CBSE Class 10 Science Chapter 2 PDF Notes – Acids, Bases and Salts 2026-27

These Class 10 Science Chapter 2 PDF Notes explain Acids, Bases and Salts step by step: indicators, how acids and bases react, the pH scale, and the preparation and uses of important salts such as baking soda, washing soda, bleaching powder and plaster of Paris.

This chapter is one of the highest-scoring in the chemistry unit. Board papers regularly ask about pH values, reactions of acids with metals and carbonates, and the formulae and uses of common salts, so learn the reactions table carefully.

Use the table of contents below to jump to the part you need, or read top to bottom for a complete revision of the chapter.

Table of Contents

• Chapter Overview
• Important Topics and Concepts
• Important Salts: Formula and Uses
• Important Definitions and Terms
• Short Answer Questions
• Long Answer Questions
• MCQs with Answers
• Quick Revision Points
• Chapter Summary
• Exam Tips and PYQ Pattern
• Frequently Asked Questions
• Conclusion

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Chapter Overview

Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue. When they react together they neutralise each other to form salt and water.

The chapter explains why acids and bases behave this way (H⁺ and OH⁻ ions in water), how we measure their strength using pH, why pH matters in our body, soil and teeth, and how common salt is the raw material for many useful chemicals.

Important Topics and Concepts

Indicators

• Natural: litmus (from lichen), turmeric (turns red in bases), red cabbage juice.
• Synthetic: phenolphthalein (colourless in acid, pink in base) and methyl orange (red in acid, yellow in base).
• Olfactory indicators change smell: onion and vanilla lose their smell in basic solution, clove oil too.

Reactions of acids and bases with metals

Acid + metal → salt + hydrogen gas, e.g. Zn + H₂SO₄ → ZnSO₄ + H₂. The hydrogen burns with a pop sound. Some bases also react with metals: 2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂.

Acids with metal carbonates and hydrogencarbonates

Metal carbonate/hydrogencarbonate + acid → salt + carbon dioxide + water. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The CO₂ turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O); excess CO₂ makes it clear again as soluble Ca(HCO₃)₂ forms.

Neutralisation

Acid + base → salt + water. NaOH + HCl → NaCl + H₂O. Metal oxides are basic (CuO + 2HCl → CuCl₂ + H₂O, blue-green solution) and non-metal oxides are acidic (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O).

What all acids and bases have in common

Acids produce H⁺ ions (as H₃O⁺, hydronium) in water; bases produce OH⁻ ions. That is why their solutions conduct electricity. Without water, an acid does not show acidic behaviour: dry HCl gas does not change dry blue litmus.

Diluting acids

Always add acid to water slowly with stirring, never water to acid. The process is highly exothermic and adding water to concentrated acid can make the mixture splash and the glass break.

Strength of acids and bases: the pH scale

pH runs from 0 to 14. pH 7 is neutral, below 7 acidic, above 7 basic. The lower the pH, the stronger the acid; the higher, the stronger the base. Universal indicator shows different colours across the range. Strong acids (HCl) give many H⁺ ions; weak acids (acetic acid) give fewer.

Importance of pH in daily life

• Our body works between pH 7.0 and 7.8; acid rain (pH below 5.6) harms aquatic life.
• Plants need soil of suitable pH; farmers treat acidic soil with lime.
• The stomach produces HCl; excess acidity is treated with antacids such as milk of magnesia (Mg(OH)₂).
• Tooth decay starts when mouth pH falls below 5.5; toothpastes are basic.
• Bee and nettle stings inject methanoic acid; baking soda or dock leaf rubbed on the sting gives relief.

Salts and their families

A salt forms from an acid and a base. Salts with the same positive or negative radical belong to the same family, e.g. NaCl and Na₂SO₄ are sodium salts. Salts of strong acid + strong base are neutral (NaCl), strong acid + weak base are acidic (NH₄Cl), and weak acid + strong base are basic (Na₂CO₃).

Chemicals from common salt

• Sodium hydroxide by the chlor-alkali process: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Chlorine forms at the anode and hydrogen at the cathode.
• Bleaching powder (CaOCl₂): Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Used to bleach cotton and paper and to disinfect drinking water.
• Baking soda (NaHCO₃): NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. On heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Used in baking powder, antacids and soda-acid fire extinguishers.
• Washing soda (Na₂CO₃·10H₂O): made by recrystallising sodium carbonate. Used in glass, soap and paper industries and to remove permanent hardness of water.

Water of crystallisation

Some salts contain a fixed number of water molecules in one formula unit: CuSO₄·5H₂O (blue), Na₂CO₃·10H₂O, CaSO₄·2H₂O (gypsum). Heating blue copper sulphate removes this water and it turns white; adding water makes it blue again.

Plaster of Paris

Made by heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. With water it sets back into hard gypsum. Used for plaster casts for fractures, toys, decorative materials and smoothing surfaces. It must be stored in moisture-proof containers.

Important Salts: Formula and Uses

Important Definitions and Terms

Short Answer Questions

Q1. Why does dry HCl gas not change the colour of dry litmus paper?

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

Q2. Why should curd and sour substances not be kept in brass and copper vessels?

Curd and sour foods contain acids that react with copper and brass to form harmful, poisonous compounds. This spoils the food and can make it unsafe to eat.

Q3. Which gas is usually liberated when an acid reacts with a metal? How will you test for it?

Hydrogen gas is liberated. Bring a burning candle near the gas: it burns with a pop sound.

Q4. A solution has pH 6 and another has pH 8. Which is acidic and which is basic?

The solution with pH 6 is acidic (below 7) and the one with pH 8 is basic (above 7).

Q5. Why does tooth decay start when the pH of the mouth falls below 5.5?

Tooth enamel is made of calcium hydroxyapatite, which starts dissolving in acids below pH 5.5. Bacteria in the mouth break down food particles into acids, lowering the pH and causing decay.

Q6. Why is plaster of Paris stored in moisture-proof containers?

Plaster of Paris absorbs water and sets into hard gypsum. If moisture reaches it, it becomes useless for making casts.

Q7. Why do HCl and HNO₃ show acidic character in water while alcohol and glucose do not?

HCl and HNO₃ release H⁺ ions in water. Alcohol and glucose contain hydrogen but do not ionise to give H⁺ ions, so they are not acidic.

Q8. What is the effect of excess carbon dioxide on lime water?

Lime water first turns milky because insoluble calcium carbonate forms. With excess CO₂, soluble calcium hydrogencarbonate forms, so the milkiness disappears.

Long Answer Questions

Q1. What is the pH scale? Explain its importance in everyday life with three examples.

The pH scale measures the hydrogen ion concentration in a solution, from 0 (very acidic) to 14 (very alkaline), with 7 as neutral.

1. Digestion: our stomach makes HCl for digestion. Too much causes indigestion and pain, which antacids such as milk of magnesia neutralise.

2. Tooth decay: when mouth pH falls below 5.5, enamel dissolves. Brushing with basic toothpaste neutralises the acid.

3. Agriculture: crops grow best within a certain soil pH. Acidic soils are treated with lime (calcium oxide or hydroxide).

Other examples: our body works in a narrow pH range of 7.0 to 7.8, acid rain harms aquatic life, and bee stings are soothed by baking soda.

Q2. How is bleaching powder prepared? Write its chemical equation and three uses.

Bleaching powder is produced by passing chlorine gas over dry slaked lime.

Equation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses: (i) bleaching cotton and linen in textiles, wood pulp in paper factories, and washed clothes in laundry; (ii) as an oxidising agent in industries; (iii) disinfecting drinking water to make it germ-free.

Q3. What is baking soda? How is it prepared, and why is it used in baking powder and antacids?

Baking soda is sodium hydrogencarbonate, NaHCO₃, a mild non-corrosive base.

Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.

In baking powder (baking soda + tartaric acid), heating or adding water releases CO₂, which makes cakes soft and spongy. Tartaric acid neutralises the sodium carbonate so the cake does not taste bitter.

As an antacid, being basic, it neutralises excess acid in the stomach. It is also used in soda-acid fire extinguishers.

MCQs with Answers

1. The pH of a neutral solution is
(a) 0 (b) 5 (c) 7 (d) 14
Answer: (c) 7

2. Phenolphthalein in basic solution turns
(a) Red (b) Pink (c) Yellow (d) Colourless
Answer: (b) Pink

3. Which salt is used to remove permanent hardness of water?
(a) Baking soda (b) Washing soda (c) Bleaching powder (d) Common salt
Answer: (b) Washing soda

4. The chemical formula of plaster of Paris is
(a) CaSO₄·2H₂O (b) CaSO₄·½H₂O (c) CaSO₄ (d) CaOCl₂
Answer: (b) CaSO₄·½H₂O

5. In the chlor-alkali process, chlorine gas is formed at the
(a) Cathode (b) Anode (c) Both electrodes (d) Neither
Answer: (b) Anode

6. Which is an olfactory indicator?
(a) Litmus (b) Turmeric (c) Vanilla essence (d) Methyl orange
Answer: (c) Vanilla essence

7. A solution turns red litmus blue. Its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Answer: (d) 10

8. Ant sting contains
(a) Acetic acid (b) Methanoic acid (c) Citric acid (d) Oxalic acid
Answer: (b) Methanoic acid

9. The number of water molecules in one formula unit of blue copper sulphate is
(a) 2 (b) 5 (c) 7 (d) 10
Answer: (b) 5

10. Which of these is a strong acid?
(a) Acetic acid (b) Citric acid (c) Hydrochloric acid (d) Carbonic acid
Answer: (c) Hydrochloric acid

Quick Revision Points

• Acids give H⁺ (H₃O⁺) and bases give OH⁻ ions in water.
• Acid + metal → salt + H₂; acid + carbonate → salt + water + CO₂.
• Acid + base → salt + water (neutralisation).
• Always add acid to water, never water to acid.
• pH < 7 acidic, 7 neutral, > 7 basic.
• Chlor-alkali: brine → NaOH + Cl₂ (anode) + H₂ (cathode).
• Learn formulas: CaOCl₂, NaHCO₃, Na₂CO₃·10H₂O, CaSO₄·½H₂O, CuSO₄·5H₂O.

Chapter Summary

Acids and bases are identified with indicators and by their reactions with metals, carbonates and each other. In water, acids release hydrogen ions and bases release hydroxide ions; the pH scale measures how strong they are, and pH matters for digestion, teeth, soil and living things.

Salts form when acids and bases neutralise each other. Common salt is the starting point for sodium hydroxide, bleaching powder, baking soda and washing soda, while gypsum gives plaster of Paris. Several salts hold water of crystallisation, which changes their colour and properties.

Exam Tips and PYQ Pattern

• Formula and uses of bleaching powder, baking soda, washing soda and plaster of Paris are asked very often.
• Expect pH-based questions such as arranging solutions by pH or choosing the one that is most acidic.
• Activity questions on testing CO₂ with lime water and on the reaction of zinc with acid are common.
• For 5-mark questions, write each equation with names and give at least two uses.

Frequently Asked Questions

What is the difference between a base and an alkali?

All alkalis are bases, but only bases that dissolve in water are called alkalis, such as NaOH and KOH.

Why is baking powder better than baking soda alone?

Baking powder contains a mild acid (tartaric acid) that neutralises the sodium carbonate formed, so the cake is not bitter.

Is the salt formed from a strong acid and a weak base acidic?

Yes. Ammonium chloride (from HCl and NH₄OH) gives an acidic solution with pH less than 7.

Conclusion

Acids, Bases and Salts links chemistry to daily life, from digestion and toothpaste to cakes and drinking water. Learn the indicator colours, the pH scale and the salts table thoroughly, and you can answer most questions from this chapter quickly and accurately.

More Notes: Class 10 Science Chapter 1: Chemical Reactions and Equations notes | Class 10 Science Chapter 3: Metals and Non-metals notes | All Class 10 Science notes

Author Bio: Written and reviewed by the Nextoper Editorial Team, a group of CBSE science educators who build chapter notes directly from the NCERT textbook and past board paper patterns.

Freshness Note: These notes follow the NCERT Class 10 Science textbook (Reprint 2026-27) and the CBSE syllabus for the 2026-27 session. Last checked on 7 November 2026.