Chapter 2

Acids, Bases and Salts

8 min video15 MCQs8 exam questions6 practice problems

Acid-base indicators tell us whether a solution is acidic or basic by a change in colour. Litmus is a PURPLE dye extracted from LICHEN — purple in a neutral solution, red in acid, blue in base. Methyl orange: red in acid, yellow in base. Phenolphthalein: colourless in acid, pink in base. Other natural indicators: turmeric, red cabbage, coloured petals of hydrangea.

Olfactory indicators change SMELL instead of colour — vanilla, onion and clove lose their smell in a basic solution.

Acid + metal → salt + hydrogen gas. Test for H₂: bring a burning candle near — it burns with a POP sound.

Acid + metal carbonate (or hydrogencarbonate) → salt + water + carbon dioxide. Test for CO₂: it turns lime water MILKY (excess CO₂ makes it clear again).

Base + metal: SOME metals (e.g. zinc, aluminium) react with strong alkalis to give hydrogen — 2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂. This does NOT happen with all metals.

Neutralisation: acid + base → salt + water (e.g. NaOH + HCl → NaCl + H₂O).

Metal oxides are BASIC — they react with acids to give salt + water. Non-metal oxides are ACIDIC — they react with bases to give salt + water.

All acids produce H⁺ (hydrogen) ions in water; all bases produce OH⁻ (hydroxide) ions in water. H⁺ ions cannot exist alone in water — they attach to water molecules forming HYDRONIUM ions: H⁺ + H₂O → H₃O⁺ (so acids are written as H₃O⁺(aq) or H⁺(aq)). Example: HCl + H₂O → H₃O⁺ + Cl⁻; NaOH(aq) → Na⁺ + OH⁻. Because their solutions contain free ions, acids and bases conduct electricity.

Acids need WATER to show acidic behaviour — dry HCl gas does NOT change the colour of DRY litmus paper, but MOIST litmus paper turns red (the water lets HCl release H⁺ ions). A base soluble in water is called an ALKALI.

SAFETY: always add acid to water slowly with stirring — never water to acid. The process is highly exothermic and can splash acid out.

Strong acids (HCl, H₂SO₄, HNO₃) ionise completely and give many H⁺ ions; weak acids (acetic/ethanoic, carbonic, citric) ionise only partly. Same idea for strong bases (NaOH, KOH) vs weak bases (NH₄OH).

pH scale runs 0–14: pH 7 is neutral, below 7 acidic (lower = stronger acid), above 7 basic (higher = stronger base). A universal indicator shows the whole range.

Tooth decay: tooth enamel is CALCIUM PHOSPHATE — the hardest substance in the body, insoluble in water — but it corrodes when the pH in the mouth falls below 5.5. Bacteria degrade leftover sugar and food to make that acid; brushing with an alkaline toothpaste neutralises it.

pH in daily life: the stomach's HCl aids digestion and antacids like milk of magnesia (Mg(OH)₂) neutralise the excess; plants need a specific soil pH; rain below pH 5.6 is ACID RAIN, which makes river water acidic and harms aquatic life. Self-defence chemistry: a BEE STING leaves an acid that causes pain — a mild base such as baking soda gives relief; the stinging hair of NETTLE leaves injects METHANOIC ACID, traditionally relieved by rubbing with dock plant leaf.

FAMILY OF SALTS: salts with the same positive radical (cation) OR the same negative radical (anion) belong to one family. NaCl and Na₂SO₄ → sodium salts; NaCl and KCl → chloride salts; CaSO₄ and Na₂SO₄ → sulphate salts.

pH of 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).

Common salt (NaCl) comes from sea water and as ROCK SALT (deposits left by dried-up ancient seas). It is the raw material for sodium hydroxide, chlorine, hydrogen, bleaching powder, baking soda and washing soda. (Plaster of Paris is NOT from common salt — it comes from gypsum.)

Chlor-alkali process: electricity through brine (concentrated NaCl solution) gives 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine at the ANODE, hydrogen at the CATHODE, sodium hydroxide in solution. Uses — NaOH: soaps/detergents, paper making, de-greasing metals; Cl₂: water treatment, PVC, disinfectants; H₂: fuel, margarine, ammonia for fertilisers.

Bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: bleaching cotton and wood pulp, oxidising agent, disinfecting drinking water.

Baking soda NaHCO₃ — a mild, non-corrosive base. Antacid; with a mild edible acid (tartaric acid) it forms BAKING POWDER, which releases CO₂ on heating to make cakes soft and spongy. On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. It is also used in soda-acid fire extinguishers — but there the CO₂ comes from NaHCO₃ reacting with sulphuric acid, not from heating.

Washing soda Na₂CO₃·10H₂O — cleaning agent, removes permanent hardness of water, used in glass, soap and paper industries.

Water of crystallisation = fixed water molecules present in one formula unit of a salt. Examples: CuSO₄·5H₂O (blue; turns white on heating and blue again on adding water), Na₂CO₃·10H₂O, CaSO₄·2H₂O (gypsum).

Plaster of Paris CaSO₄·½H₂O — made by heating gypsum to 373 K. It sets into a hard solid (gypsum again) on mixing with water, so it must be stored in a moisture-proof container. Used in plaster casts for fractured bones, and for making toys and decorations.

Try answering on paper first — then reveal the model answer. 📄

Solve in your notebook. Stuck? Take the hint before the solution. ✏️

1. Write the balanced chemical equation for the reaction of zinc with sodium hydroxide solution, and name the salt formed.

medium

2. A colourless gas is evolved when dilute hydrochloric acid is added to sodium carbonate. Name the gas, describe the test to identify it, and write the equation for the test.

medium

3. Arrange these in increasing order of pH: lemon juice, sodium hydroxide solution, pure water, milk of magnesia, gastric juice.

medium

4. A salt X dissolves in water to give a solution of pH 11. Which type of acid and base formed X? Give one example of such a salt.

hard

5. Write the equation for obtaining plaster of Paris from gypsum, and the equation for the setting of plaster of Paris.

hard

6. Fresh milk has a pH of 6. When it turns into curd, does the pH increase or decrease? Give a reason.

easy

Download worksheet & answer key (PDF)

Blue litmus paper is dipped in a solution and turns red. The solution is: