Changes of State
≈ 22 minAdding or removing energy changes the state of a substance, because it changes how strongly the particles are held together.
| Change | From → To | Energy |
|---|---|---|
| Melting | solid → liquid | taken in |
| Freezing (solidifying) | liquid → solid | given out |
| Evaporation | liquid → gas, at the surface, below boiling point | taken in |
| Boiling | liquid → gas, throughout the liquid, at the boiling point | taken in |
| Condensation | gas → liquid | given out |
| Sublimation | solid → gas, skipping the liquid | taken in |
Pure water melts at C and boils at C at sea level. Note that a change of state is a physical change: no new substance is made, and the change can be reversed.
Worked example — reading a heating curve from a table. A learner heats crushed ice and records the temperature every minute:
| Time (min) | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Temp (C) | 0 | 0 | 0 | 4 | 9 |
- Minutes 0–2: the ice warms up from C to C. Still solid.
- Minutes 2–4: the temperature is stuck at C. This is the melting. Ice and water are both present.
- Minutes 4–6: all the ice is gone, so the liquid water now warms up.
The flat part of the table is the fingerprint of a change of state.
Altitude matters. Water boils at C at the coast in Durban, but at roughly C in Johannesburg, 1 753 m above sea level, because the air pushes down on the water less. That is why a pot of samp takes longer to cook on the Highveld — the water is genuinely not as hot.
Drops of water form on the outside of a cold cooldrink bottle taken from the fridge. What change of state has taken place?
What is the name of the change of state in which a solid turns directly into a gas, without becoming a liquid first?
A beaker of crushed ice at C is heated steadily. Halfway through the experiment the beaker holds a mixture of ice and water. What temperature, in degrees Celsius, does the thermometer read at that moment?
A pot of samp takes noticeably longer to cook in Johannesburg (1 753 m above sea level) than in Durban (at the coast). What is the best scientific explanation?