Interaction between particles and states of matter

Water and ice consist of molecules of the same substance. Why does water spread, but a piece of ice holds its shape? Just knowing “particles move” is not enough: you need to find out how they hold each other and whether they can change neighbors.

Why does a piece of chalk neither fall apart by itself nor collapse to a point? Interactions between particles explain both. At short distances, attraction helps hold particles together; when they are pushed too close, repulsion rises sharply. Without attraction, it would be hard to explain why the object holds together. Without resistance to closer packing, its low compressibility would be hard to explain. Both effects matter in a real substance.

A heated metal object can still hold its shape even though its thermal motion has increased. Particle motion therefore need not mean free travel throughout the object. In a solid, particles mainly vibrate about stable positions and do not readily change neighbours. Under small disturbances, the object keeps its shape and volume. This model describes ordinary conditions; a sufficiently large applied force can noticeably deform it.

Must a solid’s particles form neat rows for it to keep its shape? No. A crystal has a repeating spatial arrangement that extends over distances much larger than the spacing between particles. An amorphous solid such as glass lacks this long-range order. Both a crystal and glass can keep their shape. The fact that an object is solid therefore does not tell us its full internal structure.

Pour water from a narrow glass into a wide bowl. The number of particles stays the same, but the shape changes. Liquid particles are close together, making a large reduction in volume difficult. They can, however, move and change neighbours, so the liquid flows. A given amount keeps approximately the same volume and takes the shape of the part of the container it occupies. A small amount of water does not expand to fill an entire large closed bottle as a gas does.

Let's open the partition between the vessel with air and the empty adjacent part. The gas particles moving towards the partition will now fly further, and the gas will occupy the entire available volume. In a sufficiently rarefied gas, the gaps are much larger than the particle sizes; between collisions they move freely. Therefore, gas does not have its own constant shape and volume. This is a model of a rarefied gas: under strong compression, interactions between particles can no longer be neglected as easily.

How can we compare water before and after pouring when the level changes? Measure the space it occupies, not just its height. This quantity is volume V, whose SI unit is the cubic metre, м³. Pouring without spilling leaves the volume almost unchanged, even though the level is lower in a wider container. A gas with the same number of particles fills a larger available container. A change in volume does not necessarily mean that more substance was added.

Seal a syringe’s tip and press the plunger. With air inside, the plunger moves noticeably: large gaps between particles shrink, without the particles themselves needing to squash like sponges. With water, movement is tiny: its particles are already close, and further approach meets strong repulsion. One particle model thus explains the difference in compressibility. School problems often treat liquids and solids as incompressible, although their real volumes do change slightly.

Why does a sealed syringe’s plunger move noticeably with air inside but hardly at all with water?

In air, the distances between particles can be significantly reduced without compressing the particles themselves.

In water, the particles are already close together, and further approach encounters strong resistance.

Air is much more compressible than water.

Shape and compressibility depend not only on the movement of particles, but also on their interaction and the ability to change neighbors.

In a liquid, the particles are close and can rearrange; in a rarefied gas they occupy all available space.

Preservation of shape does not mean immobility of particles or a mandatory crystalline structure.

Next, we will move from particles to whole objects and choose a reference for describing their motion.