KCSE 2025 Physics P1 Q5 — Why a Salt Solution's Volume Shrinks
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The Question
“When 2 cm³ of dry salt is dissolved in 10 cm³ of water, the resulting solution measures only about 10.5 cm³ instead of the expected 12 cm³. Explain why the volume of the solution is less than the sum of the two separate volumes.”
Picture the gaps between the water molecules
Matter is made of tiny particles, and in a liquid these particles are not packed together perfectly. The water molecules leave small empty spaces between one another, so a sample of water is not completely solid throughout — there is room in between.
Let the salt particles fill those spaces
When the salt dissolves, it breaks up into very small particles. These particles slot into the gaps that already exist between the water molecules, rather than pushing the water aside. Because they settle into space that was previously empty, they add almost nothing to the overall volume.
Conclude why the total is less than the sum
Since the salt particles occupy the spaces between the water molecules instead of adding fresh volume of their own, the combined solution takes up less room than the 12 cm³ you would get by simply adding 2 cm³ and 10 cm³ together.
Final Result
The salt particles fit into the spaces that already exist between the water molecules, so they take up almost no extra room. That is why the solution measures only about 10.5 cm³ instead of the 12 cm³ expected from adding the volumes.
Why this method works
This is direct evidence for the particulate (kinetic) theory of matter: liquids are made of separate particles with gaps between them, not continuous, gap-free material. When two substances mix and the total volume falls, it shows that particles of one have moved into the empty spaces of the other. It is often compared to mixing a beaker of marbles with a beaker of sand — the sand grains trickle into the gaps between the marbles, so the mixture never reaches the full combined height. Volume is therefore not always conserved on mixing, because volume depends on how the particles pack together, and dissolving lets them pack more efficiently.
The marbles-and-sand analogy confirms the idea: pour sand among marbles and the level sits below the sum of the two, exactly as the salt solution does.