We’ve said gases have lots of space between particles, solids very little. Can we prove it? Two simple activities make the spacing visible.
Activity 7.6 — The squeezable syringe Pull the plunger of a needle-less syringe fully out, seal the tip with your thumb, and push the plunger in. The trapped air compresses — its volume shrinks. Release, and the plunger springs back. So gas particles have lots of empty space between them, which a push can squeeze out. Now try the same with water: it barely moves. Liquids are practically incompressible — their particles are already close, with little space to give.
Why Gases Compress Gases are easily compressed because their particles are far apart with large gaps; pressure simply pushes them closer. Liquids and solids are nearly incompressible because their particles are already packed close together. Spacing order: gas (most) > liquid > solid (least).
A hidden gap in liquids
Activity 7.7 — Sugar that takes up no room Mark the water level (A), add two spoons of sugar (level rises to B), then stir until it dissolves and mark again (C). The final level C is lower than B — the dissolved sugar seems to take up almost no extra room! That’s because the sugar particles slip into the spaces between the water particles. (The volume of the solution is less than water + sugar separately.)
Why Sugar Dissolves but Sand Doesn’t Add sand instead and the level just rises and stays — sand does not dissolve. Its particles are held together by attractions too strong for the water particles to pull apart, so the sand simply settles, taking up its own space. Sugar’s particles, held more weakly, are pulled apart by water and slot into the gaps. Even solids have some space between their tightly-packed particles — just far less than liquids or gases.
Ever Heard Of… How Soap Cleans Oil The particle picture even explains laundry. A soap particle has two ends: one end grips oil, the other mixes with water. So soap particles surround an oily stain, latch on, and let the water lift the whole lot away. Tiny particles, big help.