Acids, Bases and Salts
Acids, Bases and Salts
This domain tests acid–base definitions, indicators, pH, neutralisation, salt formation, water of crystallisation and everyday applications.
Core rules
- Acids produce H⁺ (or H₃O⁺) in water; bases produce OH⁻; alkalis are soluble bases.
- pH scale: below 7 acidic, 7 neutral, above 7 basic. Each pH unit is a tenfold change in hydrogen-ion concentration.
- Common indicators: litmus (red in acid, blue in base), methyl orange (red in acid, yellow in base), phenolphthalein (colourless in acid, pink in base).
- Neutralisation: acid + base → salt + water.
- Acid + metal → salt + hydrogen; acid + metal carbonate → salt + water + carbon dioxide.
- Salts may be normal, acidic or basic. Water of crystallisation is fixed water in a crystal, as in CuSO₄·5H₂O.
- Strong acids and bases ionise almost completely; weak ones partly.
Worked example
Hydrochloric acid reacts with sodium hydroxide: HCl + NaOH → NaCl + H₂O. If 0.01 mol HCl is neutralised by 0.01 mol NaOH, the salt formed is sodium chloride. A 0.01 M HCl solution has pH = −log(0.01) = 2, because HCl is a strong acid and dissociates almost completely. The solution is acidic, so blue litmus turns red.
Traps
pH is logarithmic, not linear: pH 2 is ten times more acidic than pH 3. Not every base is an alkali; an alkali must be soluble. Do not write hydrogen as H instead of H₂. Salt solutions are not always neutral: sodium carbonate is basic, ammonium chloride is acidic.