The Water Cycle
The same water has been going round for billions of years. It never runs out and it never starts anywhere, it just keeps changing state.
The big idea
The water cycle is water moving between the sea, the sky and the land, changing state as it goes.
No new water is made and none is lost anywhere in the loop. The water sitting in a glass on the table has already been sea, cloud, rain and river more times than anyone could count, and it will be all of them again. That is not a poetic way of putting it. It is the literal reason the topic is called a cycle.
Everything here is the changing-state topic applied to one substance at the scale of an ocean. A child who is confident about evaporating and condensing has already done the hard part, and what remains is mostly learning to use the right four words in the right order.
The cycle
The four stages
Evaporation
The sun heats the water in seas, lakes and rivers, and particles at the surface pick up enough energy to break away and leave. What rises is water vapour, an invisible gas. Nothing is boiling and there is nothing to see.
Condensation
The vapour rises, and the higher it goes the colder the air around it becomes, until the particles have lost enough energy to stop flying apart and start clinging together again as tiny droplets of liquid. Billions of those droplets in one place are what we call a cloud.
Precipitation
Inside the cloud droplets keep bumping into each other and merging, and each merge makes them a little heavier, until eventually a droplet is too heavy for the rising air to hold up and it falls. It comes down as rain, or as snow, sleet or hail if it is cold enough on the way.
Collection
Some of that water lands straight back in the sea, some runs down hillsides into rivers that carry it there, and some soaks into the ground. Wherever it ends up, the sun can reach it and start the whole thing again.
What drives it
Ask what powers the water cycle and the answer is the sun, every time. It supplies the energy that lifts water off the sea, and without it nothing else in the loop happens. Gravity handles the return trip for free, pulling rain out of the sky and rivers back downhill.
Sun up, gravity down. Two forces, one loop.
The words that earn marks
A child can describe this whole process correctly, in their own words, and still score almost nothing. Write that the water goes up and comes down again and a mark scheme has nothing to award, because what it is looking for is the name of each stage. Learning the four words is worth more here than any extra detail.
- Water leaving the sea is evaporation, not steam and not boiling.
- Cloud forming is condensation.
- Rain, snow, sleet and hail are all precipitation.
- Water gathering again is collection.
- The gas in the air is water vapour, and it is invisible.
Common mistakes
1. Saying the cycle starts with evaporation
Treating evaporation as the beginning, which is understandable when every diagram in every book starts there. A cycle has no beginning. Evaporation is just a convenient place to start talking, and if a question asks what comes after collection, the answer is evaporation again.
2. Thinking clouds are made of gas
Calling a cloud a gas, usually straight after correctly saying that water vapour rises. But you can see a cloud, and water vapour is invisible, so the two cannot be the same thing. The gas stage happens on the way up. By the time there is anything to look at, it has already condensed into liquid droplets.
3. Thinking the sea should have emptied by now
Wondering why the sea has not run dry after all this time, which is a much better question than it looks. Water leaves the sea and water comes back to it in roughly equal amounts, so nothing is being used up. That balance is exactly why this is a cycle and not a journey.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
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For parents
Build the cycle in a bowl. A little warm water in the bottom, cling film stretched over the top, a small weight in the middle of the film to make it dip, and the whole thing left somewhere sunny. Come back in an hour and there are droplets on the underside of the film, running down to the dip and falling. Evaporation, condensation and precipitation, happening on a kitchen counter. The diagram stops being a picture in a book after that.
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