For this experiment you will need:
- Two clear containers of the same size (We used a pint pot but half of a 2L plastic bottle will probably be just as good and a little safer!)
- A small quantity of potting compost.
- Two seedlings (avoid things like beans which will grow too rapidly, we used Cosmos – it seemed an apt choice for a plant having to serve in the role of all known plant life in the universe!)
- Enough cling-film to cover one container.
- Some water!
What to do:
Fill each container with a couple of inches of compost and plant your seedlings in it. Now you need to add equal amounts of water to each container, use a measuring jug to make this a fair experiment! 50mls of water should be sufficient for a pint pot. One pot should be securely covered with cling-film so that nothing can escape. Leave the other pot open. Now place your seedlings on a sunny windowsill. Watch and observe over the next few weeks.
What happened here?
The water on earth and in the atmosphere is constantly recycled. As the temperature inside the pint pot rises water evaporates from the soil and the vapour rises. The plant transpires, which means it releases excess moisture into the atmosphere also. As rising water vapour comes into contact with the cooler cling-film it condenses. As the water vapour becomes increasingly dense, it forms drops which can no longer be held in the air and they fall down much like rain, where the water is collected in the soil, we call this precipitation. And then the water cycle starts all over again!
But what happened to the pot you didn’t cover? Did it dry out? Did you want to add extra water to keep the seedling alive? The cling-film acts like the colder layers of atmosphere forcing the water vapour, which is a greenhouse gas, to condense and fall back to earth as precipitation. Without this the cycle in the uncovered pot cannot work because the vapour escapes into space!
evaporation (and transpiration)
precipitation – not just rain, can you think of other examples?
Collection – underground, in rivers, reservoirs and the sea.
What was learned?
- That water is continually recycled in the process we call the water cycle.
- The importance of controlling variables to produce a fair test.
- The notion of using a ‘control’ in experiments.
Take it further:
Your plants will not survive for long in either pot, as you can see from the picture above our Cosmos is already outgrowing its planet. There is also the danger that it is too wet for the plant to live and that its roots will rot or it will succumb to disease. You might be able to rescue your seedlings by potting them on into bigger containers with adequate drainage. Don’t forget to keep them watered though!