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Class 10 Acids, Bases and Salts notes: NCERT Science Chapter 2 revision

CMDLearn · 3 November 2026

These are short revision notes for Chapter 2 of the NCERT Class 10 Science textbook (Reprint 2026-27), pages 17 to 36. The chapter is full of general reactions and named salts, so these notes put every general reaction in one table, then go through the chapter section by section, and end with the key words, the set-up to draw, the mistakes students make most often and 8 quick questions of our own with worked answers.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts this chapter in Unit I, Chemical Substances: Nature and Behaviour, worth 25 of the 80 theory marks, shared with Chemical Reactions and Equations, Metals and Non-metals, and Carbon and its Compounds (curriculum, PDF page 4). For this chapter the unit lists: acids and bases defined by the H⁺ and OH⁻ ions they give, indicators, chemical properties, examples and uses, neutralisation, the pH scale (no logarithm definition), pH in everyday life, and how five chemicals are made and used: plaster of Paris, washing soda, baking soda, bleaching powder and sodium hydroxide (PDF pages 4-5). It is examined in the board paper; only Periodic Classification is marked as formative-only in this unit. More on the units: CBSE Class 10 Science syllabus 2026-27.

The general reactions on one page

1. Indicators (pages 17-19)

• Acids taste sour and turn blue litmus red; bases taste bitter and turn red litmus blue. Never taste or touch laboratory chemicals.
• Natural indicators: litmus (a purple dye from lichen; purple when the solution is neutral), turmeric, red cabbage leaves, and the coloured petals of hydrangea, petunia and geranium.
• A curry (turmeric) stain turns reddish-brown with soap, which is basic, and yellow again after washing with plenty of water.
• Synthetic indicators: methyl orange and phenolphthalein.
• Olfactory indicators change their smell in acid or base: onion, vanilla essence and clove oil are the ones tested.

2. How acids and bases react (section 2.1, pages 18-22)

• Acid + metal → salt + hydrogen. The metal pushes out hydrogen from the acid. The gas burns with a pop when a burning candle is brought near a gas-filled soap bubble.
• Some metals react with a base too: zinc and sodium hydroxide give sodium zincate and hydrogen. Not every metal does this.
• Acid + carbonate or hydrogencarbonate → salt + CO₂ + water. CO₂ turns lime water milky (white CaCO₃); with excess CO₂ the milkiness goes, because soluble calcium hydrogencarbonate forms. Limestone, chalk and marble are all calcium carbonate.
• Neutralisation: acid + base → salt + water. Phenolphthalein is pink in sodium hydroxide, loses its colour as acid is added, and turns pink again if more base is added.
• Metal oxides are basic oxides: they react with acids to give salt and water, like a base.
• Non-metal oxides are acidic oxides: CO₂ reacts with the base calcium hydroxide to give a salt and water.

3. What acids and bases have in common (section 2.2, pages 22-24)

• Acid solutions in water conduct electricity (the bulb in Fig. 2.3 glows); glucose and alcohol solutions do not, though they contain hydrogen.
• Acids give H⁺(aq) ions in water. H⁺ cannot exist alone: it joins water as the hydronium ion, H₃O⁺. HCl + H₂O → H₃O⁺ + Cl⁻.
• Dry HCl gas does not change dry blue litmus; it shows acidic behaviour only once water is present.
• Bases give OH⁻ ions in water. Bases that dissolve in water are called alkalis; they feel soapy, taste bitter and are corrosive. Not every base dissolves.
• Dissolving an acid or base in water is highly exothermic. Always add the acid to water, slowly, stirring all the time; adding water to concentrated acid can make the mixture splash and cause burns, and the glass can crack.
• Dilution lowers the concentration of H₃O⁺ or OH⁻ ions per unit volume.

4. pH and strength (section 2.3, pages 25-28)

• A universal indicator is a mixture of indicators that shows different colours at different H⁺ concentrations.
• The pH scale usually runs from 0 at the most acidic end to 14 at the most alkaline. The "p" is from the German potenz, power. Neutral = 7; below 7 = acidic; above 7 = basic. The more hydronium ions, the lower the pH.
• Fig. 2.7 marks some values: gastric juice about 1.2, lemon juice about 2.2, one mark for pure water and blood labelled 7.4, milk of magnesia 10, sodium hydroxide solution about 14 (page 26).
• Strong acids give more H⁺ ions than weak acids of the same concentration (HCl compared with acetic acid).
• pH in everyday life (pages 26-27): - our body works within pH 7.0 to 7.8; - rain below pH 5.6 is acid rain, and it makes rivers harder for aquatic life; - plants need soil of a particular pH; - the stomach makes hydrochloric acid; too much causes pain, and an antacid such as milk of magnesia (magnesium hydroxide) neutralises it; - tooth decay starts below pH 5.5, when enamel (calcium hydroxyapatite) is corroded; basic toothpastes help; - a bee sting leaves acid, eased with baking soda; nettle hairs inject methanoic acid; - Venus has clouds of sulphuric acid.
• Naturally occurring acids (Table 2.3): vinegar, acetic; orange and lemon, citric; tamarind, tartaric; tomato, oxalic; curd, lactic; ant and nettle stings, methanoic.

5. Salts (section 2.4, pages 28-33)

• Families: salts with the same positive or negative radical form a family (NaCl and Na₂SO₄ are sodium salts; NaCl and KCl are chloride salts).
• pH of salts: strong acid + strong base gives a neutral salt (pH 7); strong acid + weak base gives an acidic salt (below 7); strong base + weak acid gives a basic salt (above 7).
• Common salt (sodium chloride) comes from seawater and from rock salt, mined like coal and often brown from impurities. It was the symbol of Gandhiji's Dandi March.

Chemicals from common salt

Water of crystallisation and plaster of Paris

• Water of crystallisation means the set number of water molecules held in each formula unit of a salt. Crystals that look dry still hold it.
• Copper sulphate (CuSO₄·5H₂O) is blue; heating drives the water off and it turns white; a few drops of water bring the blue back.
• Gypsum is CaSO₄·2H₂O. Heated at 373 K it becomes plaster of Paris, CaSO₄·½H₂O (calcium sulphate hemihydrate), where two formula units share one water molecule.
• Mixed with water, plaster of Paris sets back into gypsum, a hard solid. Uses: supporting fractured bones, toys, decorative materials, smoothing surfaces.

Key words at a glance

• Indicator: a dye, or mixture of dyes, whose colour (or smell) shows acid or base.
• Alkali: a base that dissolves in water.
• Hydronium ion: H₃O⁺, the form H⁺ takes in water.
• Neutralisation: acid + base → salt + water.
• pH: a number from 0 to 14 for how acidic or basic a solution is; 7 is neutral.
• Strong / weak acid: gives more / fewer H⁺ ions at the same concentration.
• Antacid: a mild base that neutralises excess stomach acid.
• Chlor-alkali process: electrolysis of brine, giving chlorine, sodium hydroxide and hydrogen.
• Water of crystallisation: the set number of water molecules held in each formula unit of a salt.

The set-up to draw: testing the gas from an acid and a metal

Board papers often ask for this diagram (Fig. 2.1, page 19). Practise it:

1. A test tube with dilute sulphuric acid and zinc granules, held in a stand.
2. A delivery tube from the cork of the test tube, dipping into a trough or dish of soap solution.
3. Bubbles forming in the soap solution as the gas comes through.
4. A burning candle held near a gas-filled bubble.
5. Label: dilute sulphuric acid, zinc granules, delivery tube, soap solution, bubbles of hydrogen gas, burning candle. The gas burns with a pop.

Mistakes that cost marks

• Adding water to concentrated acid. It is always acid into water, slowly, with stirring.
• Saying a lower pH is less acidic. Lower pH means more H⁺ ions and a stronger acid.
• Calling every base an alkali. Only bases that dissolve in water are alkalis.
• Forgetting the second lime-water step. Milky with CO₂; clear again with excess CO₂.
• Mixing up baking soda and washing soda. NaHCO₃ is baking soda; Na₂CO₃·10H₂O is washing soda.
• Mixing up gypsum and plaster of Paris. Gypsum has 2H₂O; plaster of Paris has ½H₂O.
• Writing bleaching powder from wet slaked lime. The book says dry slaked lime.
• Unbalanced equations. Count every atom on both sides; the general reactions above are a good place to practise.

Quick check: 8 questions of our own

These are our own questions, written for these notes and checked against the textbook pages named; no practising teacher has reviewed them yet. Every number and equation was re-checked by a script.

Question 1. Meera has turmeric paper and phenolphthalein, but no litmus. She tests three liquids: soap solution, lemon juice and sugar solution. (a) What will each indicator show with the soap solution? (b) Which two liquids can she not tell apart with these two indicators, and why? (c) Name a natural material from the chapter that she could use instead to tell them apart by smell.

Answer: (a) turmeric turns reddish-brown and phenolphthalein turns pink; (b) lemon juice and sugar solution, because neither indicator changes colour in an acid or in a neutral liquid; (c) onion (an olfactory indicator). (a) Soap is basic, and the chapter shows turmeric (the curry stain) turning reddish-brown with soap (page 17); phenolphthalein is pink in a base (page 21). (b) Turmeric and phenolphthalein show a change only with bases, so an acid and a neutral liquid look the same with both. (c) Onion-treated cloth strips change their smell in acid or base (pages 18-19); blue litmus would also work, since it turns red only in the acid.

Question 2. Write a word equation and a balanced equation for each, and name the general type of reaction from the chapter: (a) magnesium oxide reacts with dilute hydrochloric acid; (b) potassium hydrogencarbonate reacts with dilute nitric acid; (c) carbon dioxide is bubbled into lime water, and then (d) much more carbon dioxide is bubbled through.

Answer: (a) magnesium oxide + hydrochloric acid → magnesium chloride + water; MgO + 2HCl → MgCl₂ + H₂O; a metal oxide with an acid (page 21). (b) potassium hydrogencarbonate + nitric acid → potassium nitrate + water + carbon dioxide; KHCO₃ + HNO₃ → KNO₃ + H₂O + CO₂; acid with a metal hydrogencarbonate (page 21). (c) calcium hydroxide + carbon dioxide → calcium carbonate + water; Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; a base with a non-metal oxide, and the white calcium carbonate makes the lime water milky (pages 20 and 22). (d) calcium carbonate + water + carbon dioxide → calcium hydrogencarbonate; CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂, which dissolves, so the milkiness clears (page 21).

Question 3. (a) Rain samples from three towns have pH 4.8, 5.9 and 5.2. Which are acid rain? (b) Why does acid rain flowing into a river harm the fish in it? (c) A blood test reads pH 7.4. Is this inside the range the chapter gives for the body? (d) For indigestion, would you choose milk of magnesia (pH about 10) or lemon juice (pH about 2.2)? Give the reason.

Answer: (a) pH 4.8 and 5.2; (b) it lowers the river's pH, and living things survive only a narrow pH range; (c) yes; (d) milk of magnesia, a mild base that neutralises the excess acid. (a) Rain is acid rain when its pH is below 5.6 (page 26): 4.8 and 5.2 are below it, 5.9 is not. (b) Acid rain lowers the pH of river water, and the survival of aquatic life becomes difficult (page 26). (c) The body works between pH 7.0 and 7.8 (page 26), and 7.4 lies inside it. (d) Indigestion means too much hydrochloric acid in the stomach; an antacid such as milk of magnesia neutralises it (page 27), while lemon juice is itself acidic (Fig. 2.7, page 26).

Question 4. For each salt, name the acid and the base it can be made from, say whether each parent is strong or weak, and predict whether its solution is acidic, basic or neutral: (a) potassium chloride, KCl; (b) ammonium nitrate, NH₄NO₃; (c) potassium acetate, CH₃COOK; (d) sodium sulphate, Na₂SO₄. (e) Which two of these belong to the same family?

(Take HCl, HNO₃, H₂SO₄, NaOH and KOH as strong, and acetic acid and ammonium hydroxide as weak, the usual school values.)

Answer: (a) HCl + KOH, both strong, neutral (pH 7); (b) HNO₃ (strong) + NH₄OH (weak), acidic (below 7); (c) CH₃COOH, acetic acid (weak) + KOH (strong), basic (above 7); (d) H₂SO₄ + NaOH, both strong, neutral; (e) KCl and CH₃COOK, the potassium family. The rule is on page 29: a salt made from a strong acid with a strong base is neutral, of a strong acid and a weak base acidic, and of a strong base and a weak acid basic. The chapter itself names acetic acid as weaker than hydrochloric acid at the same concentration (page 26). Salts with the same positive or negative radical belong to one family (page 29), and these two share potassium.

Question 5. In the chlor-alkali process: (a) what is passed through brine, and which three products form? (b) Which product forms at the anode, which at the cathode, and where does sodium hydroxide collect? (c) Give one use of each product from Fig. 2.8. (d) Using the balanced equation, how many chlorine molecules can 10 formula units of sodium chloride give?

Answer: (a) electricity; sodium hydroxide, chlorine and hydrogen; (b) chlorine at the anode, hydrogen at the cathode, sodium hydroxide near the cathode; (c) for example, chlorine for water treatment, hydrogen for ammonia for fertilisers, sodium hydroxide for soaps and detergents; (d) 5. (a) and (b) are on page 30. (c) Fig. 2.8 (page 30) lists chlorine for water treatment, swimming pools, PVC, disinfectants, CFCs and pesticides; hydrogen for fuels, margarine and ammonia for fertilisers; and sodium hydroxide for de-greasing metals, soaps and detergents, paper making and artificial fibres. (d) 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂: every 2 NaCl give 1 Cl₂, so 10 give 10 ÷ 2 = 5.

Question 6. Match each use to baking soda, washing soda or bleaching powder: (a) a cake rising in the oven (as part of baking powder); (b) making glass; (c) removing permanent hardness of water; (d) a soda-acid fire extinguisher; (e) killing germs in drinking water; (f) making borax; (g) bleaching wood pulp for paper; (h) an antacid. Then: (i) when baking soda is heated, how many molecules of carbon dioxide come from 6 formula units of NaHCO₃?

Answer: baking soda: (a), (d), (h); washing soda: (b), (c), (f); bleaching powder: (e), (g); (i) 3. The uses are listed on pages 31-32. (i) 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (page 31): every 2 formula units give 1 CO₂, so 6 give 3.

Question 7. Use Cu = 63.5, S = 32, O = 16 and H = 1. (a) A student weighs 2.495 g of blue copper sulphate crystals that look perfectly dry, heats them until the mass stops changing, and is left with 1.595 g of a white powder, CuSO₄. How many grams of water were in the "dry" crystals, and how many water molecules does that give per formula unit of CuSO₄? (b) Find the formula mass of CuSO₄·5H₂O and the percentage of it that is water, to one decimal place. (c) In plaster of Paris, CaSO₄·½H₂O, how many water molecules are there for every 2 formula units of calcium sulphate? (d) What does plaster of Paris turn into when mixed with water?

Answer: (a) 0.9 g of water, which is 5 water molecules per formula unit; (b) 249.5 and 36.1%; (c) 1; (d) gypsum, CaSO₄·2H₂O, a hard solid. (a) 2.495 − 1.595 = 0.9 g of water. CuSO₄ = 63.5 + 32 + 4 × 16 = 159.5, so 1.595 g is 1.595 ÷ 159.5 = 0.01 of a formula-mass unit (0.01 mol); water is 18, so 0.9 g is 0.9 ÷ 18 = 0.05 mol; 0.05 ÷ 0.01 = 5. Crystals that look dry still hold this water of crystallisation, a fixed number of water molecules per formula unit, which is why the formula is CuSO₄·5H₂O (page 32). (b) CuSO₄ = 159.5; 5H₂O = 5 × 18 = 90; total 249.5. Water: 90 ÷ 249.5 × 100 ≈ 36.1%. (c) Two formula units of CaSO₄ share one water molecule, which is why the formula shows a half (page 33). (d) Page 33.

Question 8. For each pair, choose: (A) both true, and R explains A; (B) both true, but R does not explain A; (C) A true, R false; (D) A false, R true.

• Pair 1. Assertion: Bases that dissolve in water are called alkalis. Reason: All bases dissolve in water.
• Pair 2. Assertion: To dilute concentrated sulphuric acid, water should be poured slowly into the acid. Reason: Dissolving an acid in water is a highly exothermic process.
• Pair 3. Assertion: Baking soda on a bee sting eases the pain. Reason: Baking soda is a mild base that neutralises the acid the sting leaves.
• Pair 4. Assertion: Rock salt crystals are often brown. Reason: Sodium chloride is a neutral salt, made from a strong acid with a strong base.

Answer: Pair 1 (C); Pair 2 (D); Pair 3 (A); Pair 4 (B). Pair 1: only water-soluble bases are alkalis, and the chapter says that not all bases dissolve (page 24), so R is false. Pair 2: the acid must be added to water, never water to the acid, because the heat can make the mixture splash (page 24), so A is false and R is true. Pair 3: a bee sting leaves an acid, and a mild base such as baking soda gives relief by neutralising it (pages 27 and 31). Pair 4: rock salt is brown because of impurities (page 29); sodium chloride is indeed neutral (page 29), but that does not explain its colour.

Revise with questions

Test yourself next: Class 10 Acids, Bases and Salts, 8 practice questions with answers. The chapter before this one: Class 10 Chemical Reactions and Equations, practice questions. The other notes in this set: Metals and Non-metals, Life Processes and Heredity. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

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Sources

All checked on 3 November 2026. NCERT page numbers are the printed page numbers of the textbook (printed page = PDF page + 16).

• NCERT, Science, Textbook for Class X, Chapter 2 "Acids, Bases and Salts" (Reprint 2026-27), pages 17-36: indicators (pages 17-19); section 2.1 and Figs. 2.1-2.2 (pages 18-22); section 2.2 (pages 22-24); section 2.3, Fig. 2.7 and Table 2.3 (pages 25-28); section 2.4, Fig. 2.8 (pages 28-33); "What you have learnt" (pages 33-34). ncert.nic.in/textbook/pdf/jesc102.pdf
• NCERT, the same book's opening pages (contents, Chapter 2 at page 17): ncert.nic.in/textbook/pdf/jesc1ps.pdf
• CBSE, Science curriculum, Class X, 2026-27: unit marks (PDF page 4) and Unit I contents (PDF pages 4-5): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, equations, section numbers and figure numbers are from the documents above. The wording of these notes, the diagram steps, the list of mistakes and the 8 questions are our own.

Practise this class: Class 10 worksheets, chapter by chapter · free practice, marked instantly · timed revision papers