Water of crystallisation
Some salts contain water of crystallisation (they are hydrated). Heating drives this water off, changing the colour:
Key idea / 要点
CuSO4·5H2O(s) → CuSO4(s) + 5H2O(g)
Blue hydrated crystals turn to white anhydrous powder on heating.
Carbonates
Most metal carbonates decompose to the metal oxide and carbon dioxide on heating:
CuCO3(s) → CuO(s) + CO2(g) (green to black)
Sodium and potassium carbonates are very stable and are not decomposed by a Bunsen flame.
Nitrates
- Sodium and potassium nitrates give the nitrite + oxygen: 2KNO3(s) → 2KNO2(s) + O2(g)
- Other metal nitrates give metal oxide + brown NO2 + oxygen: 2Cu(NO3)2(s) → 2CuO(s) + 4NO2(g) + O2(g)
Worked example
A green salt is heated and turns black, releasing a gas that turns limewater milky. Identify the salt and gas.
Answer: The salt is copper(II) carbonate, CuCO3; the gas is carbon dioxide, CO2: CuCO3 → CuO + CO2.
The pattern of thermal stability follows the reactivity of the metal. Salts of very reactive metals in Group 1 hold onto their carbonate and nitrate strongly, so they resist decomposition, whereas salts of less reactive metals such as copper and zinc break down easily on gentle heating. Observing the colour of the residue and testing any gas released (limewater for CO2, a glowing splint for O2, colour and smell for NO2) helps identify the salt.
Remember
- Hydrated salt heated ⇒ loses water of crystallisation (colour change).
- Most carbonates ⇒ oxide + CO2; Na/K carbonates are stable.
- Nitrates give O2; most also give brown NO2.