Acids and alkalis
- Acids produce hydrogen ions (H⁺) in aqueous solution. Examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).
- Alkalis are soluble bases. Their aqueous solutions contain hydroxide ions (OH⁻). Example: sodium hydroxide (NaOH).
- Bases are substances that neutralise acids, such as metal oxides and metal hydroxides.
The pH scale
- The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.
- A neutral solution has pH 7. Acids have pH below 7; alkalis have pH above 7.
- pH can be measured with universal indicator (red for strong acids through green for neutral to purple for strong alkalis) or with a pH probe.
Neutralisation
- In neutralisation, hydrogen ions from the acid react with hydroxide ions from the alkali to make water: H⁺(aq) + OH⁻(aq) → H₂O(l).
- Acid + alkali → salt + water.
Acids and metals
- Acids react with metals above hydrogen in the reactivity series to form a salt and hydrogen.
- Magnesium + hydrochloric acid → magnesium chloride + hydrogen: Mg + 2HCl → MgCl₂ + H₂.
- Test for hydrogen: a lighted splint gives a squeaky pop.
- (Higher) These are redox reactions: the metal atoms lose electrons (oxidised) and the hydrogen ions gain electrons (reduced).
Strong and weak acids (Higher)
- A strong acid is completely ionised in aqueous solution: hydrochloric, nitric and sulfuric acids.
- A weak acid is only partially ionised: ethanoic, citric and carbonic acids.
- As pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10.
- Strong and weak are about how much the acid ionises; concentrated and dilute are about how much acid is in a given volume.
Key terms
- Acid
- A substance that produces H⁺ ions in aqueous solution.
- Alkali
- A soluble base that produces OH⁻ ions in aqueous solution.
- Base
- A substance that neutralises an acid, such as a metal oxide or hydroxide.
- pH scale
- A scale from 0 to 14 that measures acidity or alkalinity.
- Neutralisation
- The reaction between an acid and a base, forming a salt and water.
- Universal indicator
- A mixture of dyes that changes colour with pH.
- Strong acid
- An acid that is completely ionised in water.
- Weak acid
- An acid that is only partially ionised in water.
- Concentrated
- Containing a large amount of solute in a given volume.