When water boils
Particles move so fast that they escape as gas.
Heating water
Water in a beaker is heated, with a thermometer in it. (This is done by a teacher or an adult.)
As the water gets hotter, what happens to the movement of its particles?
What this lesson covers
The idea
The temperature at which a liquid boils and turns into vapour at atmospheric pressure is its boiling point; there the particles move so vigorously that they overcome the attractions and escape as gas.
Heating water
Water in a beaker is heated, with a thermometer in it. (This is done by a teacher or an adult.)
As the water gets hotter, what happens to the movement of its particles?
- They move faster and faster
- They slow down
- They stop moving
Heat the water
Heat the water step by step. Watch the thermometer and the particles. At what temperature does it boil?
The boiling point
The temperature at which a liquid boils and turns into vapour at atmospheric pressure is its boiling point. There the particles move so vigorously that they overcome the attractions and escape as gas.
The boiling point of water at atmospheric pressure is 100 °C.
- Water at | Its particles
- 30 °C to 90 °C | Move faster and faster as the water gets hotter
- 100 °C (boiling point) | Move so vigorously that they escape as gas: the water boils
Notes
The boiling point is the temperature at which a liquid boils and turns into vapour at atmospheric pressure. There the particles move so vigorously that they overcome the attractions and escape as gas.
Check yourself
What is the boiling point of a liquid?
At the boiling point, why do particles escape from the liquid?
Water at 30 °C is heated until it just starts to boil. By how many degrees does its temperature rise?
Answer: 70 °C
100 − 30 = 70 °C.
When water boils, what does it turn into?
- The temperature at which it boils and turns into vapour at atmospheric pressure — correct. Yes. For water it is 100 °C.
- The temperature at which it turns into a solid. A liquid turns into a solid on cooling, not at its boiling point.
- The highest temperature a thermometer can show. It is a property of the liquid, not of the thermometer.
- They move so vigorously that they overcome the attractions — correct. Yes. They move apart and escape as gas.
- The attractions between them become stronger. As the particles move apart, the attractions decrease.
- Air pushes them out of the water. No air pushes them. They escape because they move so vigorously.
- Air. It is still water, now as a gas: water vapour.
- Vapour, the gaseous state of water — correct. Yes. The particles escape as gas.
- Ice. Ice forms when water is cooled, not heated.