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This lesson covers the definitions and properties of acids, bases and alkalis as required by the Edexcel GCSE Chemistry specification (1CH0). You need to know the key definitions, be able to identify common acids and alkalis by formula, understand the role of ions in solution, and describe how indicators are used to classify substances.
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. These hydrogen ions are responsible for the characteristic properties of acidic solutions, including sour taste (though you should never taste chemicals in a lab), corrosiveness, and the ability to react with metals and bases.
The hydrogen ions are always present in aqueous solution and can be written as H⁺(aq).
| Acid | Formula | Where It Is Found / Used |
|---|---|---|
| Hydrochloric acid | HCl | Stomach acid; used in the lab |
| Sulfuric acid | H₂SO₄ | Car batteries; industrial manufacturing |
| Nitric acid | HNO₃ | Making fertilisers and explosives |
| Ethanoic acid | CH₃COOH | Vinegar |
| Citric acid | C₆H₈O₇ | Citrus fruits (lemons, oranges) |
| Carbonic acid | H₂CO₃ | Fizzy drinks |
When these acids dissolve in water, they dissociate (split up) to release H⁺ ions. For example:
HCl(aq) → H⁺(aq) + Cl⁻(aq)
H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
Exam Tip: The key definition to remember is that an acid is a substance that produces H⁺ ions in aqueous solution. This is the definition Edexcel uses in mark schemes. Make sure you include the word "aqueous" or write (aq) to show the ions are in solution.
A base is a substance that neutralises an acid. Bases react with acids to form a salt and water. Most bases are metal oxides or metal hydroxides.
| Base | Formula | Type |
|---|---|---|
| Copper oxide | CuO | Metal oxide (insoluble) |
| Zinc oxide | ZnO | Metal oxide (insoluble) |
| Magnesium oxide | MgO | Metal oxide (slightly soluble) |
| Iron(III) oxide | Fe₂O₃ | Metal oxide (insoluble) |
| Sodium hydroxide | NaOH | Metal hydroxide (soluble — an alkali) |
| Calcium hydroxide | Ca(OH)₂ | Metal hydroxide (slightly soluble) |
Not all bases dissolve in water. Those that do dissolve are called alkalis. Those that do not dissolve are simply called bases. Therefore:
All alkalis are bases, but not all bases are alkalis.
An alkali is a soluble base that produces hydroxide ions (OH⁻) when dissolved in water.
| Alkali | Formula | Notes |
|---|---|---|
| Sodium hydroxide | NaOH | Strong alkali; used in soap making and the lab |
| Potassium hydroxide | KOH | Strong alkali; used in alkaline batteries |
| Calcium hydroxide | Ca(OH)₂ | Slightly soluble; used in agriculture to neutralise acidic soil (limewater) |
| Ammonia solution | NH₃(aq) | Weak alkali; used in cleaning products |
When alkalis dissolve, they release OH⁻ ions:
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
KOH(aq) → K⁺(aq) + OH⁻(aq)
Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq)
Exam Tip: A very common exam question asks you to explain the difference between a base and an alkali. A base neutralises an acid. An alkali is a base that is soluble in water and releases OH⁻ ions in solution. Give an example of each: CuO is a base (insoluble), NaOH is an alkali (soluble base).
An indicator is a substance that changes colour depending on whether a solution is acidic, neutral or alkaline. Indicators allow us to classify substances without needing to know their chemical composition.
| Indicator | Colour in Acid | Colour When Neutral | Colour in Alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
| Universal indicator | Red/orange/yellow | Green | Blue/purple |
Litmus tells you whether something is acidic or alkaline, but it does not tell you the strength of the acid or alkali.
Universal indicator is a mixture of several indicators that produces a range of colours corresponding to the pH of the solution. It gives a more precise indication than litmus because it shows a continuous colour change across the pH range.
A pH meter (or pH probe) gives a numerical reading of pH, which is more accurate and precise than universal indicator. It can measure pH to one or two decimal places, making it useful for quantitative work.
Exam Tip: When asked to compare methods for measuring pH, state that a pH meter is more accurate and precise than universal indicator, because it gives a numerical value rather than a colour that must be matched by eye to a colour chart. Universal indicator only gives a whole number estimate.
Acidic solutions share a common set of properties because they all contain H⁺ ions:
Alkaline solutions share common properties because they all contain OH⁻ ions:
When working with acids and alkalis in the laboratory:
Exam Tip: If asked about safety precautions for an experiment involving acids or alkalis, always mention wearing safety goggles (to protect eyes) and washing any splashes with water. For concentrated reagents, mention the corrosive hazard and the need for additional care.