Acids, Bases and Salts: notes

Acids and Alkalis

• Acids: sour substances that give hydrogen ions in water. Hydrochloric acid, HCl, is an example.
• Alkalis: soluble bases that give hydroxide ions in water. Sodium hydroxide is an example.
• Neutralisation: acid plus base gives salt plus water. HCl plus NaOH gives sodium chloride plus water.

Indicators and pH

• pH scale: runs from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is alkaline.
• Litmus: turns red in acid and blue in alkali.

Strong and Weak Acids

• Strong acids: ionise fully in water, for example H2SO4.
• Weak acids: ionise only partly, for example ethanoic acid in vinegar.

Oxides

• Acidic oxides: nonmetal oxides that form acids in water. Sulfur dioxide is an example.
• Basic oxides: metal oxides that react with acids. Copper oxide plus sulfuric acid gives copper sulfate plus water.

Preparing Salts

• Soluble salts: add excess solid base to an acid, filter, then evaporate. Copper sulfate is made this way.
• Insoluble salts: mix two solutions, filter, then wash and dry the precipitate. Lead iodide is an example.
• Titration: finds exact volumes needed for neutralisation, using a pipette, burette and indicator.

Tests

• Carbonates: add dilute acid. The gas turns limewater milky.
• Chloride ions: add dilute nitric acid and silver nitrate. A white precipitate forms.
• Sulfate ions: add dilute hydrochloric acid and barium chloride. A white precipitate forms.
• Hydrogen: a lit splint gives a squeaky pop.

Fertilisers

• Nitrogen fertilisers: such as ammonium nitrate, help leaves grow.
• Lime: calcium hydroxide neutralises acidic soil.