The pH scale
- Acids have a pH below 7: hydrochloric acid, sulfuric acid, nitric acid, citric acid (in lemons) and ethanoic acid (in vinegar).
- Alkalis have a pH above 7: sodium hydroxide, soap and oven cleaner. Pure water is neutral, with a pH of 7.
- The further from 7, the stronger the acid or alkali. Strong acids and alkalis are corrosive, so wear eye protection.
Indicators
- Indicators change colour depending on pH. Litmus turns red in acids and blue in alkalis.
- Universal indicator shows the pH: red, orange or yellow for acids; green for neutral; blue or purple for alkalis. Red cabbage juice can be used as an indicator too.
- A pH probe (meter) gives a more accurate reading than an indicator.
Neutralisation and salts
- Acid + alkali → salt + water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water.
- The first part of a salt's name comes from the metal; the second part comes from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
- Uses of neutralisation: indigestion tablets neutralise extra stomach acid, and farmers spread lime to neutralise acidic soil.
Acids and metal carbonates
- Acid + metal carbonate → salt + water + carbon dioxide. You see fizzing as the gas is given off.
- For example, hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide.
- Test for carbon dioxide: bubble it through limewater, which turns milky (cloudy).
- To make a salt: warm the acid, add the solid metal carbonate or oxide until no more reacts (excess), filter off the extra solid, then evaporate some water and leave the solution to crystallise. Write methods as numbered steps with equipment, amounts and safety.
Key terms
- Acid
- A substance with a pH below 7.
- Alkali
- A substance that dissolves in water and has a pH above 7.
- Neutral
- Neither acidic nor alkaline; pH 7.
- pH scale
- A scale from 0 to 14 showing how acidic or alkaline a solution is.
- Indicator
- A substance that changes colour depending on pH.
- Universal indicator
- An indicator with a range of colours that show the pH.
- Neutralisation
- The reaction between an acid and an alkali (or base).
- Salt
- A compound made when an acid reacts with a metal compound.
- Corrosive
- Can destroy living tissue and other materials.
- Metal carbonate
- A compound such as calcium carbonate that fizzes with acid.
- Limewater
- A solution that turns milky when carbon dioxide passes through it.