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ScienceGrade 8 5 min read

🌊 The Water Cycle & Earth's Oceans: Earth's Great Recycling Machine

Explore how water moves endlessly between oceans, atmosphere, and land through evaporation, condensation, and precipitation, and learn to quantify these flows using simple proportions.

Learning Goals

By the end of this lesson, you will be able to:

✓
Describe the stages of the water cycle and explain the energy transfers driving each stage.
✓
Explain why Earth's oceans store the vast majority of surface water.
✓
Calculate proportions and percentages of water distribution using given data.
✓
Relate evaporation and condensation to observable weather in the local environment.

Before You Begin

Before this lesson, you should know:

→
Understanding that matter exists as solid, liquid, and gas states.
→
Knowing that heat energy causes changes of state (melting, boiling, condensing).
→
Ability to calculate basic percentages and simple fractions.

Notation & Symbols

Key notation and symbols used in this topic:

%

Percent — a proportion expressed out of 100, e.g.

97%

of water is in the oceans

^{\circ} C

Degrees Celsius — the unit of temperature, e.g. water boils at

10 0^{\circ} C

\approx

Approximately equal to — used when a value is rounded

\times

Multiplication symbol, used in scaling quantities of water

\frac{a}{b}

Fraction — a part of a whole, e.g.

\frac{1}{40}

of water is fresh

How Water Moves Around the Planet

The water cycle is the continuous movement of water between the oceans, atmosphere, and land, powered by energy from the Sun and the force of gravity.

Evaporation: The Sun's heat gives liquid water molecules enough energy to escape into the air as water vapour. Most evaporation occurs over the oceans, which cover about $71%$ of Earth's surface.
Condensation: As water vapour rises, it cools and changes back into tiny liquid droplets, forming clouds. This releases heat energy to the surrounding air.
Precipitation: When droplets in clouds grow heavy enough, gravity pulls them down as rain, snow, or hail, returning water to land and sea.
Collection and Runoff: Water on land flows downhill into rivers and lakes, then back to the oceans, completing the cycle. Oceans hold about $97%$ of all Earth's water.

Example

If a puddle contains 500 mL of water and the Sun evaporates

20%

of it in an hour, then

500 \times 0.20 = 100

mL becomes water vapour, leaving 400 mL in the puddle.

100%

The diagram shows the Sun driving evaporation from the ocean, condensation into clouds, precipitation, and runoff returning water to the sea.

Worked Example

Basic

A puddle contains

500 mL

of water. On a sunny day,

20%

of it evaporates. How much water evaporates, and how much remains?

Worked Example

Intermediate

Earth holds about

1.4 \times 1 0^{9} km^{3}

of water, and roughly

97%

of it is salt water in the oceans. How much water is fresh water (not in the oceans)?

Worked Example

Advanced

A region receives

800 mm

of rain per year over an area of

50 km^{2}

. If

35%

of this returns to the atmosphere by evapotranspiration, what volume (in

m^{3}

) stays on the land?

Why Water Cycles Without Being Used Up

Understanding the reasoning behind the rule:

The water cycle conserves mass: the amount of water on Earth stays almost constant, it just changes state and location. Here is why the cycle is a closed loop.

Your Turn

Try this one yourself — use the hint if you get stuck:

A lake holds

2.0 \times 1 0^{6} m^{3}

of water. During a hot month,

8%

evaporates but

1.5 \times 1 0^{5} m^{3}

of rain falls in. What is the net change in the lake's volume?

Watch Out For...

✗

Thinking evaporation only happens from oceans and large lakes.

Evaporation occurs from any exposed water surface — puddles, soil, even leaves (transpiration). Together these form evapotranspiration, which contributes significantly to atmospheric water vapour.

✗

Confusing condensation with precipitation.

Condensation is water vapour changing into liquid droplets (forming clouds). Precipitation is when those droplets become heavy enough to fall as rain, snow, or hail. One leads to the other but they are distinct steps.

✗

Believing the ocean's salt also evaporates into the air.

Only the water (

H_{2} O

) evaporates; dissolved salts are left behind. This is why rain is fresh water and why oceans stay salty.

Extended Practice

Test yourself with a mix of multiple choice and short-answer questions:

Q1.

Which process changes liquid water into water vapour?

MCQ

Q2.

Approximately what fraction of Earth's surface is covered by oceans?

MCQ

Q3.

Explain why rain is fresh even though most of Earth's water is salty ocean water.

When ocean water evaporates, only the water molecules (

H_{2} O

) rise as vapour; the dissolved salts are left behind in the ocean. The vapour condenses into clouds and falls as fresh rain.

Q4.

A puddle holds 500 mL of water. After a sunny day,

\frac{3}{5}

of it has evaporated. How much water remains?

Evaporated:

\frac{3}{5} \times 500 = 300

mL. Remaining:

500 - 300 = 200

mL.

Q5.

Which sequence correctly orders water cycle stages?

MCQ

Extension

Advanced

A harder challenge that connects to the next skill:

A town receives 1200 mm of rainfall per year over a catchment area of

5 km^{2}

. If

40%

of the rain becomes usable runoff collected in a reservoir, what volume of water (in cubic metres) is collected each year?

Key Takeaways

1.The water cycle moves water continuously through evaporation, condensation, precipitation, and collection, driven by energy from the Sun.
2.Oceans cover roughly 71% of Earth's surface and act as the main reservoir that feeds evaporation.
3.Only water molecules evaporate — dissolved salts stay behind, which is why oceans are salty but rain is fresh.
4.Understanding the cycle lets us predict weather, manage reservoirs, and harvest resources like salt.

Printable Cutouts

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Ready to Practice?

Now that you've learned The Water Cycle & Earth's Oceans, put it to the test with a quick quiz!

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