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Grade 6 Beginner About 15 min Version 1 Awaiting review Premium

Evaporation and condensation

When water is warmed, its particles gain energy and some escape from the surface as an invisible gas called water vapour. This is evaporation. When water vapour touches something cold, it loses energy and turns back into tiny drops of liquid water. This is condensation. Dissolved 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

  1. Dish A holds 30 mL of salt water. Beaker B holds 50 mL of water with a cold lid of ice on top.
  2. Heat Dish A and use the timer until all the water has evaporated.
  3. Look at what is left in Dish A.
  4. 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.