Copper sulphate crystals are blue but turn white on strong heating, and white Plaster of Paris sets hard when…

Question

Copper sulphate crystals are blue but turn white on strong heating, and white Plaster of Paris sets hard when mixed with water. What common chemical principle underlies both these changes, and in which direction does each process go with respect to water of crystallisation?

Accepted Answer

## Model Answer Both changes involve **water of crystallisation**. - **Copper sulphate** (CuSO₄·5H₂O) is blue. On strong heating, it **loses** water of crystallisation and becomes anhydrous CuSO₄, which is white — water is **removed**. - **Plaster of Paris** (CaSO₄·½H₂O) **gains** water when mixed with water, forming gypsum (CaSO₄·2H₂O), which sets hard — water is **absorbed**. Thus, both processes involve loss or gain of water of crystallisation; they proceed in **opposite directions**. --- ## Explanation The examiner expects students to name "water of crystallisation" as the common principle and clearly state the direction for each case — loss of water on heating (CuSO₄) and gain of water on setting (Plaster of Paris). Mixing them up is a common error. Note: the source passages confirm the concept of water of crystallisation in hydrated salts (e.g., FeSO₄·7H₂O losing water on heating), which underpins both examples here.

Science (086) — AI-generated practice question