pH, neutralisation and strong and weak acids
The pH scale
The runs from 0 to 14 and shows how acidic or alkaline a solution is:
- pH below 7 — acidic (the lower the pH, the more acidic)
- pH 7 — neutral, e.g. pure water
- pH above 7 — alkaline (the higher the pH, the more alkaline)
pH can be measured with , whose colour is matched against a chart, or more accurately with a pH probe (pH meter).
| Indicator | In acid | In alkali |
|---|---|---|
| Litmus | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
Key terms
Neutralisation
In water, produce and produce hydroxide ions (OH⁻).
In , the hydrogen ions from the acid react with the hydroxide ions from the alkali to make water:
H⁺(aq) + OH⁻(aq) → H₂O(l)
This is the same for every acid–alkali reaction. The other ions stay in solution and form the salt, for example Na⁺ and Cl⁻ in HCl + NaOH → NaCl + H₂O.
Key terms
Strong and weak acids (Higher tier only)
A is completely ionised in water: every molecule splits up into ions. Hydrochloric, nitric and sulfuric acids are strong.
HCl → H⁺ + Cl⁻
A is only partially ionised: most molecules stay whole. Ethanoic, citric and carbonic acids are weak. The ⇌ shows the reaction is reversible:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
At the , a strong acid has more H⁺ ions, so it has a than a weak acid.
Key terms
Concentration and pH (Higher tier only)
Strong and weak describe how much an acid ionises. describe how much acid is dissolved in a given volume. They are different ideas: you can have a dilute strong acid or a concentrated weak acid.
The pH scale is based on factors of 10. As the pH goes down by 1, the concentration of .
Example: a solution of pH 3 has 10 times the H⁺ concentration of pH 4, and 100 times (10 × 10) that of pH 5.