Acids, Bases and Salts — Practice

Question 1

What colour does phenolphthalein turn in (a) an acid and (b) a base?

Accepted Answer

Phenolphthalein is colourless in acid and pink in base.

✦ Answer: (a) colourless, (b) pink.

Question 2

Which gas is released when zinc reacts with dilute sulphuric acid, and how is it tested?

Accepted Answer

Hydrogen gas is released. Test: a burning splint brought to the mouth of the tube burns the gas with a 'pop' sound.

✦ Answer: H₂; gives a pop sound with a lit splint.

Question 3

Give the chemical formula and one use of washing soda.

Accepted Answer

Washing soda is Na₂CO₃·10H₂O.

Use (any one): cleaning, removing permanent hardness of water, or manufacturing glass/soap/paper.

✦ Answer: Na₂CO₃·10H₂O; used for cleaning / softening hard water.

Question 4

Write the equation for the reaction between sodium hydroxide and hydrochloric acid, and name the type of reaction.

Accepted Answer

Step 1 — Acid + base → salt + water.

Step 2 — NaOH + HCl → NaCl + H₂O.

✦ Answer: NaOH + HCl → NaCl + H₂O; neutralisation reaction.

Question 5

What happens when dilute hydrochloric acid is added to sodium carbonate? Write the equation and the confirmatory test for the gas.

Accepted Answer

Step 1 — Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑.

Step 2 — Brisk effervescence of CO₂; on passing it through limewater, the limewater turns milky (CaCO₃ forms).

✦ Answer: CO₂ is released; it turns limewater milky.

Question 6

A farmer finds his field soil is too acidic. What should he add and why?

Accepted Answer

Step 1 — Acidic soil has pH below the ideal range for crops.

Step 2 — Adding a base such as quicklime (CaO), slaked lime [Ca(OH)₂] or chalk neutralises the excess acid and raises the pH.

✦ Answer: Add lime (a base) to neutralise the acidity and raise soil pH.

Question 7

Blue copper sulphate crystals turn white on heating and blue again on adding water. Explain.

Accepted Answer

Step 1 — CuSO₄·5H₂O contains 5 water molecules of crystallisation, giving the blue colour.

Step 2 — Heating drives off this water: CuSO₄·5H₂O → CuSO₄ (white) + 5H₂O.

Step 3 — Adding water restores the hydrate, so the blue colour returns.

✦ Answer: The blue colour is due to water of crystallisation; heating removes it (white), adding water restores it (blue).

Question 8

Explain why tooth decay starts when the pH of the mouth falls below 5.5, and how toothpaste helps.

Accepted Answer

Step 1 — Bacteria in the mouth break down leftover sugar into acids, lowering the pH.

Step 2 — Below pH 5.5, this acid dissolves tooth enamel (made of calcium phosphate), starting decay.

Step 3 — Toothpastes are mildly basic; brushing neutralises the excess acid and stops the enamel from dissolving.

✦ Answer: Acid below pH 5.5 dissolves enamel; basic toothpaste neutralises the acid.

Question 9

How is bleaching powder prepared? Write its formula and two uses.

Accepted Answer

Step 1 — Chlorine is passed over dry slaked lime.

Step 2 — Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Formula: CaOCl₂.

Step 3 — Uses (any two): bleaching cotton/linen and wood pulp, disinfecting drinking water, oxidising agent in industry.

✦ Answer: Cl₂ + dry slaked lime → CaOCl₂; used for bleaching and disinfecting water.

Question 10

Classify the following salts as acidic, basic or neutral and justify: (a) NaCl, (b) Na₂CO₃, (c) NH₄Cl.

Accepted Answer

Step 1 — Identify the parent acid and base of each.

(a) NaCl: from strong acid HCl + strong base NaOH → neutral (pH 7).
(b) Na₂CO₃: from weak acid H₂CO₃ + strong base NaOH → basic (pH > 7).
(c) NH₄Cl: from strong acid HCl + weak base NH₄OH → acidic (pH < 7).

✦ Answer: (a) neutral, (b) basic, (c) acidic.

Question 11

(a) Name the raw material and the reaction used to make plaster of Paris. (b) Write its formula. (c) Why must it be stored in a moisture-proof container? (d) Give two uses.

Accepted Answer

Step 1 (a) — Gypsum (CaSO₄·2H₂O) is heated to about 373 K: CaSO₄·2H₂O →(373 K) CaSO₄·½H₂O + 1½H₂O.

Step 2 (b) — Formula: CaSO₄·½H₂O (calcium sulphate hemihydrate).

Step 3 (c) — On contact with moisture it sets into hard gypsum, becoming useless; so it must be kept dry/airtight.

Step 4 (d) — Uses (any two): setting fractured bones (plaster casts), making statues and decorative items, smoothing surfaces before painting.

✦ Answer: heat gypsum → CaSO₄·½H₂O; store dry because it sets with water; used for casts and statues.

Practice — Acids, Bases and Salts

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