Energy decides the state

The idea

The thermal energy of particles sets their distance and hence the strength of their attractions, so it determines the state: low in solids (only vibrations), enough to overcome fixed positions in liquids, and enough to move freely in gases.

Ice, water and vapour

The same substance, water, can be ice, liquid water or vapour.

What decides which state it is in?

• The thermal (heat) energy of its particles
• The shape of its container
• Its colour

Change the energy

Give the particles more thermal energy, or take some away. Watch how far apart they are and how they move.

Thermal energy and state

The thermal energy of particles sets the distance between them, and so the strength of their attractions. So thermal energy determines the state: it is low in solids (only vibrations); at the melting point it is enough to overcome the fixed positions (liquid); in gases it is enough to move freely in all directions.

• State | Thermal energy and particles
• Solid | Low: close together, strong attractions, only small vibrations
• Liquid | Enough to leave fixed positions: a little farther apart, move within a limited space
• Gas | Enough to overcome the attractions: move freely in all directions

Notes

Thermal energy sets the distance between particles and so the strength of their attractions: it determines the state. Low: solid (only vibrations). More: liquid. Enough to move freely: gas.

Check yourself

In which state do the particles have the lowest thermal energy?

At the melting point, what is the thermal energy used for?

Why do gas particles move freely in all directions?

In general, as particles get more thermal energy, the distance between them...

• Solid — correct. Yes. They stay close and only vibrate.
• Liquid. Liquid particles have enough energy to leave their fixed positions.
• Gas. Gas particles have the most energy. They move freely in all directions.
• To overcome the attractive forces holding particles in fixed positions — correct. Yes. The particles can then move around.
• To make the particles bigger. Thermal energy changes how particles move, not their size.
• To stop the particles moving. More thermal energy makes particles move more, not stop.
• They have the strongest attractions. Attractions are weakest in gases.
• They have enough energy to overcome the attractions between them — correct. Yes. Their attractions become negligible.
• They are held in fixed positions. That is a solid.
• stays the same. The distance changes: it increases as the energy increases.
• decreases, so the attractions get stronger. More energy moves particles farther apart, not closer.
• increases, so the attractions get weaker — correct. Yes. Even a slight increase in distance weakens the attraction a lot.