Question

hich 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

Here’s each step classified with a brief reason:

- Crushing calcium carbonate — Physical change. Only particle size changes; no new substance formed.
- Mixing calcium carbonate and HCl — Chemical change. Acid reacts: CaCO3 + 2HCl → CaCl2 + CO2 + H2O (gas evolution, new products).
- Boiling water — Physical change. Phase change (liquid → gas), no new substance.
- Heating copper(II) sulfate pentahydrate (CuSO4·5H2O) — Change of hydration (loss of water). This changes composition (blue hydrated → white anhydrous CuSO4) and is often called a dehydration reaction; some sources treat it as a chemical change because the chemical formula changes, while others treat it as a reversible physical change (removal of water of crystallization).
- Separating iron filings and sulfur — Physical change. Mechanical separation (magnet, sieving, etc.), mixture components remain unchanged.
- Mixing potassium iodide and lead nitrate — Chemical change. Precipitation reaction: Pb(NO3)2 + 2KI → PbI2(s) + 2KNO3 (yellow solid precipitate forms).
- Combining magnesium and HCl — Chemical change. Metal reacts with acid: Mg + 2HCl → MgCl2 + H2 (hydrogen gas released).
- Burning the candle — Chemical change. Combustion of wax produces CO2, H2O, light and heat; substances are chemically transformed.

If you want, I can mark which of these would show obvious signs (bubbling, color change, precipitate, heat) so you can match to your observations.