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Acids, Bases and Salts – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 2. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Acids give H⁺ ions in water and bases give OH⁻ ions. They neutralise each other to form salt and water. The pH scale (0–14) measures how acidic or basic a solution is.

Properties

Acids are sour, turn blue litmus red; bases are bitter, soapy and turn red litmus blue.
Acid + metal → salt + hydrogen gas; acid + metal carbonate → salt + water + CO₂.
Acid + base → salt + water (neutralisation).

pH scale

pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
Our body works in pH 7.0–7.8; tooth decay starts below pH 5.5.
Antacids (like milk of magnesia) neutralise excess stomach acid.

Important salts

Bleaching powder CaOCl₂: disinfecting water, bleaching.
Baking soda NaHCO₃: cooking, antacid; washing soda Na₂CO₃·10H₂O: removes hardness of water.
Plaster of Paris CaSO₄·½H₂O: sets into gypsum with water; used for casts and statues.

Key terms

Indicator: a substance that changes colour in acid or base (litmus, phenolphthalein)
Neutralisation: reaction of an acid with a base to give salt and water
Water of crystallisation: fixed water molecules in a salt crystal (e.g. CuSO₄·5H₂O)

Formulas

NaOH + HCl → NaCl + H₂O
Zn + H₂SO₄ → ZnSO₄ + H₂
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating)

Important questions with answers

Why does dry HCl gas not change the colour of dry litmus paper?2 marks

Acidic behaviour needs H⁺ ions, which form only in water. Dry HCl does not ionise, so it cannot change dry litmus.

What is the pH scale? A solution turns red litmus blue and its pH is 10. Is it acidic or basic, and how will its pH change on adding water?3 marks

The pH scale (0–14) measures H⁺ ion concentration; 7 is neutral, below 7 acidic, above 7 basic. The solution is basic (pH 10 > 7). On dilution OH⁻ concentration falls, so the pH decreases towards 7.

How is bleaching powder prepared? Write its uses. Also explain how baking soda is made and why it is used in baking powder.5 marks

Bleaching powder is made by passing chlorine over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: bleaching cotton and linen, disinfecting drinking water, as an oxidising agent in industry. Baking soda (NaHCO₃) is made by the reaction NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. On heating it releases CO₂ (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂), which makes dough rise; baking powder adds a mild acid such as tartaric acid to neutralise the bitter sodium carbonate.

Quick revision

Acid turns blue litmus red; base turns red litmus blue.
pH 7 is neutral.
Chlor-alkali process gives NaOH, Cl₂ and H₂ from brine.

Exam tips

Always add acid to water, not water to acid (the reaction is highly exothermic).
Phenolphthalein is colourless in acid and pink in base.

Practice MCQs

A solution turns red litmus blue. Its pH is likely
1. 1
2. 4
3. 7
4. 10
Show answer
Answer: (d) 10. Turning red litmus blue means basic, pH > 7.
Chemical formula of baking soda is
1. Na₂CO₃
2. CaCO₃
3. NaCl
4. NaHCO₃
Show answer
Answer: (d) NaHCO₃. Sodium hydrogencarbonate.
Acid + base →
1. salt + hydrogen
2. salt + water
3. only water
4. oxygen
Show answer
Answer: (b) salt + water. Neutralisation.
Which gas is released when zinc reacts with dilute HCl?
1. oxygen
2. carbon dioxide
3. chlorine
4. hydrogen
Show answer
Answer: (d) hydrogen. Zn + 2HCl → ZnCl₂ + H₂.
Plaster of Paris is
1. CaSO₄·2H₂O
2. CaOCl₂
3. CaCO₃
4. CaSO₄·½H₂O
Show answer
Answer: (d) CaSO₄·½H₂O. Calcium sulphate hemihydrate.
The pH of a neutral solution at room temperature is
1. 0
2. 7
3. 14
4. 1
Show answer
Answer: (b) 7. Neutral means pH 7.
Lime water turns milky when a gas from an acid–carbonate reaction is passed. The gas is
1. H₂
2. O₂
3. CO₂
4. SO₂
Show answer
Answer: (c) CO₂. CO₂ forms insoluble CaCO₃.
The products of the chlor-alkali process are
1. NaCl and H₂O
2. NaOH, Cl₂ and H₂
3. Na₂CO₃ and CO₂
4. CaOCl₂ only
Show answer
Answer: (b) NaOH, Cl₂ and H₂. Electrolysis of brine gives NaOH, chlorine and hydrogen.

All chapters of Class 10 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.