The water cycle, also known as the hydroponic cycle, shows the different stages that water on our planet goes through. As the sun shines on water on the ground or in the sea, lakes or rivers, the water heats up and becomes water vapour. This vapour rises until it begins to cool. As the vapour cools, it condenses into water droplets that merge together to form clouds. When the clouds gain too much water, they become heavy and the water is precipitated as rain. The rain falls and either evaporates again or is collected in lakes, rivers and the sea. So our water cycle consists of three main stages --- evaporation, condensation and precipitation, followed by the possible fourth stage --- collection.
The Plastic Baggie Experiment
This easy experiment demonstrates the water cycle in a highly visual way. Place about half a cup of water into a plastic baggy, seal it and tape it to a window. Use a marker pen to mark the water level in the baggie. After the baggie has been left in the sun, the water level will have fallen due to evaporation and droplets of water will have condensed at the top of the baggie. When enough droplets have condensed, they will run together and become heavy enough to fall back into the water below, demonstrating precipitation.
The Colored Water Experiment
In this experiment food colouring is used to create more of a contrast between the original water and the precipitate. Half fill a transparent plastic cup with water and colour the water with a few drops of food colouring. Tape another plastic cup upside down on top of the first to create an enclosed system and leave it on a sunny windowsill. The water will evaporate, condense and precipitate, as in the Plastic Baggie Experiment, but the food colouring will not, remaining only in the original water and not the droplets that have condensed. They will be clear.
The Empty Cup Experiment
This simple experiment can capture children's attention since the empty cup appears to fill by magic! Place a large bowl half full of water in a sunny place with an empty cup in the centre. Cover the bowl with cling film and weight down the centre over the cup with a small pebble. After a period of time, the water will evaporate from the bowl and condense on the cling film. The weight will funnel the droplets together and they will precipitate from the centre of the cling film into the empty cup, filling it with water.
Individual Stage Experiments
If you wish to demonstrate the stages of the water cycle separately, these experiments are ideal. To demonstrate evaporation, boil a kettle and explain that the steam is water that has been heated and become vapour. To demonstrate condensation, simply pour a glass of iced water in a warm room. As water droplets condense on the outside of the glass, explain that these are the water molecules in air cooling down when they come into contact with the cold glass. To demonstrate precipitation, hold up a length of cotton thread and use a pipette to carefully drip water on to it, one drop at a time. The first drop or two will cling to the bottom of the thread but adding more drops will make this droplet too heavy and it will fall, precipitating just as rain does from clouds.