About this experiment
Water is the easiest material to see in all three states: solid ice, liquid water and gas called steam or water vapour. Put a flask of ice at -20 °C on a hot plate and heat it. Watch the thermometer, the ice cubes, the bubbles and the balloon that catches the steam, and zoom in to see what the particles are doing. The heating curve shows something surprising: while ice melts and while water boils, the temperature does not change at all.
Learning goals
- Name the three states of matter and give an example of each
- Describe how particles are arranged and move in a solid, a liquid and a gas
- Use the words melting, freezing, boiling, evaporating and condensing correctly
- Recall that pure water melts at 0 °C and boils at 100 °C at normal air pressure
- Explain that the temperature stays the same while the state is changing
How to use it
- 1Press Start heating. Watch the thermometer rise until it reaches 0 °C. What happens to the ice cubes and to the particles?
- 2Keep heating while the ice melts. Look at the temperature. Does it go up while there is still ice in the flask?
- 3Heat the water to 100 °C. Describe the bubbles. What is inside them?
- 4Watch the balloon as the water boils. Why does it grow, even though nothing is added to the flask?
- 5Switch the hot plate to Cool. Run the film backwards: where does the water vapour go, and when does ice appear again?
- 6Look at the heating curve. Which takes more heat: melting the ice or boiling the water away?
The model
Assumptions
- The flask is closed with a balloon, so no water escapes and the steam stays at normal air pressure.
- All the heat from the hot plate goes into the water. In a real lab some heat is lost to the air and the glass.
- Values used: specific heat 2.1 J/(g·°C) for ice, 4.18 for water and 2.0 for steam; 334 J/g to melt ice and 2257 J/g to boil water.
- The particle picture shows each particle as a small ball. It is a model: real water molecules are far too small to see and there are vastly more of them.
Try this in class
- Q1Ice cream melts in your hand but not in the freezer. What is different?
- Q2A puddle disappears on a sunny day without ever reaching 100 °C. Where did the water go?
- Q3Why do drops of water appear on the outside of a cold glass of juice?
- Q4Is steam the white cloud you see above a kettle? Explain what the cloud really is.
- Q5Why does it take much longer to boil a pan dry than to bring it to the boil?
Frequently asked questions
Why does the temperature stop rising while the ice melts?
In ice the particles are held together in a pattern. Melting means breaking that pattern, and all the heat from the hot plate is used for that job instead of making the particles move faster. Only when all the ice has melted does the temperature start rising again.
What is the difference between boiling and evaporating?
Both turn liquid water into gas. Evaporating happens slowly at the surface at any temperature. Boiling happens all through the liquid at 100 °C, which is why bubbles of steam form at the bottom.
Is steam visible?
No. Real steam, water as a gas, is invisible. The white cloud above a kettle is tiny droplets of liquid water that form when steam cools in the air.
Does water always boil at 100 °C?
Only at normal air pressure at sea level. On a high mountain the air pressure is lower and water boils at a lower temperature, around 70 °C on the top of Mount Everest.