Acids, Bases, and Salts
This domain tests the properties of acids and bases, the pH scale, indicators, neutralisation, and the preparation and uses of salts.
Core rules
- Acids release hydrogen ions, H+, in water; bases release hydroxide ions, OH−, in water. Alkalis are soluble bases.
- The pH scale commonly runs from 0 to 14. pH below 7 is acidic, 7 is neutral, and above 7 is basic. Each pH unit represents a tenfold change in H+ concentration.
- Indicators such as litmus, methyl orange, and phenolphthalein change colour over particular pH ranges. Universal indicator gives a colour matching a pH value.
- Neutralisation is acid + base → salt + water. The salt depends on the acid and base used. Some salts contain water of crystallisation, such as CuSO4·5H2O.
Worked example
Predict the products of hydrochloric acid reacting with sodium hydroxide. The acid supplies Cl− and the base supplies Na+, so the salt is sodium chloride. The remaining H+ and OH− combine to form water. The equation is HCl + NaOH → NaCl + H2O. If equal amounts react, the resulting solution is neutral, pH 7. If acid is in excess, the solution remains acidic.
Traps that cost marks
Do not say a strong acid is the same as a concentrated acid; strength is the extent of ionisation, concentration is amount per volume. When diluting an acid, add acid to water, not water to acid. Do not assume all bases are alkalis; copper oxide is a base but not soluble. Remember that pH 3 is ten times more acidic than pH 4, not three times.
Key points
- Acids give H+ in water; bases give OH− in water.
- pH below 7 is acidic, 7 neutral, above 7 basic.
- Neutralisation forms a salt and water.
- Strong versus concentrated is a common distinction.