Acids, Bases and Salts: Class 10 Quick Revision Notes

These Acids, Bases and Salts Class 10 revision notes cover everything the board exam expects in one place: indicators, key reactions with balanced equations, the pH scale and the important salts. Use them for last-minute revision after reading the NCERT chapter.

1. Acids and Bases: Basic Properties

Property	Acids	Bases
Taste	Sour	Bitter
Feel	—	Soapy
Blue litmus	Turns red	No change
Red litmus	No change	Turns blue
Ions in water	H⁺ (as H₃O⁺)	OH⁻
Examples	HCl, H₂SO₄, HNO₃, CH₃COOH	NaOH, KOH, Ca(OH)₂, NH₄OH

Alkalis are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis.

2. Indicators

Indicator	In acid	In base
Litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Turmeric	No change (yellow)	Reddish-brown

Olfactory indicators change smell. Onion, vanilla essence and clove oil lose their smell in basic solutions but keep it in acidic solutions.

3. Key Reactions of Acids

a) Acid + Metal → Salt + Hydrogen gas
Zn + H₂SO₄ → ZnSO₄ + H₂
Test: a burning splinter near the gas burns with a "pop" sound.

b) Acid + Metal carbonate/hydrogencarbonate → Salt + Water + Carbon dioxide
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Test: CO₂ turns lime water milky. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

c) Acid + Base → Salt + Water (Neutralisation)
NaOH + HCl → NaCl + H₂O

d) Acid + Metal oxide → Salt + Water
CuO + 2HCl → CuCl₂ + H₂O (the solution turns blue-green)

4. Key Reactions of Bases

a) Base + Metal → Salt + Hydrogen (only with some metals, like zinc)
2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂

b) Base + Non-metal oxide → Salt + Water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — this shows that non-metal oxides are acidic in nature.

5. Acids and Bases in Water

• Acids give H⁺ ions in water; bases give OH⁻ ions.
• Dissolving an acid or base in water is highly exothermic (it gives out heat).
• Always add acid to water slowly, never water to acid. Adding water to concentrated acid can make the mixture splash out and cause burns.

6. The pH Scale

• pH runs from 0 to 14.
• pH 7 = neutral (pure water)
• Below 7 = acidic (the lower the pH, the stronger the acid)
• Above 7 = basic (the higher the pH, the stronger the base)
• Universal indicator shows different colours at different pH values.

pH in Everyday Life

Situation	What happens
Our body	Works within a narrow pH range, roughly 7.0–7.8
Stomach	Produces hydrochloric acid for digestion; antacids neutralise excess acid
Tooth decay	Starts when mouth pH falls below 5.5; toothpastes are basic to neutralise acid
Soil	Plants need a suitable pH; farmers adjust it with lime or other treatments
Bee sting	Contains methanoic acid; baking soda (a mild base) gives relief
Nettle sting	Also contains methanoic acid; traditionally rubbed with dock leaves

7. Salts

A salt is formed when an acid reacts with a base.

Formed from	Nature of salt	Example
Strong acid + strong base	Neutral (pH 7)	NaCl
Strong acid + weak base	Acidic (pH < 7)	NH₄Cl
Weak acid + strong base	Basic (pH > 7)	CH₃COONa, Na₂CO₃

Important Chemicals from Common Salt

Chemical	Formula	How it is made	Uses
Sodium hydroxide	NaOH	Chlor-alkali process: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)	Soaps, paper, degreasing metals
Bleaching powder	CaOCl₂	Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O	Bleaching cloth, disinfecting drinking water
Baking soda	NaHCO₃	NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃	Baking powder, antacids, fire extinguishers
Washing soda	Na₂CO₃·10H₂O	Recrystallisation of sodium carbonate	Removing water hardness, cleaning, glass/soap making
Plaster of Paris	CaSO₄·½H₂O	Heating gypsum (CaSO₄·2H₂O) at 373 K	Plastering fractured bones, toys, decoration

On heating baking soda: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. This CO₂ makes cakes rise.
Plaster of Paris + water: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (sets into hard gypsum).

8. Water of Crystallisation

Some salts contain a fixed number of water molecules in their crystals. Copper sulphate: CuSO₄·5H₂O is blue; on heating it loses water and turns white, and adding water makes it blue again. Gypsum: CaSO₄·2H₂O. Washing soda: Na₂CO₃·10H₂O.

Last-Minute Checklist

• Colours of all four indicators
• The four reactions of acids, with equations
• Lime water test for CO₂ and the "pop" test for H₂
• pH examples from daily life
• Formulas, preparation and uses of the five important chemicals
• Why you add acid to water, not water to acid

Frequently Asked Questions

What is the pH of pure water?
7, which is neutral.

Why does the "pop" test confirm hydrogen gas?
Hydrogen burns rapidly with oxygen in air, producing a small pop sound near a burning splinter.

What is the chemical formula of Plaster of Paris?
CaSO₄·½H₂O (calcium sulphate hemihydrate).

Why is baking soda used in cakes?
On heating, it gives out carbon dioxide, which makes the cake rise and become soft and spongy.

What is the chlor-alkali process?
The electrolysis of brine (a concentrated solution of sodium chloride), which gives sodium hydroxide, chlorine and hydrogen.