Acids, bases and indicators
An acid gives H⁺ ions in water and a base gives OH⁻ ions (Arrhenius). Indicators such as litmus show which is which by changing colour.
Acids and bases around us
| Where you find it | What it contains | Acid or base |
|---|---|---|
| Lemon | Citric acid and ascorbic acid (vitamin C) | Acid |
| Vinegar | Ethanoic acid (acetic acid) | Acid |
| Tamarind | Tartaric acid | Acid |
| Sour milk, curd | Lactic acid | Acid |
| Tea | Tannic acid | Acid |
| Soft drinks | Carbonic acid, H₂CO₃ | Acid |
| Gastric juice in the stomach | Hydrochloric acid, HCl | Acid |
| Lime water, whitewash | Calcium hydroxide, Ca(OH)₂ | Base |
| Milk of magnesia | Magnesium hydroxide, Mg(OH)₂ | Base |
| Soaps | Made with sodium hydroxide (caustic soda) or potassium hydroxide | Base |
| Hair dyes | Ammonium hydroxide, NH₄OH | Base |
The first tests for acids and bases
Robert Boyle listed these properties in the seventeenth century. Never taste or touch a laboratory acid or base: many are harmful and can burn the skin.
The Arrhenius idea
In the late nineteenth century, the Swedish scientist Svante Arrhenius said that many compounds split into ions when they dissolve in water. An acid gives hydrogen ions (H⁺). A base gives hydroxide ions (OH⁻). These ions give acids and bases their properties. A base that dissolves in water is called an alkali, for example NaOH and KOH. Not every compound with hydrogen is an acid: ethyl alcohol and glucose contain hydrogen but give no H⁺ ions in water.
Indicator colours
| Indicator | In acid | In neutral solution | In base |
|---|---|---|---|
| Litmus (natural, from lichens) | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
| Turmeric (natural) | Yellow | Yellow | Red |
Natural indicators
- Litmus is a natural dye from certain lichens, small plant-like growths on rocks and tree bark. It was the earliest indicator used.
- Turmeric is yellow. A turmeric or curry stain turns red when soap, which is basic, is rubbed on it.
- Phenolphthalein and methyl orange are made in laboratories.
- A universal indicator is a mixture of indicators that shows a different colour at each pH (see the pH topic).
Activity: test household items with litmus
- 1Collect packaged curd, a fruit juice from a tetra pack, soap solution and lime water.
- 2Put a drop of each on blue litmus paper, then on red litmus paper.
- 3★Record the colour changes.Curd and juice turn blue litmus red: acidic. Soap solution and lime water turn red litmus blue: basic
★ key step
Families clean copper vessels with tamarind or lemon, because their acids remove the dull coating. Pickles are kept in glass or ceramic jars, not metal ones, because their acids attack metals.
1 marks: “What colour does phenolphthalein show in a basic solution?” Pink.
2 marks: “Define an acid and a base according to Arrhenius, with one example of each.” Acid: gives H⁺ ions in water, e.g. HCl(aq) → H⁺(aq) + Cl⁻(aq). Base: gives OH⁻ ions in water, e.g. NaOH(aq) → Na⁺(aq) + OH⁻(aq).
2 marks: “A yellow curry stain on a shirt turns red when it is scrubbed with soap. Name the indicator in the stain and explain the change.” Turmeric is a natural indicator. Soap is basic, so turmeric turns red. Rinsing away the soap brings back the yellow colour.
BRA: Blue turns Red in Acid. Red litmus turns blue in a base. Phenolphthalein turns pink only in a base.