Acids, Bases and SaltsAQA GCSE Chemistry: Revision notes
Section 1
How Do Acids React With Metals?
Acids react with some metals to produce a salt and hydrogen gas:
metal + acid → salt + hydrogen
At GCSE this is limited to the reactions of magnesium, zinc and iron with hydrochloric acid and sulfuric acid. The more reactive the metal, the faster the reaction (more vigorous fizzing).
Magnesium + hydrochloric acid → magnesium chloride + hydrogen
Section 2
How Do Acids React With Alkalis, Bases and Carbonates?
Acids are neutralised by different substances, each producing different products:
| Reacts with | Products |
|---|---|
| Alkalis (soluble metal hydroxides) | Salt + water |
| Bases (insoluble metal hydroxides/oxides) | Salt + water |
| Metal carbonates | Salt + water + carbon dioxide |
The salt produced depends on both the metal (or base) used and the acid:
- Hydrochloric acid → chloride salts
- Nitric acid → nitrate salts
- Sulfuric acid → sulfate salts
Formulae of common ions can be used to work out the formula of the salt produced.
Match the acid to its salt ending: hydrochloric → chloride, nitric → nitrate, sulfuric → sulfate.
Section 3
How Do We Make a Pure, Dry Sample of a Soluble Salt?
To make a pure, dry sample of a soluble salt from an insoluble oxide or carbonate:
- Add the insoluble oxide/carbonate to the acid in small amounts, stirring, until it is in excess (no more reacts)
- Filter to remove the unreacted excess solid, leaving the salt solution
- Gently heat/evaporate the solution to reduce its volume, forming a saturated solution
- Leave the saturated solution to cool so that the salt crystallises; filter off and dry the crystals
Section 4
What Makes a Solution Acidic or Alkaline?
- Acids produce hydrogen ions (H⁺) when dissolved in water
- Alkalis produce hydroxide ions (OH⁻) when dissolved in water
- The pH scale (0–14) measures how acidic or alkaline a solution is: pH < 7 is acidic, pH = 7 is neutral, pH > 7 is alkaline
- pH can be measured using universal indicator (colour change) or a pH probe (numerical reading)
The neutralisation reaction itself can be written as an ionic equation:
H⁺(aq) + OH⁻(aq) → H₂O(l)
Section 5
How Do We Find Reacting Volumes Precisely?
Titrations are used to find the volume of acid or alkali needed to exactly neutralise a given volume of the other. At GCSE, titrations are limited to strong acids and strong alkalis only (hydrochloric, sulfuric and nitric acids).
(HT only) From titration results, calculate quantities in mol/dm³ and g/dm³ using the volumes and concentrations involved.
(HT only) Strong acids (hydrochloric, nitric, sulfuric) are fully ionised in water, while weak acids (ethanoic, citric, carbonic) are only partially ionised. As pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10.
Strong/weak refers to how fully an acid ionises, not to its concentration — a dilute strong acid can still fully ionise, and a concentrated weak acid still only partially ionises.
Must Know
- Metal + acid → salt + hydrogen (Mg, Zn, Fe with HCl or H₂SO₄)
- Alkali/base + acid → salt + water; carbonate + acid → salt + water + carbon dioxide
- Salt naming: hydrochloric → chloride, nitric → nitrate, sulfuric → sulfate
- Making a pure soluble salt from an insoluble base: react in excess, filter, evaporate, crystallise
- Acids give H⁺ ions, alkalis give OH⁻ ions; pH 0–14 measured by indicator or probe; H⁺(aq) + OH⁻(aq) → H₂O(l)
- (HT) Strong acids are fully ionised, weak acids partially ionised; a pH drop of 1 means [H⁺] increases ×10
That's the notes covered.
Carry on to the next subtopic.