The Water Cycle in Four Stages
The water cycle is the journey water makes, over and over again, between the seas, the air and the land. No new water is made and none is used up: the water that falls as rain in Singapore today may once have been in a river in another country, or in the sea far away. What changes is where the water is and what state it is in. Pupils usually learn the cycle in four stages:
- Evaporation. Heat from the sun warms the water in seas, rivers, lakes and puddles. The water gains heat and changes from a liquid into water vapour, a gas that we cannot see, which rises into the air.
- Condensation. Higher up, the air is cooler. The rising water vapour loses heat and condenses into tiny water droplets. Millions of these droplets together make a cloud.
- Precipitation. The tiny droplets in a cloud join together into bigger, heavier drops. When they are too heavy to float, they fall as rain. Rain, and the snow and hail that fall in colder countries, are all called precipitation.
- Collection. The rainwater flows over the land into drains, streams and rivers, soaks into the ground, or collects in lakes, reservoirs and the sea. From there it can evaporate again, and the cycle carries on.
Heat, and the Three States of Water
The water cycle is really a story about changes of state. Water can be a solid (ice), a liquid (water) or a gas (water vapour). When water gains heat it melts from ice into water, or evaporates from water into water vapour. When it loses heat it condenses from water vapour into water, or freezes from water into ice. In the water cycle, evaporation needs heat from the sun, and condensation happens when the water vapour gives heat away to cooler air or a cold surface. The box under the water cycle diagram lets children tap each change and see whether heat is gained or lost.
Evaporation happens at the surface of the water and at any temperature, not only when water boils. It is faster when the temperature is higher, when it is windy, when the air is less humid and when the water is spread over a larger exposed surface area. That is why clothes dry quickly on a hot, windy day and slowly on a still, damp one.
The Life Cycle of a Flowering Plant
A life cycle is the series of stages a living thing goes through from the start of its life until it can make young of its own. For a flowering plant, the stages on this page are:
| Stage | What happens |
|---|---|
| Seed | A seed holds a tiny young plant and a store of food, protected by the seed coat. |
| Germination | With water, air and warmth, the seed swells, the root grows out first and then the shoot grows upwards. |
| Seedling | The first green leaves open and the young plant begins to make its own food using light. |
| Adult plant | The plant grows taller, with more leaves and roots, until it is able to reproduce. |
| Flowering | The adult plant produces flowers. Pollen is carried between flowers by insects, birds or the wind. |
| Fruit and seed dispersal | After pollination and fertilisation, the flower develops into a fruit with seeds, and the seeds are carried away from the parent plant. |
The Grow it slider shows the whole life cycle as one plant: the seed swelling and splitting, the root and shoot appearing, the seed leaves giving way to true leaves, and finally flowers and fruits. Each seed that lands in a good place can germinate, so the cycle begins again.
Seed Dispersal: Wind, Water, Animals and Splitting
Seeds are dispersed, carried away from the parent plant, so that the young plants do not have to compete with the parent and with each other for space, water and sunlight. The four ways in the primary syllabus are:
- By wind: the seeds or fruits are light, or have wing-like parts (such as the angsana fruit) or hair-like parts (such as the dandelion and the lalang) that let the wind carry them far.
- By water: the fruit can float. The coconut has a fibrous husk that traps air and a waterproof outer layer.
- By animals: juicy, colourful fruits such as the rambutan and the chiku are eaten and the seeds are dropped or passed out in droppings far away; other fruits, like those of the love grass, have hooks or bristles that stick to fur and clothes.
- By splitting: the fruit dries and splits open suddenly, flinging the seeds away, as in the balsam, the rubber tree and the pea pod.
How to Use This Page
- Choose a diagram with the two big buttons: the water cycle or the plant life cycle.
- Explore. Tap each stage of the water cycle to read what happens, whether heat is gained or lost and how the state of water changes. For the plant, tap a stage or move the slider to watch it grow.
- Choose your year and a quiz. Label the diagram by dragging each word to its number, or by tapping a word and then a number. Put the stages in order, or answer true or false. A mixed round does all three.
- Check and read the feedback. Wrong labels and wrong orders are marked in red with a short reason, and anything missed comes back in a fix round. Stars for each quiz and year are shown on the quiz buttons.
- Print a labelling worksheet with a blank diagram, a word bank, an ordering task and true or false questions, followed by an answer page.
At P3 the labelling uses the four main stage names and a shorter set of stages; at P4 there are more labels, such as water vapour and tiny water droplets; at P5 the word bank includes extra words that do not belong, and the ordering uses full sentences that describe each change.
Common Misconceptions
- “Clouds and steam are water vapour.” Water vapour is invisible. Clouds, mist and the white “steam” above a kettle are tiny droplets of liquid water that have already condensed.
- “Water only evaporates when it boils.” A puddle disappears on a warm day without ever boiling. Boiling is fast evaporation throughout the liquid; ordinary evaporation happens at the surface at any temperature.
- “The water droplets on a cold glass leaked through it.” They came from water vapour in the air, which lost heat to the cold glass and condensed.
- “Seeds need light and soil to germinate.” They need water, air and warmth. Light becomes important once the leaves open and the seedling makes its own food.
- “Plants grow from their fruit.” The new plant grows from the seed inside the fruit; the fruit protects the seed and helps it to be dispersed.
Links to the Primary Science Syllabus
Both diagrams belong to the Cycles theme of the Singapore primary science syllabus. The plant life cycle is met first in the lower block, when pupils compare the life cycles of plants and animals, and reappears in the upper block with pollination, fertilisation, seed dispersal and germination in reproduction in plants. The water cycle builds on the study of matter and the three states of water, and links to heat gain and heat loss and to why water is a precious resource that Singapore collects in its reservoirs. The wording on this page, such as evaporation, condensation, water vapour, germination and dispersal by wind, water, animals and splitting, follows the terms pupils are expected to use in their answers.
Tips for Parents and Teachers
- Try it at home. Put a cold can on a table and watch droplets appear on the outside; leave a saucer of water in the sun and another in the shade and compare.
- Grow a bean. Place a green bean or red bean on damp cotton wool and watch the root appear before the shoot. Try one seed with no water, as a fair comparison.
- Collect fruits. On a walk, collect fallen seeds and fruits and sort them by how they are dispersed.
- Insist on the full answer. “The water gains heat from the sun and evaporates into water vapour” earns more marks than “the sun dries the water”.
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This page runs entirely in your browser on any phone, tablet or computer, with no account and no download. Your year, the diagram you chose and your stars are kept only in your own browser; answers are never sent to us.
Frequently Asked Questions
Water in the sea, rivers and lakes gains heat from the sun and evaporates into water vapour. The water vapour rises, loses heat and condenses into tiny water droplets that form clouds. The droplets join and fall as rain (precipitation). The rainwater collects in rivers, reservoirs and the sea, and the cycle starts again.
No. Water vapour is an invisible gas. Clouds are made of millions of tiny droplets of liquid water that form when water vapour loses heat and condenses. The white mist above a kettle is also tiny water droplets, not water vapour.
A seed needs water, air and warmth to germinate. It does not need light or soil, because the young plant uses the food stored in the seed until its first leaves open. That is why seeds can germinate on damp cotton wool in a dark cupboard.
Seeds are dispersed by wind (light seeds, or fruits with wing-like or hair-like parts), by water (fruits that float, such as the coconut), by animals (juicy fruits that are eaten, or fruits with hooks that stick to fur), and by splitting (fruits such as the balsam that burst open and fling their seeds out).