Chapter 2: Acids, Bases and Salts

1. Introduction

All the substances around us have a particular taste and chemical nature. Some taste sour like lemon and tamarind, while others taste bitter like soap. This difference in taste and nature is because some substances are acids and some are bases. When acids and bases react with each other, they form salts and water.

2. Acids

Definition:  Substances which produce hydrogen ions (H⁺) when dissolved in water are called acids.

Example:
 HCl → H⁺ + Cl⁻
H₂SO₄ → 2H⁺ + SO₄²⁻

Types of Acids

  1. Strong acids: Completely ionize in water.
    Example – Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃)
  2. Weak acids: Partially ionize in water.
    Example – Acetic acid (CH₃COOH), Carbonic acid (H₂CO₃)

Sources of Acids

  • Natural acids: Found in fruits and plants (citric acid in lemon, lactic acid in curd, tartaric acid in tamarind).
  • Mineral acids: Made in laboratories (HCl, H₂SO₄, HNO₃).

Physical Properties of Acids

  • Sour in taste.
  • Turn blue litmus red.
  • Corrosive in nature (can damage skin or metals).
  • Conduct electricity in aqueous solution.

3. Bases

Definition:  Substances which produce hydroxide ions (OH⁻) when dissolved in water are called bases.

Example:
 NaOH → Na⁺ + OH⁻
 KOH → K⁺ + OH⁻

Types of Bases

  1. Strong bases: Fully ionize in water.
     Example – NaOH, KOH, Ca(OH)₂
  2. Weak bases: Partially ionize.
     Example – NH₄OH

Physical Properties of Bases

  • Bitter in taste.
  • Soapy or slippery to touch.
  • Turn red litmus blue.
  • Conduct electricity in aqueous form.

4. Indicators

Indicators are substances which show a change in colour or smell in presence of an acid or a base.

Indicator

In Acid

In Base

Litmus

Red

Blue

Methyl orange

Red

Yellow

Phenolphthalein

Colourless

Pink

Turmeric

Yellow

Reddish brown

5. Reaction of Acids and Bases with Different Substances

(a) Reaction with Metals

  • Acids react with metals to produce hydrogen gas and a salt.
    Example:
     Zn + 2HCl → ZnCl₂ + H₂ ↑

Observation: Gas bubbles are seen; hydrogen gas burns with a pop sound.

  • Bases react with certain metals like zinc or aluminium to produce hydrogen gas.
    Example:
     2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂ ↑

(b) Reaction with Metal Carbonates and Bicarbonates

  • Acids react with carbonates or bicarbonates to form salt, water and carbon dioxide gas.

    Example:
     Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ ↑
     NaHCO₃ + HCl → NaCl + H₂O + CO₂ ↑

    Observation: Effervescence due to CO₂ gas.

(c) Reaction with Metal Oxides

  • Metal oxides are basic in nature. They react with acids to form salt and water.

    Example:
     CuO + 2HCl → CuCl₂ + H₂O

    Observation: Black CuO turns blue-green (formation of copper chloride).

(d) Reaction Between Acid and Base (Neutralization Reaction)

  • When acid reacts with base, salt and water are formed.

    Example:
     NaOH + HCl → NaCl + H₂O

    Observation: The solution becomes neutral (neither acidic nor basic).

6. What Happens to an Acid or Base in Water?

  • Acids and bases always ionize in water to release ions.
  • Acid gives H⁺ ions
  • Base gives OH⁻ ions

Without water, acids and bases cannot show their chemical properties.

Example:
 HCl (gas) → No change on dry litmus
 HCl (aq) → Turns blue litmus red

7. Strength of Acids and Bases (pH Scale)

  • The pH scale is used to measure the strength of acids or bases.

    • Scale ranges from 0 to 14.
    • pH < 7: Acidic
    • pH = 7: Neutral
    • pH > 7: Basic

    Examples:

    • Stomach acid: pH 1–2
    • Pure water: pH 7
    • Soap solution: pH 9–10

8. Importance of pH in Everyday Life

  1. In our digestive system:
     Our stomach contains HCl for digestion. Too much acid causes acidity; it is cured using bases called antacids (Mg(OH)₂).

  2. Tooth decay and pH:
     Acids formed by bacteria lower pH in mouth below 5.5, leading to tooth decay. Brushing neutralizes acid.

  3. pH of soil:
     Plants grow well in specific pH range. Acidic soil is treated with slaked lime (Ca(OH)₂).

In our blood:
 Human blood has pH around 7.4. A slight change can be harmful.

9. Salts

When acid reacts with base, the product formed is called salt (along with water).
 Salts can be neutral, acidic, or basic depending on the strength of acid and base used.

Example:
 HCl + NaOH → NaCl (neutral salt) + H₂O
 H₂SO₄ + NH₄OH → (NH₄)₂SO₄ (acidic salt) + H₂O

10. Family of Salts

All salts formed from the same acid belong to one family.

Example:

    • NaCl, KCl, CaCl₂ → Family of HCl
    • Na₂SO₄, K₂SO₄ → Family of H₂SO₄

11. Common Salts and Their Uses

  1. Sodium chloride (NaCl): Common salt, used in food.
  2. Sodium hydroxide (NaOH): Used in soap making.
  3. Baking soda (NaHCO₃): Used in baking, medicine for acidity.
    • Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
    • Uses: Making baking powder, antacid.
  4. Washing soda (Na₂CO₃·10H₂O): Used in cleaning and removing hardness of water.
    • Preparation: From baking soda by heating.
  5. Bleaching powder (CaOCl₂): Used for disinfecting water and clothes.
    • Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
  6. Plaster of Paris (CaSO₄·½H₂O): Used for making casts and statues.
    • Preparation: 2(CaSO₄·2H₂O) → (CaSO₄)₂·H₂O + 3H₂O (on heating gypsum).

12. Crystals of Salts

Many salts contain water molecules in fixed proportion; these are called hydrated salts.

Example:
 CuSO₄·5H₂O – Blue vitriol
 FeSO₄·7H₂O – Green vitriol
 Na₂CO₃·10H₂O – Washing soda

When these salts are heated, they lose water and become anhydrous.

Example:
 CuSO₄·5H₂O (blue) → CuSO₄ (white) + 5H₂O

13. Water of Crystallization

The number of water molecules present in one formula unit of a salt is called water of crystallization.

Example:
 In CuSO₄·5H₂O, water of crystallization = 5.

Use:
 Blue colour of copper sulphate is due to water of crystallization.

14. Summary

  • Acids give H⁺ ions; bases give OH⁻ ions in water.
  • Acid + Base → Salt + Water (Neutralization)
  • pH scale shows strength of acids and bases.
  • Salts are formed from neutralization and have wide industrial uses.
  • Many salts contain water of crystallization which gives them colour and shape.
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