States of matter, melting and boiling points
When a solid lets go
1. General properties: mass, radius and metallic property
2. Ice and iron
Ice melts in your hand. An iron pan stays solid on a hot stove flame.
Pick the right answer
Predict: what decides how high a substance's melting point is?
- A. How strongly its particles hold each other
- B. The size of the piece
- C. How fast you heat it
Explanation
To melt, the particles must partly break free, so the melting point depends on how strongly they hold each other. The size of the piece and how fast you heat it do not change the temperature at which melting starts.
3. Melting point
- Solid → Liquid (melting)
- Liquid → Gas (boiling)
Melting point
The temperature at which a solid changes into a liquid.
Pure ice melts at 0 °C. Below it, water is solid; above it, liquid.
4. Heat the water
Boiling point
The temperature at which a liquid changes into a gas (vapour).
Heat pure water at normal air pressure. Watch its particles.
temperature -30 °C
5. Stronger forces, higher values
| Substance | Melting point (°C) |
|---|---|
| Oxygen | -218 |
| Water (ice) | 0 |
| Aluminum | 660 |
| Table salt | 801 |
To melt or boil, particles must break free from their neighbours.
So the stronger the forces between the particles, the higher the melting and boiling points.
6. Read the numbers
Pick the right answer
Substance A melts at 45 °C. Substance B melts at 1085 °C. Which has stronger forces between its particles?
- A. B
- B. A
- C. You can't tell without the mass of each sample
Explanation
B melts at a much higher temperature, so its particles need far more heating to break free: the forces between them are stronger. A melting point does not depend on the mass of the sample.
Fill in the blanks
A substance melts at 45 °C and boils at 220 °C. At 25 °C, it is a _____ .
Explanation
25 °C is below the melting point (45 °C), so the substance is still solid.
7. Summary
- Solid → Liquid (melting point (water: 0 °C))
- Liquid → Gas (boiling point (water: 100 °C))
- Matter is solid, liquid or gas.
- Melting point: solid → liquid. Boiling point: liquid → gas.
- Stronger forces between particles → higher melting and boiling points.
- Next in Chapter 1: transition metals melt at very high temperatures. Why?
Can you explain why a high melting point means strong forces between particles?