Learning objectives
- Explain the four main stages of the water cycle.
- Distinguish evaporation from condensation.
- Connect clouds and rainfall to changes of state.
- Identify practical ways to conserve water.
How water moves through nature
The water cycle is the continuous movement of water between Earth's surface and the atmosphere. Solar energy warms water in oceans, rivers, lakes and wet soil. Some of that liquid water changes into water vapour and rises into the air.
Higher in the atmosphere, the air is cooler. Water vapour loses heat and condenses into tiny droplets. Large groups of droplets form clouds. When droplets combine and become heavy, water returns to the ground as rain, snow or hail.
Collection, runoff and groundwater
After precipitation, some water flows across the surface as runoff and enters streams, rivers and lakes. Some soaks into the soil. Water that moves deeper through soil and rock becomes groundwater.
Plants also return water vapour to the atmosphere through transpiration. Their roots absorb water from soil, and tiny openings in leaves release some of it as vapour.
Why conservation matters
The total amount of water on Earth is large, but only a small part is easily available as clean fresh water. Leaking taps, polluted water bodies and unnecessary use reduce the amount available for people, farms and ecosystems.
Simple actions such as repairing leaks, using a bucket instead of a running hose, collecting rainwater and reusing suitable water can reduce waste.
Read the water cycle as a system
What changes?
Liquid water becomes vapour during evaporation and vapour becomes liquid droplets during condensation. These are physical changes, not the creation of new water.
What drives it?
Solar energy supplies much of the energy for evaporation, while gravity moves precipitation and runoff back toward lower ground and water bodies.
What can vary?
Temperature, wind, vegetation, soil and land use can change how quickly water evaporates, infiltrates or runs off.
Observation task: follow one puddle
After rain, choose a safe puddle or wet patch. Record its approximate size at the same time on two or three occasions. Note sunlight, shade, wind and temperature. The goal is not to obtain perfect measurements; it is to connect evaporation with real conditions.
| Observation | What to record | What it may suggest |
|---|---|---|
| Area becoming smaller | Time and approximate dimensions | Water is leaving the surface, mainly by evaporation or drainage |
| Faster change in sun | Sun/shade condition | More available energy can increase evaporation |
| Water disappears into soil | Surface type | Infiltration moves water below the surface |
Follow one drop of water: a worked example
Imagine a drop of water in a lake on a warm day. Energy from sunlight helps some surface water evaporate into invisible water vapour. As the vapour rises and cools, it can condense onto tiny particles in the air. Many tiny droplets together form a cloud. When droplets grow large enough, they may fall as rain. The water may run into a stream, soak into the soil or be taken up by a plant; it does not necessarily return directly to the lake.
This is a simplified cycle. In nature, runoff, infiltration and transpiration connect several pathways.
Why a covered glass behaves differently
If a little warm water is placed in a transparent container and the inside of its lid is cooler, water vapour can condense as droplets on the lid. This is a small-scale example of a change of state, not a complete model of weather. Ask an adult before handling hot water; ordinary room-temperature observations are sufficient.
Apply the idea
Scenario: After a short shower, one puddle disappears quickly from a paved path, while another remains in the shade. The water has not all been destroyed. Some may have evaporated, some may have flowed away, and some may have entered nearby soil.
Think it through: Why can a puddle disappear even when the day is cloudy?
Evaporation can occur without direct sunshine. Air temperature, humidity and moving air all influence how quickly liquid water enters the air as vapour.
Challenge: Name a route by which rain can become groundwater.
Rainwater infiltrates soil and percolates downward through connected pores and cracks. Some collects in saturated underground layers and can supply wells.
Case study: where did the puddle go?
Observe a puddle at two times of day without touching it. Record sunlight, wind, shade and nearby surfaces. A careful explanation links evaporation rate to conditions; it does not say that water “disappeared.” Ask where vapour could move next and what would make condensation more likely.
This also opens a conservation discussion: the water cycle moves water, but clean, accessible freshwater is not equally available everywhere.
Cycle as a system
Evaporation adds vapour to the air, condensation forms droplets or ice, precipitation returns water, and collection, runoff and groundwater move it through landscapes. The same water can change location and state repeatedly.
Practice questions with explanations
Try each question before opening the answer. The explanation shows the reasoning, not only the final response.
Q1. What provides most of the energy that drives the water cycle?
Answer: The Sun.
Explanation: Solar energy heats surface water and causes evaporation.
Q2. What is evaporation?
Answer: The change of liquid water into water vapour.
Explanation: It occurs when water molecules gain enough energy to escape into the air.
Q3. What is condensation?
Answer: The change of water vapour into liquid droplets.
Explanation: Cooling removes energy from vapour, allowing droplets to form.
Q4. Why do clouds form?
Answer: Water vapour cools and condenses around tiny particles in the air.
Explanation: The resulting droplets or ice crystals gather into visible clouds.
Q5. What is precipitation?
Answer: Water falling from clouds as rain, snow, sleet or hail.
Explanation: It happens when droplets or ice crystals become too heavy to remain suspended.
Q6. What is runoff?
Answer: Water flowing over the land surface.
Explanation: Runoff carries water toward streams, rivers, lakes and oceans.
Q7. How is groundwater formed?
Answer: Water infiltrates soil and moves into spaces in rock.
Explanation: This stored underground water can supply wells and springs.
Q8. What is transpiration?
Answer: The release of water vapour from plant leaves.
Explanation: Plants absorb water through roots and release part of it through leaf openings.
Q9. Why can wet clothes dry on a cloudy day?
Answer: Evaporation can still occur without direct sunshine.
Explanation: Air movement and available heat allow water molecules to escape from the cloth.
Q10. Give two practical ways to conserve water at home.
Answer: Repair leaks and turn off taps while brushing.
Explanation: Both actions stop clean water from being wasted unnecessarily.