Evaporation and condensation
Heat salt water until the water evaporates and leaves salt behind, and watch water vapour condense on a cold lid.
Core concept
In short: Heat water to make vapour, then cool the vapour back into drops.
Evaporation turns liquid water into vapour; condensation turns vapour back into liquid
Topic: Matter and its properties
When water is warmed, its particle: A tiny piece that matter is made of, far too small to see. All key words gain energy and some escape from the surface as an invisible gas called water vapour. This is evaporation: When a liquid slowly turns into a gas (vapour), like puddles drying in the sun. All key words. When water vapour touches something cold, it loses energy and turns back into tiny drops of liquid water. This is condensation: When a gas cools and turns back into a liquid, like drops on a cold glass. All key words. dissolve: To mix into a liquid so completely that you can no longer see the pieces. All key words substances such as salt do not evaporate, so they are left behind.
What you will learn
- Describe evaporation and condensation as changes of state.
- Show that dissolved salt is left behind when water evaporates.
- Explain why water droplets form on a cold surface above warm water.
Before you start
- Know that salt dissolves in water. Try "Dissolving: how much can water hold?" first.
Materials
- Salt water (5 g of salt in 30 mL of water)
- Water
- Ice
Equipment
- Evaporating dish
- Beaker with a watch-glass lid
- Hot plate
- Thermometer
Procedure
- Dish A holds 30 mL of salt water. Beaker B holds 50 mL of water with a cold lid of ice on top.
- Heat Dish A and use the timer until all the water has evaporated.
- Look at what is left in Dish A.
- Warm Beaker B and watch the underside of the cold lid.
How this simulation is simplified
- Evaporation times are shortened to a few minutes.
- Water is shown evaporating only while it is heated. In real life it also evaporates slowly at room temperature.
- All the water vapour from Beaker B condenses on the lid and drips back, so its water level does not change.