Acids, bases and indicators — return questions key

1. MR1 · Return

Model answer. Thymolphthalein is blue only in an alkali. Alkalis contain hydroxide ions, OH⁻(aq). In an alkali litmus is blue and methyl orange is yellow, and the pH is above 7.

A full answer includes

• (1) hydroxide (OH⁻)
• (2) blue
• (3) yellow
• (4) above (more than)

Also accept

• OH⁻; blue; yellow; above

Watch for

• “Litmus turns red in an alkali.” — It is the other way round: an acid turns litmus red, and an alkali turns it blue.
• “An alkali contains H⁺ ions.” — Acids contain H⁺ ions. Alkalis contain OH⁻ ions.

2. MR2 · Return

Options as numbered on the pupil sheet: 1 — a salt, water and ammonia; 2 — a salt and hydrogen; 3 — a salt and water; 4 — a salt, water and carbon dioxide.

Model answer. Zinc oxide is a base: base + acid → salt + water. Magnesium carbonate is a carbonate: salt + water + carbon dioxide. Ammonium nitrate is an ammonium salt and sodium hydroxide a base: warmed, they give a salt, water and ammonia. Aluminium is a metal: salt + hydrogen.

A full answer includes

• 1. Zinc oxide + hydrochloric acid: a salt and water.
• 2. Magnesium carbonate + nitric acid: a salt, water and carbon dioxide.
• 3. Ammonium nitrate + sodium hydroxide, warmed: a salt, water and ammonia.
• 4. Aluminium + hydrochloric acid: a salt and hydrogen.

Also accept

• salt + water; salt + water + carbon dioxide; salt + water + ammonia; salt + hydrogen

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.
• “A metal oxide and an acid give hydrogen.” — Hydrogen comes from the metal itself. A metal oxide is a base: it gives a salt and water.

3. MR3 · Return

Model answer. Benzoic acid is weak, so it is only partially dissociated, with a reversible arrow: C₆H₅COOH(aq) ⇌ H⁺(aq) + C₆H₅COO⁻(aq). Like every acid, it gives H⁺ ions, protons, to a base: it is a proton donor. The hydroxide ion takes the protons and becomes water: it is the proton acceptor.

A full answer includes

• (1) ⇌
• (2) H⁺(aq)
• (3) partially
• (4) donor
• (5) acceptor

Also accept

• ⇌; H⁺(aq); partially; donor; acceptor

Watch for

• “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.
• “A weak acid is a proton acceptor.” — Every acid, weak or strong, is a proton donor. Bases are proton acceptors.