Class 10 Science Chapter 2: Acids, Bases and Salts

Acids, Bases and Salts is Chapter 2 of NCERT Class 10 Science. It explains how indicators identify acids and bases, how these substances react, what the pH scale tells us, and how important salts like bleaching powder, baking soda, washing soda and plaster of Paris are made. Watch the six video lessons below, then revise the reactions and solved examples for your CBSE board exam.

What you will learn

Natural, synthetic and olfactory indicators and the colours they show
Chemical properties of acids and bases with balanced equations
Why acids give H⁺ ions and bases give OH⁻ ions in water
The pH scale, strong and weak acids, and the importance of pH in daily life
Preparation and uses of NaOH, bleaching powder, baking soda, washing soda and plaster of Paris

Video Lessons

Acids, Bases and Salts - Part 1

Acids, Bases and Salts - Part 2

Acids, Bases and Salts - Part 3

Acids, Bases and Salts - Part 4

Acids, Bases and Salts - Practice Questions Part 1

Acids, Bases and Salts - Practice Questions Part 2

Key Concepts and Formulas

Acid-Base Indicators

An indicator changes colour or smell to show whether a substance is acidic or basic.

Litmus (from lichen): blue litmus turns red in acid; red litmus turns blue in base
Methyl orange: red in acid, yellow in base; phenolphthalein: colourless in acid, pink in base
Turmeric turns reddish-brown in a base
Olfactory indicators such as onion, vanilla and clove oil lose their smell in a basic solution

Reactions of Acids and Bases

Acids react with metals, carbonates, bases and metal oxides; strong bases also react with some metals and with non-metal oxides.

Acid + metal → salt + H₂
Zn + H₂SO₄ → ZnSO₄ + H₂
2NaOH + Zn → Na₂ZnO₂ + H₂
Acid + carbonate → salt + CO₂ + H₂O
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
Acid + base → salt + water
NaOH + HCl → NaCl + H₂O
CuO + 2HCl → CuCl₂ + H₂O

Hydrogen gas burns with a pop sound; Na₂ZnO₂ is sodium zincate
CO₂ turns lime water milky; excess CO₂ makes it clear again by forming soluble Ca(HCO₃)₂
The acid-base reaction is called neutralisation
Metal oxides are basic (CuO gives a blue-green solution with HCl); non-metal oxides are acidic, as the lime water reaction shows

Acids and Bases in Water

Acids produce hydrogen ions in water and bases produce hydroxide ions, so their solutions conduct electricity.

HCl + H₂O → H₃O⁺ + Cl⁻
NaOH(aq) → Na⁺ + OH⁻
H⁺ + OH⁻ → H₂O

H⁺ ions cannot exist alone and form hydronium ions (H₃O⁺)
Dry HCl gas does not change the colour of dry litmus because no H⁺ ions form without water
Alkalis are bases that dissolve in water, such as NaOH and KOH
Mixing acid or base with water is exothermic; always add acid to water slowly, never water to acid

The pH Scale

The pH scale (0 to 14) measures hydrogen ion concentration; universal indicator shows a different colour at each pH.

pH = 7  → neutral
pH < 7  → acidic
pH > 7  → basic
Human body: pH 7.0 to 7.8
Acid rain: pH < 5.6
Tooth decay starts: mouth pH < 5.5

More H⁺ ions means lower pH; more OH⁻ ions means higher pH
Strong acids (HCl, H₂SO₄, HNO₃) give many H⁺ ions; weak acids (acetic acid, carbonic acid) give fewer
Stomach acidity is relieved by antacids such as milk of magnesia, Mg(OH)₂
Basic toothpaste neutralises the acid that causes tooth decay

Salts and Their pH

Salts sharing the same positive or negative radical form a family, such as NaCl and Na₂SO₄ (sodium salts).

Strong acid + strong base → neutral salt (NaCl, pH 7)
Strong acid + weak base → acidic salt (NH₄Cl, pH < 7)
Weak acid + strong base → basic salt (Na₂CO₃, CH₃COONa, pH > 7)

Chemicals from Common Salt

Common salt is the raw material for several important chemicals. In the chlor-alkali process, Cl₂ forms at the anode and H₂ at the cathode.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂   (on heating)
Washing soda = Na₂CO₃·10H₂O
Blue vitriol = CuSO₄·5H₂O
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O   (373 K)

Bleaching powder (CaOCl₂): used for bleaching cotton and linen, disinfecting drinking water and as an oxidising agent
Baking soda (NaHCO₃): used in baking powder (with tartaric acid), antacids and soda-acid fire extinguishers
Washing soda: made by recrystallising sodium carbonate; used in glass, soap and paper industries and to remove permanent hardness of water
Water of crystallisation: fixed number of water molecules in a salt crystal; blue copper sulphate crystals turn white on heating
Plaster of Paris is made by heating gypsum; it sets into hard gypsum again when mixed with water

Solved Examples

Example 1: Solutions P, Q, R and S have pH 2, 6, 11 and 7. Arrange them in increasing order of H⁺ ion concentration.

Lower pH → more H⁺
pH order (high to low): R 11, S 7, Q 6, P 2
∴ R < S < Q < P (P is the strongest acid)

Example 2: 15 mL of a NaOH solution is exactly neutralised by 12 mL of a HCl solution. How much of the same HCl is needed for 40 mL of the NaOH?

15 mL NaOH needs 12 mL HCl
40 mL NaOH needs 12 × 40 ÷ 15
∴ HCl needed = 32 mL

Example 3: A white powder X used in cake making gives a gas that turns lime water milky on heating, leaving a residue Y that softens hard water. Identify X and Y.

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
∴ X = NaHCO₃, Y = Na₂CO₃

Example 4: Predict whether solutions of NH₄Cl, Na₂CO₃ and KNO₃ are acidic, basic or neutral.

NH₄Cl: strong acid + weak base → acidic
Na₂CO₃: weak acid + strong base → basic
KNO₃: strong acid + strong base → neutral
∴ Acidic, basic and neutral respectively

Example 5: How many water molecules does one formula unit of gypsum lose when it turns into plaster of Paris?

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
Water lost = 2 − ½
∴ 1½ molecules of water

Example 6: A farmer finds that his soil has a pH of 4.8. What should he add, and why?

Answer: The soil is too acidic, so he should add a base such as quicklime (CaO), slaked lime (Ca(OH)₂) or chalk (CaCO₃) to neutralise the excess acid.

Common Mistakes to Avoid

Writing "water to acid" while describing dilution; the correct and safe method is acid to water, slowly with stirring.
Forgetting that a lower pH means a higher H⁺ concentration; pH 2 is more acidic than pH 5.
Mixing up formulas: baking soda is NaHCO₃, washing soda is Na₂CO₃·10H₂O, and bleaching powder is CaOCl₂.
Not balancing equations or skipping state symbols in the chlor-alkali reaction; balanced equations carry marks in the board exam.

Quick Revision

Acid + base → salt + water
H⁺ + OH⁻ → H₂O
pH < 7 acidic, 7 neutral, > 7 basic
Acid rain: pH < 5.6
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Plaster of Paris = CaSO₄·½H₂O
Copper sulphate = CuSO₄·5H₂O