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Answers · Acids, bases and indicators

Acids, bases and indicators — final check key

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

1. MK1 · Final check

Model answer. Red is pH 0 to 2, yellow 5 to 6, green 7 and purple 11 to 14. L, red, has the lowest pH: it is the most acidic and has the highest concentration of H+ ions. J, green, is neutral. K, purple, has the highest pH: the most alkaline. M, yellow, is slightly acidic. Row B.

A full answer includes

  • I chose row B.
  • I took L (red) to be the most acidic, with the highest concentration of H+ ions.
  • I took J (green) to be neutral.
  • I took K (purple) to be the most alkaline.
  • I did not take yellow M to be neutral.

Also accept

  • Row B

Watch for

  • “Row A.” — Yellow is only slightly acidic, pH 5 or 6. Red is further below 7, so L is the most acidic.
  • “Row C.” — Yellow is pH 5 or 6, slightly acidic. Only green is neutral.
  • “Row D.” — That turns the scale round: purple is the alkaline end and red the acid end.

2. MK2 · Final check

Model answer. (a) E is acidic, pH 2: litmus red, methyl orange red, thymolphthalein colourless. (b) F is alkaline, pH 11: litmus blue, thymolphthalein blue. (c) Alkalis contain hydroxide ions, OH−(aq). (d) Methyl orange is yellow in an alkali and also in a neutral solution, and even in a very weakly acidic one, so yellow does not tell these apart. Litmus or thymolphthalein turning blue would show that F is an alkali.

A full answer includes

  • (a) E: litmus red.
  • (a) E: methyl orange red.
  • (a) E: thymolphthalein colourless.
  • (b) F: litmus blue.
  • (b) F: thymolphthalein blue.
  • (c) The hydroxide ion, OH−.
  • (d) Methyl orange is yellow in a neutral solution as well as in an alkali, so yellow does not tell them apart.

Also accept

  • red, red, colourless; blue, blue; OH−; yellow in neutral too

Watch for

  • “Thymolphthalein turns blue in an acid.” — Thymolphthalein is blue only in an alkali. In an acid, and in a neutral solution, it is colourless.
  • “Methyl orange is red in an alkali.” — Methyl orange is red in an acid. In an alkali, and in a neutral solution, it is yellow.

3. MK3 · Final check

Model answer. (a) A gas that turns limewater milky is carbon dioxide, so X is a carbonate. (b) An alkaline gas from a base and a warmed solid is ammonia, so X is an ammonium salt. (c) Of the three, sodium carbonate would give no ammonia and ammonium chloride no carbon dioxide: X is ammonium carbonate. (d) ammonium carbonate + hydrochloric acid → ammonium chloride + water + carbon dioxide. (e) A metal and an acid give a salt and hydrogen, which burns with a squeaky pop when a lighted splint is held to it.

A full answer includes

  • (a) Carbon dioxide.
  • (b) Ammonia.
  • (c) Ammonium carbonate.
  • (d) ammonium carbonate + hydrochloric acid → ammonium chloride + water + carbon dioxide.
  • (e) The gas: hydrogen.
  • (e) The test: a lighted splint.
  • (e) The result: a squeaky pop.

Also accept

  • carbon dioxide; ammonia; ammonium carbonate; hydrogen, squeaky pop

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.
  • “The gas that turns red litmus blue is an acid gas.” — Red litmus turning blue shows an alkali: ammonia is an alkaline gas.

4. MK4 · Final check

Model answer. (a) Sodium hydroxide is a metal hydroxide, so it is a base; it dissolves in water to give OH− ions, so it is an alkali too. (b) sodium hydroxide + nitric acid → sodium nitrate + water. (c) H+(aq) + OH−(aq) → H2O(l). (d) Once the OH− ions are used up, each extra drop of acid adds H+ ions that stay in the solution, so the pH falls below 7.

A full answer includes

  • (a) It is a metal hydroxide, so it is a base.
  • (a) It dissolves in water, so it is an alkali.
  • (b) sodium hydroxide + nitric acid → sodium nitrate + water.
  • (c) H+(aq) + OH−(aq) → H2O(l).
  • (d) Below 7: there are H+ ions left over, with no OH− ions to react with them.

Also accept

  • metal hydroxide that dissolves; sodium nitrate + water; H+(aq) + OH−(aq) → H2O(l); below 7

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.
  • “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.

5. MK5 · Final check

Model answer. R has only ions, H+ and X−: it is completely dissociated, a strong acid. S is mostly whole HY molecules with one H+ and one Y−: partially dissociated, a weak acid. At the same concentration R has more H+ ions, so its pH is lower. Row A.

A full answer includes

  • I chose row A.
  • I took R to be the strong acid: it is completely dissociated, with no whole molecules.
  • I took R to have the higher concentration of H+ ions and so the lower pH.
  • I did not choose row D: the same concentration does not mean the same strength.

Also accept

  • Row A

Watch for

  • “A strong acid is a concentrated acid.” — Strong describes how completely the acid dissociates into ions. Concentrated describes how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid concentrated.
  • “Row B.” — S has more particles drawn as whole molecules, but those molecules have not given up their H+. The acid with only ions is the strong one.
  • “Row C.” — The solution with more H+ ions has the lower pH, not the higher.

6. MK6 · Final check

Model answer. (a) Hydrochloric acid is strong, completely dissociated, and would give pH 0.3 at this concentration. The descaler has pH 2.5, so it has a much lower concentration of H+ ions: its acid is only partially dissociated, a weak acid. It is ethanoic acid. (b) HCl(aq) → H+(aq) + Cl−(aq); CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq). (c) The acid gives its H+ ions: it is the proton donor. The carbonate ions take them: they are the proton acceptor, so calcium carbonate acts as a base.

A full answer includes

  • (a) Ethanoic acid.
  • (a) Its pH is higher than that of hydrochloric acid at the same concentration, so it has fewer H+ ions.
  • (a) So it is a weak acid, only partially dissociated.
  • (b) HCl(aq) → H+(aq) + Cl−(aq), with a one-way arrow.
  • (b) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq), with a two-way arrow.
  • (c) The acid is the proton donor.
  • (c) The carbonate ion is the proton acceptor.

Also accept

  • Ethanoic acid, weak; HCl(aq) → H+(aq) + Cl−(aq) and CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq); acid donor, carbonate ion acceptor

Watch for

  • “A strong acid is a concentrated acid.” — Strong describes how completely the acid dissociates into ions. Concentrated describes how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid concentrated.
  • “A weak acid has no H+ ions.” — A weak acid is partially dissociated: a few of its molecules have split into H+ ions and negative ions, so its solution has some H+ ions, just fewer than a strong acid at the same concentration.
  • “The carbonate ion is a proton donor.” — The carbonate ions take H+ ions from the acid: they are the proton acceptor. Only the acid gives protons here.