Question

Which reactions performed in the experiment involved chemical changes?

crushing calcium carbonate

mixing calcium carbonate and HCl

boiling water

heating copper (II) sulfate pentahydrate

separating iron filing and sulfur

mixing potassium iodide and lead nitrate

combining magnesium and HCl

burning the candle

Answer 1

Chemical changes:
- Mixing calcium carbonate and HCl — chemical reaction: CaCO3 + 2HCl → CaCl2 + CO2 + H2O (effervescence, new products).
- Mixing potassium iodide and lead nitrate — chemical reaction: PbI2(s) precipitates (yellow solid) formed.
- Combining magnesium and HCl — chemical reaction: Mg + 2HCl → MgCl2 + H2 (hydrogen gas evolution, heat).
- Burning the candle — chemical reaction (combustion): wax + O2 → CO2 + H2O (light, heat, new gaseous products).

Not chemical changes (physical changes):
- Crushing calcium carbonate — only a change of size/shape.
- Boiling water — phase change (liquid → gas).
- Heating copper(II) sulfate pentahydrate — typically a physical dehydration (loss of water of crystallization; reversible on rehydration).
- Separating iron filings and sulfur — physical separation of a mixture (no new substances formed).