarbonate of soda.

Carbon dioxide turns lime water milky, because calcium carbonate, a white precipitate, is formed.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
If excess carbon dioxide is passed, the milkiness disappears, because soluble calcium hydrogen carbonate is formed.
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

Reaction of acids with marble and eggshell: Marble and eggshell are made of calcium carbonate. When acid is poured over them, bubbles of carbon dioxide are formed.

USES OF ACIDS

1. Sulphuric acid, called the king of chemicals, is used in car batteries and in the preparation of many other compounds.
2. Nitric acid is used to make ammonium nitrate, which is used as a fertiliser.
3. Hydrochloric acid is used as a cleaning agent in toilets.
4. Tartaric acid is a constituent of baking powder.
5. Sodium benzoate, a salt of benzoic acid, is used for food preservation.
6. Carbonic acid is used in aerated drinks.

BASES

A base is a substance which releases hydroxide ions when dissolved in water. Bases are bitter in taste and soapy to touch, for example washing soda, caustic soda and caustic potash. They turn red litmus blue. They are pink with phenolphthalein and yellow with methyl orange.

CLASSIFICATION OF BASES

1. Based on ionisation:
Strong bases ionise completely in aqueous solution, for example NaOH and KOH.
Weak bases ionise partially in aqueous solution, for example NH₄OH and Ca(OH)₂.

2. Based on acidity:
Monoacidic base gives one hydroxide ion per molecule, for example NaOH and KOH.
Diacidic base gives two hydroxide ions per molecule, for example Ca(OH)₂ and Mg(OH)₂.
Triacidic base gives three hydroxide ions per molecule, for example Al(OH)₃ and Fe(OH)₃.

3. Based on concentration: Bases are classified as concentrated alkalis and dilute alkalis, depending on the amount of base dissolved in water.

Concentrated alkali: an alkali having a relatively high percentage of alkali in its aqueous solution.
Dilute alkali: an alkali having a relatively low percentage of alkali in its aqueous solution.

REACTION OF BASES WITH METALS: Alkalis react with some metals to give a salt and hydrogen gas.
Alkali + Metal → Salt + Hydrogen

Examples:
2NaOH + 2Al + 2H₂O → 2NaAlO₂ + 3H₂ (sodium aluminate is formed)
2NaOH + Zn → Na₂ZnO₂ + H₂ (sodium zincate is formed)

REACTION OF BASES WITH NON-METAL OXIDES: Non-metal oxides are acidic in nature. For example, carbon dioxide dissolves in water to form carbonic acid. A base reacts with a non-metal oxide to give a salt and water.
Base + Non-metal oxide → Salt + Water

Examples:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
2NaOH + CO₂ → Na₂CO₃ + H₂O

USES OF BASES

1. Sodium hydroxide is used in the manufacture of soap.
2. Calcium hydroxide is used for whitewashing buildings.
3. Magnesium hydroxide is used as a medicine for stomach troubles.
4. Ammonium hydroxide is used to remove grease stains from clothes.

NEUTRALISATION REACTION

An acid and a base react with each other to give a salt and water. Because each neutralises the other, this is called a neutralisation reaction.
Acid + Base → Salt + Water

Examples:
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O (sodium sulphate and water are formed)
HNO₃ + NaOH → NaNO₃ + H₂O (sodium nitrate and water are formed)
HCl + NaOH → NaCl + H₂O (sodium chloride and water are formed)
2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O (calcium chloride and water are formed)

REACTION OF ACIDS WITH METAL OXIDES: Metal oxides are basic in nature. An acid reacts with a metal oxide to give a salt and water.
Acid + Metal oxide → Salt + Water

Examples:
2HCl + CaO → CaCl₂ + H₂O
H₂SO₄ + ZnO → ZnSO₄ + H₂O (zinc sulphate and water are formed)
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

INTEXT QUESTIONS PAGE NO. 22

Question 1: Why should curd and other sour substances not be kept in brass and copper vessels?

Answer:
Curd and other sour substances contain acids. When they are kept in brass or copper vessels, the metal reacts with the acid, producing hydrogen gas and harmful compounds, which spoil the food.

Question 2: 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:
Hydrogen gas is usually liberated when an acid reacts with a metal. For example, zinc granules react with dilute H₂SO₄ to give hydrogen. Pass the gas into a soap solution to form soap bubbles that contain hydrogen. When a burning candle is brought near the bubbles, the hydrogen burns with a pop sound.

H₂SO₄ + Zn → ZnSO₄ + H₂↑

Question 3: 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:
Compound A is calcium carbonate, which gives carbon dioxide (the gas that extinguishes a candle).
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

COMMON PROPERTIES OF ACIDS

All acids give hydrogen gas when they react with metals, which shows that all acids contain hydrogen. Examples are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃).

When an acid dissolves in water, it dissociates to give hydrogen ions. This is the common property that gives acids their acidic behaviour.

HCl (aq) → H⁺ + Cl⁻
H₂SO₄ (aq) → 2H⁺ + SO₄²⁻
HNO₃ (aq) → H⁺ + NO₃⁻
CH₃COOH (aq) ⇌ CH₃COO⁻ + H⁺

The hydrogen ion produced by an acid combines with a water molecule to form the hydronium ion (H₃O⁺) in aqueous solution. That is why hydrogen ions are written as H⁺(aq).
HCl + H₂O → H₃O⁺ + Cl⁻

H₂SO₄ + H₂O → H₃O⁺ + SO₄²⁻

Acids conduct electricity in aqueous solution because of the hydrogen ions and other ions they form. Dry acids such as dry HCl do not change the colour of dry blue litmus paper, because a dry acid does not dissociate into ions. This is why moist litmus paper is used to test whether a gas is acidic or basic.

Acidic behaviour of carbon dioxide: Carbon dioxide dissolves in water to form carbonic acid, which dissociates into hydrogen ions and carbonate ions.
CO₂ + H₂O → H₂CO₃ ⇌ 2H⁺ + CO₃²⁻

Are all compounds that contain hydrogen acids?
No. Glucose (C₆H₁₂O₆) and methyl alcohol (CH₃OH) contain hydrogen but are not acids, because they do not dissociate to give hydrogen ions in aqueous solution.

COMMON PROPERTIES OF BASES

A base dissociates to give hydroxide ions in water, and this is responsible for its basic behaviour.
NaOH (aq) → Na⁺ + OH⁻
KOH (aq) → K⁺ + OH⁻

NEUTRALISATION REACTION (ION VIEW): When an acid reacts with a base, the hydrogen ion of the acid combines with the hydroxide ion of the base to form water.
OH⁻ + H⁺ → H₂O
Example: NaOH + HCl → Na⁺ + OH⁻ + H⁺ + Cl⁻ → NaCl + H₂O

DILUTION OF ACIDS AND BASES: The concentration of acid is the number of hydrogen ions per unit volume, and the concentration of a base is the number of hydroxide ions per unit volume. Adding water to an acid or base reduces the concentration of these ions. This is called dilution.

Dilution of an acid or base is exothermic. So the acid or base must always be added to water, and water must never be added to the acid or base. Adding water to a concentrated acid or base generates a lot of heat, which can make the liquid splash out and cause severe burns, because concentrated acids and bases are highly corrosive.

INTEXT QUESTIONS PAGE NO. 25

Question 1: Why do HCl and HNO₃ show acidic character in aqueous solution, while solutions of alcohol and glucose do not?

Answer:
HCl and HNO₃ dissociate in water to form hydrogen ions, which combine with water to form hydronium ions (H₃O⁺). Alcohol and glucose contain hydrogen but do not dissociate in water to form hydrogen ions, so they do not show acidic character.

HCl (aq) → H⁺ + Cl⁻
H⁺ + H₂O → H₃O⁺

Question 2: Why does an aqueous solution of an acid conduct electricity?

Answer:
Acids dissociate in aqueous solution to form ions, and these ions carry the electric current.

Question 3: Why does dry HCl gas not change the colour of dry litmus paper?

Answer:
Litmus changes colour because of hydrogen ions. Dry HCl gas does not contain H⁺ ions, because an acid dissociates only in aqueous solution. Here neither the HCl is in aqueous form nor is the litmus paper wet, so the colour does not change.

Question 4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer:
Dissolving an acid in water is exothermic. If water is added to the acid, the large amount of heat produced can make the mixture splash out and cause burns. Acid should therefore always be added to water.

Question 5: How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?

Answer:
When an acid is diluted, the concentration of hydronium ions per unit volume decreases, so the strength of the acid decreases.

Question 6: How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer:
The concentration of hydroxide ions increases when excess base is dissolved in a solution of sodium hydroxide.

STRENGTH OF ACIDS AND BASES

A strong acid is one in which the hydrogen ions dissociate completely. A strong base is one in which the hydroxide ions dissociate completely. Mineral acids such as hydrochloric, sulphuric and nitric acid are strong acids. Organic acids do not dissociate completely, so they are weak acids. Alkalis are water-soluble bases, and they dissociate completely, so they are strong bases.

Complete dissociation is shown by a single arrow (→). Incomplete dissociation is shown by a double arrow (⇌).

Complete dissociation: NaOH (aq) → Na⁺ + OH⁻; HCl → H⁺ + Cl⁻
Incomplete dissociation: Mg(OH)₂ ⇌ Mg²⁺ + 2OH⁻; CH₃COOH ⇌ CH₃COO⁻ + H⁺

Although acetic acid is a weak acid, concentrated acetic acid is corrosive and can damage the skin.

pH: MEASUREMENT OF STRENGTH OF ACIDS AND BASES

pH stands for the power of hydrogen ion concentration in a solution. It tells whether a solution is acidic, basic or neutral. The pH scale was introduced by S. P. L. Sørensen.
pH = −log₁₀[H⁺]

For a neutral solution, [H⁺] = 10⁻⁷ M and pH = 7.
For an acidic solution, [H⁺] > 10⁻⁷ M and pH < 7.
For a basic solution, [H⁺] < 10⁻⁷ M and pH > 7.

When hydroxide ions are taken into account, the expression used is pOH = −log₁₀[OH⁻].

A solution is acidic if the hydrogen ion concentration is greater than the hydroxide ion concentration, basic if it is smaller, and neutral if the two are equal.

The pH scale runs from 0 to 14. As the hydrogen ion concentration increases, the pH decreases. A pH of 0 means the highest hydrogen ion concentration, and a pH of 14 means the lowest. Distilled water is neutral with a pH of 7. Acidic solutions have a pH below 7, and basic solutions have a pH above 7.

IMPORTANCE OF pH IN EVERYDAY LIFE

In "pH", the "p" stands for the German word 'potenz', meaning power or potential, and "H" stands for hydrogen. So pH means the power of hydrogen.

1. pH in the human body:
Body pH helps to predict health. At a pH of about 6.9, the body becomes prone to viral infections such as colds, coughs and flu. Cancer cells thrive at a pH of about 5.5.
The pH of healthy human skin is 4.5 to 6. Proper skin pH is essential for a healthy complexion.
The pH of stomach fluid is about 2.0. This acidic fluid is essential for digestion.
Human blood has a pH of 7.35 to 7.45. Any increase or decrease beyond this range leads to disease. The ideal pH of blood is 7.4.
The pH of normal saliva is 6.5 to 7.5.
The white enamel coating of our teeth is calcium phosphate, the hardest substance in our body. It does not dissolve in water. If the pH of the mouth falls below 5.5, the enamel is corroded. Toothpastes are generally basic. They neutralise excess acid and help prevent tooth decay.

2. pH in soil: Soil pH is very important in agriculture. Citrus fruits need slightly alkaline soil, rice needs acidic soil, and sugarcane needs neutral soil.

3. pH in rain water: The pH of rain water is about 7, which shows its purity and neutrality. If rain water is polluted by SO₂ and NO₂, acid rain forms, with a pH below 7.

INDICATORS

Substances which show whether another substance is acidic or basic by a change in colour are called indicators. Types of indicators are natural, olfactory, synthetic and universal.

Natural indicators: Indicators obtained from natural sources, such as litmus, turmeric, red cabbage and China rose.

Litmus is obtained from lichens. Its solution is purple. Litmus paper comes in two colours, blue and red. An acid turns blue litmus red. A base turns red litmus blue.

Turmeric is yellow. Turmeric turns reddish brown with a base and does not change colour with an acid.

Red cabbage juice is originally purple. It turns reddish with an acid and greenish-yellow with a base.

Olfactory indicators: Substances whose smell changes when mixed with an acid or base, such as onion, vanilla and clove. Onion and vanilla lose their smell in a base but keep it in an acid. Olfactory indicators allow visually impaired students to take part in laboratory work.

Synthetic indicators: Indicators made in the laboratory, such as phenolphthalein and methyl orange. Phenolphthalein is colourless in acid and pink in base. Methyl orange is orange originally, red in acid and yellow in base.

Universal indicator: Litmus paper, phenolphthalein and methyl orange show only whether a solution is acidic or basic. They do not show its strength. The universal indicator shows a range of colours for pH values from 1 to 14. It is available as strips and as a solution. It is a mixture of many indicators, such as water, propanol, phenolphthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt and thymol blue monosodium salt. A colour chart supplied with it shows the colour for each pH value.

[Table: Indicator colours. Red litmus: no change in acid, turns blue in base. Blue litmus: turns red in acid, no change in base. Turmeric: no change in acid, reddish brown in base. Red cabbage juice: reddish in acid, greenish-yellow in base. Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base. Onion and vanilla: smell does not change in acid, smell vanishes in base.]

INTEXT QUESTIONS PAGE NO. 28

Question 1: Two solutions, A and B, have pH values of 6 and 8. Which has more hydrogen ion concentration? Which is acidic and which is basic?

Answer:
A pH below 7 indicates an acidic solution, and a pH above 7 indicates a basic solution. So solution A (pH 6) is acidic, solution B (pH 8) is basic, and solution A has more hydrogen ion concentration.

Question 2: What effect does the concentration of H⁺(aq) ions have on the nature of the solution?

Answer:
As the H⁺ ion concentration increases, the solution becomes more acidic. As it decreases, the solution becomes more basic.

Question 3: Do basic solutions also have H⁺(aq) ions? If yes, why are they basic?

Answer:
Yes, basic solutions also have H⁺ ions, but their concentration is lower than that of OH⁻ ions, which makes the solution basic.

Question 4: Under what soil condition would a farmer treat the soil with quicklime (CaO), slaked lime (Ca(OH)₂) or chalk (CaCO₃)?

Answer:
When the soil is acidic and unsuitable for cultivation, the farmer treats it with quicklime, slaked lime or chalk to increase its basicity.

SALTS

Salts are ionic compounds formed by the neutralisation reaction between an acid and a base. Salts are electrically neutral. Sodium chloride is the most common salt. It is also called table salt or common salt, and it is used to enhance the taste of food.

Characteristics of salts:
1. Most salts are crystalline solids.
2. Salts may be transparent or opaque.
3. Most salts are soluble in water.
4. Solutions of salts conduct electricity. Salts also conduct electricity in the molten state.
5. Salts can be salty, sour, sweet, bitter or umami (savoury) in taste.
6. Neutral salts are odourless.
7. Salts can be colourless or coloured.

Classification of salts:

1. Normal salts are obtained by complete neutralisation of an acid by a base.
NaOH + HCl → NaCl + H₂O

2. Acid salts are formed by partial replacement of the hydrogen ions of a polybasic acid by a metal. They are obtained when a calculated amount of base is added to a polybasic acid.
NaOH + H₂SO₄ → NaHSO₄ + H₂O

3. Basic salts are formed by partial replacement of the hydroxide ions of a diacidic or triacidic base by an acid radical. A basic salt can react further with an acid to give a normal salt.
Pb(OH)₂ + HCl → Pb(OH)Cl + H₂O

4. Double salts are formed by crystallising a solution that contains two simple salts in equimolar ratio, for example potash alum.

Family of salts: Salts that share a common acidic or basic radical belong to the same family. For example, NaCl and CaCl₂ belong to the chloride family. CaCl₂ and CaSO₄ belong to the calcium family. ZnCl₂ and ZnSO₄ belong to the zinc family.

ACIDIC, BASIC AND NEUTRAL SALTS

Neutral salts are formed from a strong acid and a strong base. Their pH is 7. Examples are sodium chloride, sodium sulphate and potassium chloride.
NaOH + HCl → NaCl + H₂O (sodium chloride)
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O (sodium sulphate)
KOH + HCl → KCl + H₂O (potassium chloride)

Acidic salts are formed from a strong acid and a weak base. Their pH is less than 7. Examples are ammonium chloride and ammonium sulphate.
NH₄OH + HCl → NH₄Cl + H₂O
2NH₄OH + H₂SO₄ → (NH₄)₂SO₄ + 2H₂O

Basic salts are formed from a weak acid and a strong base. Their pH is more than 7. Examples are sodium carbonate and sodium acetate.
H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O (carbonic acid is a weak acid)
CH₃COOH + NaOH → CH₃COONa + H₂O (acetic acid is a weak acid)

Cause of acidic, basic and neutral salts: A strong acid with a weak base gives an acidic salt, because the base cannot fully neutralise the acid. A strong base with a weak acid gives a basic salt, because the acid cannot fully neutralise the base. Equally strong acid and base fully neutralise each other and give a neutral salt.

pH value of salts:
Neutral salt: pH is about 7.
Acidic salt: pH is less than 7.
Basic salt: pH is more than 7.

COMMON SALT (SODIUM CHLORIDE)

Sodium chloride is formed by the reaction of sodium hydroxide and hydrochloric acid. It is a neutral salt with a pH of about 7. It is used to enhance the taste of food and in the manufacture of many chemicals.

Important chemicals from sodium chloride:

Sodium hydroxide (NaOH): Sodium hydroxide is a strong base, also known as caustic soda or lye. It is made by the electrolysis of brine (concentrated aqueous sodium chloride). Chlorine is produced at the anode and hydrogen at the cathode, and these are by-products. This whole process is called the chlor-alkali process.
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Uses of the products of brine electrolysis:
1. Hydrogen is used as fuel, in making margarine, and in making ammonia for fertilisers.
2. Chlorine is used in water treatment, in making PVC, disinfectants, CFCs and pesticides, and in making bleaching powder and hydrochloric acid.
3. Sodium hydroxide is used for degreasing metals and in making paper, soap, detergents, artificial fibres and bleaches.

Bleaching powder (CaOCl₂): Bleaching powder is also called chloride of lime. It is a yellowish-white solid with a strong smell of chlorine. It is made by reacting slaked lime (calcium hydroxide) with chlorine.
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Its aqueous solution is basic. Bleaching works by oxidation, and the chlorine in bleaching powder is responsible for the bleaching effect.

Uses of bleaching powder:
1. As a disinfectant to clean water, as a moss remover and as a weedkiller.
2. For bleaching cotton in the textile industry and wood pulp in the paper industry.
3. As an oxidising agent in many industries.

Baking soda (NaHCO₃): Baking soda is another important product made from the by-products of the chlor-alkali process. Its chemical name is sodium hydrogen carbonate, also called sodium bicarbonate. Other names include bread soda, cooking soda, bicarbonate of soda and bicarb. It is made by the Solvay process, which reacts brine with carbon dioxide and ammonia.
NaCl + CO₂ + NH₃ + H₂O → NH₄Cl + NaHCO₃
In this process, calcium carbonate is the source of carbon dioxide, and the calcium oxide produced from it is used to recover ammonia from ammonium chloride.

Properties of sodium bicarbonate:
1. It is a white crystalline solid that appears as a fine powder.
2. It is amphoteric in nature.
3. It is sparingly soluble in water.
4. On heating, it decomposes to give sodium carbonate, carbon dioxide and water. This is a thermal decomposition reaction.
2NaHCO₃ + heat → Na₂CO₃ + CO₂ + H₂O
On further heating, sodium carbonate decomposes to sodium oxide and carbon dioxide.
Na₂CO₃ → Na₂O + CO₂

Uses of baking soda:
1. It is used in making baking powder. The carbon dioxide it releases makes batter soft and spongy.
2. It is used as an antacid.
3. It is used in toothpaste, which helps to whiten teeth and remove plaque.
4. It is used for cleaning silver ornaments.
5. Because it gives carbon dioxide on strong heating, it is used in fire extinguishers.

Baking powder: Baking powder is a mixture of baking soda and a mild edible acid, usually tartaric acid. When it is heated or mixed with water, carbon dioxide is released, which makes cakes and bread soft and spongy. Baking soda alone is not used in cooking, because on heating it forms sodium carbonate, which makes food taste bitter.
2NaHCO₃ + heat → Na₂CO₃ + CO₂ + H₂O
NaHCO₃ + C₄H₆O₆ → CO₂ + H₂O + NaC₄H₅O₆ (sodium hydrogen tartrate)
The sodium carbonate formed on heating is neutralised by the tartaric acid, giving a salt with a pleasant smell and taste, which makes the food tasty.

Washing soda (Na₂CO₃.10H₂O): Sodium carbonate is made by the thermal decomposition of sodium hydrogen carbonate obtained from the Solvay process.
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
The dry sodium carbonate obtained is called soda ash, or anhydrous sodium carbonate. Washing soda is obtained by adding water to anhydrous sodium carbonate.
Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O
Because it contains 10 water molecules, washing soda is called sodium carbonate decahydrate. It is a crystalline solid and is soluble in water, unlike most carbonates.

Uses of sodium carbonate:
1. It is used for cleaning clothes, especially in rural areas.
2. It is used in making detergent cakes and powders.
3. It removes the permanent hardness of water.
4. It is used in the glass and paper industries.

Water of crystallisation: Many salts contain water molecules. These are called hydrated salts, and the water molecules are called water of crystallisation.

Copper sulphate pentahydrate (CuSO₄.5H₂O): The blue colour of copper sulphate is due to its 5 water molecules. On heating, it loses the water and becomes grey-white anhydrous copper sulphate. Adding water turns it blue again.
CuSO₄.5H₂O + heat → CuSO₄ + 5H₂O

Ferrous sulphate heptahydrate (FeSO₄.7H₂O): The green colour of ferrous sulphate is due to its 7 water molecules.

Plaster of Paris: Plaster of Paris is obtained by heating gypsum, a hydrated salt of calcium.
2CaSO₄.2H₂O + heat → 2CaSO₄.H₂O + 3H₂O
When water is added, Plaster of Paris sets back into gypsum.
2CaSO₄.H₂O + 3H₂O → 2CaSO₄.2H₂O
Plaster of Paris is used to make toys and designer false ceilings. Doctors use it to set fractured bones.

INTEXT QUESTIONS PAGE NO. 33

Question 1: What is the common name of the compound CaOCl₂?

Answer:
The common name of CaOCl₂ is bleaching powder.

Question 2: Name the substance which on treatment with chlorine yields bleaching powder.

Answer:
Calcium hydroxide, Ca(OH)₂, yields bleaching powder when treated with chlorine.

Question 3: Name the sodium compound which is used for softening hard water.

Answer:
Washing soda (Na₂CO₃.10H₂O) is used for softening hard water.

Question 4: What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.

Answer:
Sodium carbonate, water and carbon dioxide gas are formed.
2NaHCO₃ + heat → Na₂CO₃ + CO₂ + H₂O

Question 5: Write an equation to show the reaction between Plaster of Paris and water.

Answer:
2CaSO₄.H₂O + 3H₂O → 2CaSO₄.2H₂O (gypsum)

EXERCISE QUESTIONS PAGE NO. 34 AND 35

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. Bases turn red litmus blue, and basic solutions have a pH above 7.

Question 2: A solution reacts with crushed eggshells to give a gas that turns lime water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl

Answer: (b) HCl. The gas is carbon dioxide, which is produced when an acid reacts with calcium carbonate in the eggshell.

Question 3: 10 mL of a NaOH solution is completely neutralised by 8 mL of a given HCl solution. How much of the same HCl solution is needed to neutralise 20 mL of the same NaOH solution? (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer: (d) 16 mL. The volume needed is directly proportional to the volume of NaOH, so 20 mL needs 2 × 8 = 16 mL of HCl.

Question 4: Which type of medicine is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Answer: (c) Antacid.

Question 5: Write word equations and then balanced equations for the reactions 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) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen; H₂SO₄ + Zn → ZnSO₄ + H₂
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen; 2HCl + Mg → MgCl₂ + H₂
(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen; 3H₂SO₄ + 2Al → Al₂(SO₄)₃ + 3H₂
(d) Hydrochloric acid + Iron → Iron(II) chloride + Hydrogen; 2HCl + Fe → FeCl₂ + H₂

Question 6: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Answer:
Fit two nails into a cork and place it in a 100 mL beaker. Connect the nails to the two terminals of a 6-volt battery through a bulb and a switch. Pour in some dilute HCl and switch the current on. Repeat the experiment with glucose solution and then with alcohol solution.
Result: The bulb glows with HCl, because HCl dissociates into H⁺ and Cl⁻ ions that conduct electricity. The bulb does not glow with glucose or alcohol, because they do not dissociate into ions.
Conclusion: All acids contain hydrogen, but not all compounds containing hydrogen are acids.

Question 7: Why does distilled water not conduct electricity, whereas rain water does?

Answer:
Distilled water is pure and has no ionic species, so it does not conduct electricity. Rain water is impure and contains ionic species such as acids, so it conducts electricity.

Question 8: Why do acids not show acidic behaviour in the absence of water?

Answer:
Acids dissociate to give hydrogen ions only in the presence of water. The hydrogen ions are responsible for acidic behaviour.

Question 9: Five solutions A, B, C, D and E give pH values of 4, 1, 11, 7 and 9 with universal indicator. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH values in increasing order of hydrogen ion concentration.

Answer:
(a) Neutral: solution D (pH 7)
(b) Strongly alkaline: solution C (pH 11)
(c) Strongly acidic: solution B (pH 1)
(d) Weakly acidic: solution A (pH 4)
(e) Weakly alkaline: solution E (pH 9)
Increasing order of hydrogen ion concentration: C (11) < E (9) < D (7) < A (4) < B (1)

Question 10: Equal lengths of magnesium ribbon are put into test tubes A and B. Hydrochloric acid is added to A and acetic acid to B. In which test tube will the fizzing be more vigorous, and why?

Answer:
The fizzing will be more vigorous in test tube A, with hydrochloric acid. HCl is a stronger acid than acetic acid, so it releases hydrogen gas faster.

Question 11: Fresh milk has a pH of 6. How will the pH change as it turns into curd? Explain.

Answer:
The pH will fall, because curd is acidic. The acids produced during curd formation lower the pH.

Question 12: A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

Answer:
(a) In an alkaline condition, milk does not set into curd easily.
(b) The milk is slightly basic, so the acids produced for setting the curd are neutralised by the base. Therefore, it takes longer for the curd to set.

Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why.

Answer:
Plaster of Paris is a powder that absorbs moisture and sets into a hard solid called gypsum, so it must be kept dry.

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster of Paris + Water → Gypsum)

Question 14: What is a neutralisation reaction? Give two examples.

Answer:
A neutralisation reaction is one in which an acid and a base react to give a salt and water. Heat is released in the reaction.
Examples: (i) HCl + NaOH → NaCl + H₂O; (ii) during indigestion, caused by excess hydrochloric acid in the stomach, an antacid such as milk of magnesia (Mg(OH)₂, a base) neutralises the excess acid and gives relief.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Question 15: Give two important uses each of washing soda and baking soda.

Answer:
(1) Washing soda: (a) It is used in the glass, soap and paper industries. (b) It removes the permanent hardness of water.
(2) Baking soda: (a) It is used to make baking powder, which is a mixture of baking soda and a mild acid, tartaric acid. When it is heated or mixed with water, it releases CO₂, which makes bread or cake soft and spongy. (b) It is used in soda-acid fire extinguishers.

Inorganic acids:- Acids from rocks and minerals are inorganic acids or mineral acids eg. HCl, HNO₃, H₂SO₄ (Strong acids).

Strong acids:- These are acids which ionise completely in water eg.HCl

Weak acids:- These are acids which ionise partially in water eg. CH₃COOH

Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic. This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

Examples:

Ø Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate.

Ø Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate.

Ø Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate.

Ø Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate.

MgCO₃ + 2HCl → MgCl2 + CO₂ + H2O

Ø Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate.

Examples:

Ø Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with

Ø Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate.

2NaHCO₃ + H2SO₄ → Na2SO₄ + 2CO₂ + 2H₂O

The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.

But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.

REACTION OF ACID WITH MARBLE AND EGG SHELL:

Strong bases:- These are bases which ionise completely in aqueous solution

eg.NaOH, KOH.