Question
Explain the water cycle with its four main stages. Why does the water cycle never stop? What role do the Sun and oceans play?
(CBSE Class 6-7 Science)
Solution — Step by Step
The Sun heats water in oceans, rivers, lakes, and even puddles. This heat energy converts liquid water into water vapour (gas). The vapour rises up into the atmosphere because it is lighter than air.
About 86% of all evaporation on Earth happens from the oceans. Plants also release water vapour through their leaves — this is called transpiration.
As water vapour rises higher, it cools down. Cool air cannot hold as much moisture. The vapour condenses (changes back to tiny water droplets) around dust particles in the air. These tiny droplets gather together to form clouds.
This is the same process you see when a cold glass of water gets water droplets on the outside — the air near the glass cools, and moisture condenses.
When cloud droplets combine and grow heavy enough, they fall as precipitation — rain, snow, sleet, or hail depending on the temperature. Most precipitation falls back into the oceans, but some falls on land.
Water that falls on land flows into rivers, streams, and lakes (this flow is called runoff). Some water seeps into the ground and becomes groundwater. Eventually, this water reaches the oceans again, and the whole cycle starts over.
Water Cycle Stages Flowchart
flowchart TD
A["Sun heats water bodies"] --> B["Evaporation: liquid to water vapour"]
B --> C["Water vapour rises into atmosphere"]
C --> D["Condensation: vapour to tiny droplets forming clouds"]
D --> E["Precipitation: rain / snow / hail falls"]
E --> F["Collection: rivers, lakes, groundwater"]
F --> G["Water flows back to oceans"]
G --> A
H["Plants release vapour via transpiration"] --> CWhy This Works
The water cycle is driven by solar energy. The Sun provides the heat that evaporates water, and gravity pulls precipitation back down. Since neither the Sun nor gravity will stop anytime soon, the water cycle keeps running continuously.
The total amount of water on Earth stays roughly the same — it just changes form (liquid, gas, solid) and location (ocean, atmosphere, underground). The water you drink today may have once been in a river millions of years ago.
Common Mistake
Many students think clouds are made of water vapour. They are not — clouds are made of tiny liquid water droplets or ice crystals. Water vapour is invisible. The white fluffy cloud you see is already condensed water. This distinction between vapour (invisible gas) and droplets (visible cloud) is a common exam question.
Remember the four stages in order: E-C-P-C (Evaporation, Condensation, Precipitation, Collection). Each stage changes the form or location of water, but never creates or destroys it.