A sealed syringe containing only air can have its plunger pushed in, noticeably reducing the volume of air inside. In contrast, a solid metal rod of the same mass cannot be compressed into a smaller volume, no matter how hard it is squeezed. (a) Using the kinetic particle model,…

Answer: ((a)) In a gas, particles are, on average, very far apart, with large empty spaces between them, so pushing the plunger in mainly reduces these empty spaces, bringing particles closer together. In a solid, particles are already packed closely together (touching, in a fixed regular arr… ((b)) The density of the air increases. The mass of air stays the same, but it now occupies a smaller volume, and density = mass ÷ volume, so the same mass packed into a smaller volume gives a greater density. ((c)) In a liquid, the particles are already close together (touching their neighbours), much like in a solid, so there is very little empty space left between them that could be removed by compressing it.. Part (a): Comparing compressibility of a gas and a solid In the syringe, the air particles are, on average, very far apart compared with their own size, with large amounts of empty space between them. Pushing the plunger in mostly squeezes…

IGCSE Physics 0625: Explaining Compressibility with the Particle Model

Part (a): Comparing compressibility of a gas and a solid

In the syringe, the air particles are, on average, very far apart compared with their own size, with large amounts of empty space between them. Pushing the plunger in mostly squeezes out this empty space, bringing the particles closer together. There is plenty of “spare” room to lose, so the volume can shrink a great deal.

In the metal rod, the particles are already held tightly together in a regular, close-packed arrangement, essentially touching their neighbours, with almost no empty space between them. Squeezing the rod cannot push the particles any closer together than they already are, so its volume barely changes.

Part (b): Effect on density

Density is defined as mass divided by volume:

$\rho = \frac{m}{V}$

The mass of air in the syringe stays constant as the plunger is pushed in, but the volume $V$ decreases. Since the same mass now occupies a smaller volume, the density of the air increases.

Part (c): Why liquids resist compression too

Although a liquid’s particles are free to move around and slide past one another (unlike a solid), they remain close together (touching their neighbours, much as in a solid) rather than being spread apart with large empty spaces as in a gas. Because there is very little empty space left between liquid particles, there is very little room to squeeze them any closer together, so liquids, like solids, are very difficult to compress.

Final answers

(a) A gas compresses easily because its particles have large empty spaces between them that can be reduced; a solid’s particles are already packed together with no such space to lose.
(b) Density increases, since the same mass of air now occupies a smaller volume.
(c) Liquid particles, like solid particles, are already close together with very little empty space between them, so there is little room left to compress.