Question

10. Choose one example and explain how mass is conserved:
• Melting ice
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• Dissolving sugar in water
• Rusting metal
• Cooking food

Answer 1

I'll use rusting metal.

What’s happening: iron reacts with oxygen in air to form iron oxide (rust). Chemical equation (one common form):
4 Fe + 3 O2 → 2 Fe2O3

How mass is conserved:
- The law of conservation of mass says the total mass of reactants equals the total mass of products.
- In the equation above, the mass of the iron plus the mass of the oxygen that combines with it equals the mass of the rust produced. No atoms are created or destroyed — they are just rearranged into new molecules.

Numerical check (approximate atomic/molecular masses):
- 4 Fe ≈ 4 × 55.85 = 223.4 g
- 3 O2 ≈ 3 × 32.00 = 96.0 g
- Total reactants = 319.4 g
- 2 Fe2O3 ≈ 2 × 159.69 = 319.4 g
They match, so mass is conserved.

Practical note about measurements:
- In a closed system (no gases can enter or leave the container), weighing before and after rusting shows no net mass change for the whole system.
- If you only weigh the metal piece in open air, its mass will increase as oxygen from the air becomes part of the iron oxide. Conversely, if bits of rust flake off, the metal piece might lose mass while the total mass of metal + flakes + air remains constant (assuming the air is part of the system).

Bottom line: rusting does not violate conservation of mass — the metal gains the mass of the oxygen it chemically binds, and the total mass of reactants equals the total mass of products.