Acid Base and Salt Short Notes - Class 10 Science Chapter 2

Acids, Bases and Salts – Complete Notes

Class 10 Science Chapter 2 notes covering indicators, acids, bases, pH scale, salts, common salt products and water of crystallisation.

1. Introduction and Indicators

Chemical substances were earlier classified by taste as acids, bases and salts.

• Acids: Sour in taste.
• Bases: Bitter in taste.
• Salts: Salty or sometimes poisonous.
• Organic compounds: Obtained from plants and animals.
• Inorganic compounds: Obtained from minerals and rocks.

Indicators

Indicators are substances that change colour or smell in acidic or basic medium.

Natural Visual Indicators

• Litmus: Purple dye from lichens. Acids turn blue litmus red. Bases turn red litmus blue.
• Turmeric: Remains yellow in acid and neutral medium, but turns reddish-brown in base.
• Red Cabbage: Turns red or pink in acid and green in base.

Synthetic Visual Indicators

• Phenolphthalein: Colourless in acid and pink in base.
• Methyl Orange: Red in acid and yellow in base.

Olfactory Indicators

Olfactory indicators change smell in acidic or basic medium. Examples are onion, vanilla essence and clove oil. They retain smell in acid but lose smell in base.

2. Acids

Acids are substances that release H⁺ ions or hydronium ions, H₃O⁺, in aqueous solution.

HCl + H₂O → H₃O⁺ + Cl⁻

Classification of Acids

• Organic Acids: Naturally occurring and generally weak. Examples: acetic acid in vinegar, citric acid in lemon.
• Mineral Acids: Prepared in laboratories and generally strong. Examples: HCl, H₂SO₄, HNO₃.

Physical Properties of Acids

• Sour in taste.
• Turn blue litmus red.
• Highly corrosive.
• Conduct electricity in aqueous solution.

Chemical Properties of Acids

Reaction with Metals

Metal + Acid → Salt + Hydrogen Gas

Zn(s) + H₂SO₄(dil) → ZnSO₄(aq) + H₂(g)

Note: Hydrogen gas is usually not evolved with nitric acid because HNO₃ is a strong oxidising agent. Exception: Mg and Mn react with very dilute HNO₃ to give H₂ gas.

Reaction with Metal Oxides

Acid + Metal Oxide → Salt + Water

CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

Reaction with Carbonates and Bicarbonates

Metal Carbonate/Bicarbonate + Acid → Salt + Water + CO₂

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

Ca(OH)₂ + CO₂ → CaCO₃(s) + H₂O

Carbon dioxide turns lime water milky due to formation of calcium carbonate.

Neutralisation Reaction

Acid + Base → Salt + Water

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

3. Bases

Bases are substances that release hydroxide ions, OH⁻, in aqueous solution.

Alkalies are bases that are soluble in water. Examples: NaOH and KOH.

Physical Properties of Bases

• Bitter in taste.
• Soapy to touch.
• Turn red litmus blue.
• Corrosive in nature.
• Conduct electricity in solution.

Chemical Properties of Bases

Reaction with Metals

Only some metals react with strong bases to release hydrogen gas.

Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g)

The salt formed is sodium zincate.

Reaction with Non-metallic Oxides

Base + Non-metallic Oxide → Salt + Water

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

4. Dilution and Behaviour in Water

Acids show acidic properties only in the presence of water because they produce H⁺ ions only in aqueous solution.

Dilution Rule

Dissolving acid or base in water is highly exothermic. Always add acid to water slowly with constant stirring. Never add water to acid because it may cause splashing or breakage of the container.

5. pH Scale and Strength

The pH scale was developed by Sorensen. It measures H⁺ ion concentration and ranges from 0 to 14.

pH < 7 : Acidic solution

pH = 7 : Neutral solution

pH > 7 : Basic solution

• Lower pH means stronger acid.
• Higher pH means stronger base.
• Pure water has pH 7.

Universal Indicator

Universal indicator is a mixture of indicators that shows different colours at different pH values. It shows red for strong acid, green for neutral solution and blue or violet for basic solution.

Importance of pH in Everyday Life

• Digestion: Stomach produces HCl. Excess acid causes indigestion and is treated with antacids like milk of magnesia, Mg(OH)₂.
• Tooth Decay: Tooth decay starts when mouth pH falls below 5.5. Alkaline toothpaste helps to neutralise acid.
• Soil pH: Plants need a suitable pH range. Acidic soil is treated by liming using CaO, Ca(OH)₂ or CaCO₃.
• Self Defence: Bee sting contains formic acid and is treated with baking soda. Wasp sting is alkaline and is treated with vinegar.

6. Chemicals from Common Salt

Common salt, NaCl, is obtained from seawater or rock salt. It is used to prepare many important chemicals.

Sodium Hydroxide

Sodium hydroxide, NaOH, is also called caustic soda. It is prepared by the chlor-alkali process, which is electrolysis of brine.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Washing Soda

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. It is used for cleaning and removing permanent hardness of water.

Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O

Baking Soda

Baking soda is sodium hydrogen carbonate, NaHCO₃. It is used in baking and as an antacid.

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Bleaching Powder

Bleaching powder, CaOCl₂, is prepared by passing chlorine gas over dry slaked lime.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Plaster of Paris

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is prepared by heating gypsum at 373 K.

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O

Note: Heating above 373 K produces dead burnt plaster, which is anhydrous CaSO₄.

7. Water of Crystallisation

Water of crystallisation is the fixed number of water molecules chemically attached to one formula unit of a salt in crystalline form.

Example: Hydrated copper sulphate is CuSO₄·5H₂O.

Effect of Heat

On heating, hydrated salts lose water of crystallisation and become anhydrous. Blue copper sulphate crystals become white on heating.

CuSO₄·5H₂O (Blue) → CuSO₄ (White) + 5H₂O