Question

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

Answer 1

Chosen example: dissolving sugar in water.

Explanation:
- Law involved: Conservation of mass (mass is neither created nor destroyed in a chemical or physical process in a closed system).
- What happens physically: Sugar crystals separate into individual sugar molecules (or ions for some solutes) that disperse among the water molecules. No atoms are created or destroyed — only their arrangement changes.
- Mass accounting: If you start with a closed container, the total mass before mixing equals the total mass after mixing:
mass(water) + mass(sugar) + mass(container) = mass(solution) + mass(container).
In other words, mass(solution) = mass(water) + mass(sugar).
- Why volume can be misleading: The solution’s volume is not always the arithmetic sum of water volume + sugar volume because molecules fit into the spaces between each other; volume can contract or expand while mass still adds exactly.
- Practical demonstration: Weigh an empty beaker, add a known mass of water and record, then add a known mass of sugar and reweigh. In a closed system (no evaporation, no sugar sticking to something outside the scale), the final mass equals the sum of the separate masses.
- Apparent exceptions: If you observe a mass change it’s usually because the system was not closed — for example, water evaporated, or gas escaped, or some sugar spilled — not because mass was lost or created by the dissolution itself.

Bottom line: Dissolving sugar in water is a physical process that redistributes existing matter; the total mass remains the same when the system is closed.