1Internal energy
A material's particles store energy as they move and as they're held together: its internal energy. Heating gives them more energy, so they move faster and it gets hotter.
internal energy
Lesson 29.4 notes
Energy in matter
Internal energy, expansion, changing state, and why mass is conserved.
2Temperature isn't energy
A bath at 40 °C stores more internal energy than a cup of tea at 80 °C: it has far more particles. Temperature tells you how fast they move on average.
3Expansion
Heat a solid and its particles vibrate more, pushing a little further apart, so it expands. A ball that fits through a ring when cold gets stuck when hot.
expands
4Room to expand
Bridges and railway lines have small gaps so they can expand on hot days without buckling. A thermometer works because its liquid expands as it warms up.
5Changing state
While ice melts it stays at 0 °C: the energy breaks particles out of their pattern instead of speeding them up. While water boils at 100 °C, it pulls them apart into a gas.
6Mass is conserved
When ice melts or salt dissolves, no particles are made or lost, so the total mass stays the same: conservation of mass.
conservation of mass
7Physical or chemical?
Melting, boiling, dissolving and sublimation (solid straight to gas) are physical changes: no new substance forms, and they can be reversed. Burning and rusting are chemical changes: new substances are made.
physical changeschemical changes
Explore: Energy lab
Heat ice until it boils and watch the temperature and the particles. Then heat a metal bar to see it expand, and weigh changes on a balance.
Common mistake: Dissolved salt is still there
Dissolving doesn't make salt disappear. Its particles spread out among the water particles, too small to see, so the mass is the water's plus the salt's.
Remember
- Internal energy is the energy stored by a material's particles. Heating gives them more, so they move faster or break away.
- Most materials expand when heated: their particles move more and push a little further apart. The particles don't get bigger.
- In changes of state and dissolving, no particles are made or lost, so mass is conserved. These physical changes can be reversed.
Going further at GCSE