All matter is made of particles, and how they are arranged and how fast they move explains the three states, why things change state, and what happens when substances mix.
The particles that make up matter are atoms and molecules. An atom is the smallest particle of an element; a molecule is two or more atoms joined together.
Particles are far too small to see, so the particle model is a model, built to explain what can be observed.
In a solid the particles are closely packed and held in fixed positions, vibrating on the spot, so a solid keeps its shape and volume and is usually the densest of the three states. In most solids that packing is a regular arrangement.
In a liquid the particles still touch but are irregularly arranged and can slide past one another, so a liquid keeps its volume but takes the shape of its container.
In a gas the particles are far apart and move quickly in all directions, so a gas has neither a fixed shape nor a fixed volume, is much less dense, and can be compressed because of the space between particles.
Heating a substance makes its particles move faster, vibrating more in a solid and moving more quickly in a liquid or gas.
It also usually pushes them slightly further apart, so most materials expand when heated. The particles themselves never change size; only their spacing and speed change.
The energy stored by the particles in a material, both their movement and their positions relative to each other, is its internal energy.
Heating a material increases its internal energy. This can make the particles move faster, raising the temperature, and it can also change how far apart they are. During a change of state the energy transferred mainly changes the separation of the particles rather than their speed, so the temperature stays the same.
Melting, freezing, evaporating, condensing and subliming are all changes of state, and can normally be reversed by changing the conditions. A dissolved solute can often be recovered by removing the solvent.
In every case the mass is conserved. Nothing is destroyed when ice melts or sugar dissolves; the particles are simply rearranged, so a sealed container weighs the same before and after.
A physical change alters only the arrangement of the particles: no new substance is made. That, not reversibility, is what defines it. Grinding a rock is a physical change but is not easily undone, and some chemical reactions can be reversed.
A chemical change makes a new substance with different properties, and is usually difficult to reverse: burning, rusting, or a reaction that fizzes or changes colour permanently.
Looking at smoke in air under a microscope, the visible smoke particles jiggle about along random zigzag paths. This is Brownian motion.
It is caused by the much smaller, invisible air molecules colliding with them unevenly from all sides, and it is direct evidence that the particles of a gas are real and constantly moving.
Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration, because of their random motion. It is why a smell fills a room.
It is readily seen in liquids and gases, where the particles are free to move; in a solid it is extremely slow. It is faster in gases than liquids because the particles move faster and further, and faster at higher temperatures for the same reason.
