Same particles, different behavior
A substance can usually exist in several states, and which one you see depends on temperature and pressure. Water is the familiar example: ice, liquid water and steam are the same molecules, H2O, behaving differently. The difference lies in how fast the particles are moving and how strongly they attract one another.
The four familiar states
Solids hold their shape and volume. Their particles are packed closely and held in place, though they still vibrate. In crystals, such as salt or ice, the particles sit in an orderly repeating pattern. In amorphous solids, such as glass, the arrangement is disordered but still rigid.
Liquids have a fixed volume but take the shape of their container. The particles are about as close as in a solid but can slide past one another. This is why liquids flow, and why they are very hard to compress.
Gases have neither a fixed shape nor a fixed volume. The particles are far apart compared with their size, move quickly in all directions and mostly fly past each other, interacting only briefly. A gas expands to fill whatever space it has, and it can be compressed easily.
Plasma is a gas that has been energized enough that electrons are stripped from atoms, leaving a mix of charged particles. Because it contains free charges, plasma conducts electricity and responds to magnetic fields. It is rare in everyday life on Earth but extremely common in the universe, since stars are made largely of it. Closer to home, plasma is found in lightning strikes, the glow of neon signs and auroras. Atoms in plasma are broken into the pieces described in the topic of subatomic particles.
Heat and motion
Temperature is a measure of the average kinetic energy of the particles. Adding heat generally makes particles move faster, which can loosen them enough to change state. The coldest possible temperature, absolute zero, is the point at which particle motion is at its minimum, and it cannot be quite reached in practice.
The phase changes
- Melting: solid to liquid. Ice melts at 0 degrees Celsius at normal atmospheric pressure.
- Freezing: liquid to solid.
- Vaporization: liquid to gas. This includes evaporation, which happens at the surface at many temperatures, and boiling, which happens throughout the liquid at the boiling point. Water boils at 100 degrees Celsius at sea level.
- Condensation: gas to liquid, as when water droplets form on a cold glass.
- Sublimation: solid directly to gas. Dry ice, which is solid carbon dioxide, does this at ordinary pressure at about minus 78 degrees Celsius. Snow can also slowly sublimate in cold, dry air.
- Deposition: gas directly to solid, as with frost forming on a cold window.
Each of these changes involves energy moving in or out. Melting and boiling absorb energy, while freezing and condensing release it. This is a simple, concrete case of energy conservation: the energy is not lost, but moved around.
Why the temperature stays flat
When you heat ice at its melting point or water at its boiling point, the temperature holds steady even though heat keeps flowing in. The energy is being used to break the attractions between particles rather than to speed them up. This energy is called latent heat. It is why a pot of boiling water stays near 100 degrees Celsius until it has all turned to steam, and why sweat cools you down: evaporating water takes energy from your skin.
Pressure matters too
Boiling points depend on pressure. At high altitude the air pressure is lower, so water boils at a lower temperature, which is why cooking times can differ in the mountains. A pressure cooker does the reverse, raising pressure so water boils above 100 degrees Celsius and food cooks faster.
Water has an unusual property: ice is less dense than liquid water, so it floats. Most substances are denser as solids. This behavior ties into upward buoyant force and into why lakes freeze from the top down, which matters for life underwater.
Beyond the four
Scientists also describe more exotic states. A Bose-Einstein condensate forms when certain atoms are cooled to extremely close to absolute zero and behave as a single quantum object. Supercritical fluids, at high temperature and pressure, are neither clearly liquid nor gas. These are the territory of specialist labs, and the key point is that the familiar categories are useful simplifications.
Phase changes in the world
The weather depends on them constantly. Evaporation, condensation and freezing drive the water cycle and shape weather systems, with clouds forming as water vapor condenses. Stars go through changes of their own, with matter in extreme conditions as part of the stellar life cycle. Whenever matter shifts from one state to another, the same story applies: particles gaining or losing energy and the attractions between them winning or losing out.