Acids, bases and indicators sheet3-mark answer frameWhat are acids and bases according to Arrhenius? Name three indicators and give their colours in an acid and in a base.
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define an acid. An acid gives H⁺ ions in water: HCl(aq) → H⁺(aq) + Cl⁻(aq).
- 2 · Define a base. A base gives OH⁻ ions in water: NaOH(aq) → Na⁺(aq) + OH⁻(aq). A base that dissolves in water is an alkali.
- 3 · Give the indicators.
- Litmus: red in acid, blue in base
- Phenolphthalein: colourless in acid, pink in base
- Methyl orange: red in acid, yellow in base
- 4 · Real life. A turmeric stain turns red when soap, a base, is rubbed on it.
- 5 · Close in one line. So indicators let us tell acids from bases without tasting them.
Scoring tip: A small table with three columns (indicator, in acid, in base) is the quickest way to give the colours.
