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InkFlaskCambridge IGCSE
Contains the answers

Answers · Acids, bases and indicators

What acids and bases do — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. MQ8 · Practice

Model answer. An acid and a metal make a salt and hydrogen: zinc + hydrochloric acid → zinc chloride + hydrogen. Hydrogen burns with a squeaky pop when a lighted splint is held near it.

A full answer includes

  • (1) chloride
  • (2) hydrogen
  • (3) lighted (burning)
  • (4) squeaky pop

Also accept

  • chloride; hydrogen; lighted; squeaky pop

Watch for

  • “The gas is carbon dioxide.” — Carbon dioxide comes from a carbonate. A metal and an acid give hydrogen.
  • “A glowing splint relights.” — A glowing splint relighting is the test for oxygen. Hydrogen pops with a lighted splint.

2. MQ9 · Practice

Model answer. An acid and a carbonate give a salt, water and carbon dioxide: copper(II) carbonate + nitric acid → copper(II) nitrate + water + carbon dioxide. Carbon dioxide turns limewater milky. Row B.

A full answer includes

  • I chose row B: carbon dioxide turns limewater milky.
  • I did not choose hydrogen, which comes from a metal.

Also accept

  • Row B

Watch for

  • “The gas from a carbonate and an acid is hydrogen.” — A carbonate gives carbon dioxide, which turns limewater milky. Hydrogen comes from an acid and a metal.
  • “Row C.” — Oxygen is not made here, and a glowing splint is its test. A carbonate gives carbon dioxide.
  • “Row D.” — Carbon dioxide does not burn, so it gives no pop. Its test is limewater turning milky.

3. MQ10 · Practice

Options as numbered on the pupil sheet: 1 — not a base; 2 — a base, not an alkali; 3 — an alkali.

Model answer. Metal oxides and hydroxides are bases. Nickel(II) oxide and iron(III) hydroxide do not dissolve, so they are bases but not alkalis. Lithium hydroxide is a base that dissolves, so it is an alkali, and its solution contains OH− ions. Sodium nitrate is neither an oxide nor a hydroxide: it is a salt, not a base.

A full answer includes

  • 1. Nickel(II) oxide: a base, not an alkali.
  • 2. Iron(III) hydroxide: a base, not an alkali.
  • 3. Lithium hydroxide: an alkali.
  • 4. Sodium nitrate: not a base.

Also accept

  • base; base; alkali; not a base

Watch for

  • “Bases and alkalis are the same thing.” — Every alkali is a base, but only a base that dissolves in water is an alkali. Copper(II) oxide is a base that does not dissolve, so it is not an alkali.
  • “Lithium hydroxide is a base, so it cannot be an alkali.” — An alkali is a kind of base, one that dissolves. Lithium hydroxide is both.

4. MQ11 · Practice

Model answer. K2CO3 has 2 potassium atoms, so 2KCl. That needs 2 chlorine atoms, so 2HCl. Now 2 hydrogen atoms go into one H2O, and the carbonate gives one CO2: K2CO3 + 2HCl → 2KCl + H2O + CO2.

Lowest whole-number coefficients: K2CO3(aq) + 2HCl(aq) → 2KCl(aq) + H2O(l) + CO2(g)

A full answer includes

  • K2CO3 + 2HCl → 2KCl + H2O + CO2.

Also accept

  • 1, 2, 2, 1, 1.

Watch for

  • “Change KCl to K2Cl.” — A formula never changes to balance an equation: change only the numbers in front.
  • “The gas from a carbonate and an acid is hydrogen.” — A carbonate gives carbon dioxide, which turns limewater milky. Hydrogen comes from an acid and a metal.

5. MQ12 · Practice

Model answer. A base and an ammonium salt, warmed, give a salt, water and ammonia. (a) Ammonia, NH3. (b) ammonium nitrate + calcium hydroxide → calcium nitrate + water + ammonia. (c) Ammonia is an alkaline gas, so it turns damp red litmus paper blue: that is the test for ammonia.

A full answer includes

  • (a) Ammonia.
  • (b) ammonium nitrate + calcium hydroxide → calcium nitrate + water + ammonia.
  • (c) The litmus paper turns blue.
  • (c) Ammonia is an alkaline gas.

Also accept

  • Ammonia; ammonium nitrate + calcium hydroxide → calcium nitrate + water + ammonia; litmus turns blue

Watch for

  • “The gas is hydrogen.” — A base and an ammonium salt give ammonia. Hydrogen comes from an acid and a metal.
  • “The products are a salt and water only, as with an acid.” — With an ammonium salt the base also drives off ammonia gas.

6. MQ13 · Practice

Model answer. (a) The water in a neutralisation of an acid by an alkali forms in the same way: H+(aq) + OH−(aq) → H2O(l). (b) Barium ions, Ba2+, come from the alkali and chloride ions, Cl−, from the acid. They are dissolved before and after and take no part: they are spectator ions.

A full answer includes

  • (a) H+(aq) + OH−(aq) → H2O(l).
  • (b) Barium ions, Ba2+, and chloride ions, Cl−.

Also accept

  • H+(aq) + OH−(aq) → H2O(l); Ba2+ and Cl−

Watch for

  • “The ionic equation is Ba2+ + 2Cl− → BaCl2.” — Barium chloride stays dissolved as separate Ba2+ and Cl− ions, so nothing forms between them. The only change is H+ and OH− joining to make water.

7. MQ14 · Practice

Model answer. At 10 cm³ only half the acid has been neutralised, so H+ ions are left over: pH below 7. At 20 cm³ the acid and alkali exactly cancel: pH 7. At 30 cm³ there is 10 cm³ of alkali with no acid to react with, so OH− ions are left over: pH above 7. Row B.

A full answer includes

  • I chose row B.
  • I took the pH to be below 7 at 10 cm³, with acid left over.
  • I took the pH to be 7 at 20 cm³.
  • I took the pH to be above 7 at 30 cm³, with alkali left over.

Also accept

  • Row B

Watch for

  • “Neutralisation always leaves a solution at pH 7.” — With an acid such as hydrochloric acid and an alkali such as sodium hydroxide, the pH is 7 only when they exactly cancel. With acid left over the pH is below 7; with alkali left over it is above 7.
  • “Row A.” — The solution starts as acid, so its pH starts below 7 and rises as alkali is added.
  • “Row D.” — At 10 cm³ only half the acid is neutralised: H+ ions are left over, so the pH is below 7.

8. MT2 · Apply

Model answer. (a) Aluminium hydroxide is a metal hydroxide, so it is a base: it neutralises acid. An alkali is a base that dissolves in water, and aluminium hydroxide does not. (b) magnesium carbonate + hydrochloric acid → magnesium chloride + water + carbon dioxide. (c) The carbon dioxide is a gas, and some of it comes back up as a burp.

A full answer includes

  • (a) It is a metal hydroxide, so it is a base.
  • (a) It does not dissolve in water, so it is not an alkali.
  • (b) magnesium carbonate + hydrochloric acid → magnesium chloride + water + carbon dioxide.
  • (c) The carbon dioxide gas made in the stomach comes back up.

Also accept

  • Insoluble hydroxide: base, not alkali; magnesium chloride + water + carbon dioxide; the carbon dioxide

Watch for

  • “Bases and alkalis are the same thing.” — Every alkali is a base, but only a base that dissolves in water is an alkali. Copper(II) oxide is a base that does not dissolve, so it is not an alkali.
  • “The gas from a carbonate and an acid is hydrogen.” — A carbonate gives carbon dioxide, which turns limewater milky. Hydrogen comes from an acid and a metal.