Water on Earth: salt water and freshwater
After a heavy shower Meena sees the school tap, the pond and the puddle all full of water. She wonders: can we drink all of it? Not really — most water on Earth is salty.
Most of Earth's water is salt water (oceans and seas) and cannot be drunk or used to grow crops. Only a small part is freshwater — in rain, rivers, ponds, lakes, wells and snow. All land plants, animals and people depend on this small part.
In the figure: If all the water on Earth filled one big bucket, the freshwater would be only about a small cupful of it.
Key point 1: Where freshwater comes from
- Easy : Rain, rivers and ponds are freshwater sources.
- Medium : A well gives freshwater from under the ground.
- Exam-type : A village near a sea still needs rivers, wells or rain, because sea water cannot be drunk.
Key point 2: Why salt water is still useful
- Easy : Sea water gives us salt.
- Medium : In shallow salt pans, sun-heat dries sea water and the salt is left behind.
- Exam-type : Sea water cannot be used for drinking or farming, but it can be used for salt-making, ships and sea food.
Freshwater is limited, so every drop is precious.
Thinking that because Earth is mostly covered with water, there is plenty to drink.
Salt water = oceans and seas. Everything else (rain, river, pond, well) = freshwater. Memory clue: "Sea is Salty" – both start with S.
Spot the difference
| Point | Salt water | Freshwater |
|---|---|---|
| Found in | oceans, seas | rain, rivers, lakes, wells |
| Drinkable? | no | yes (if clean) |
| Share on Earth | very large | very small |
Practice questions
- 1Name two sources of freshwater.
- 2Is sea water fit for drinking?
- 3Name one use of sea water.
- 4Why can farmers not water crops with sea water?
- 5A salt pan is built beside a sea. Why there?
- 6Sort into salt water/freshwater: well, ocean, pond, sea.
- 7A friend says "Earth has so much water that we can waste it." Reply in two lines.
- 8How would you explain the small share of freshwater with a bucket and a cup?
Show answers (try them yourself first)
- 1Any two of rain, river, pond, lake, well.
- 2No. It is too salty.
- 3Making salt (also ships, fishing).
- 4Salt harms most crop plants.
- 5Sea water is right there and the sun dries it to leave salt.
- 6Salt: ocean, sea. Fresh: well, pond.
- 7Most water is salty; freshwater is a very small share, so it must be saved.
- 8Bucket = all water; one cup = freshwater, the rest is salty.
Three forms of water and how it changes
Ramu's mother puts ice in a jug of juice. Soon the jug looks wet outside, and by evening the ice is gone. Water has quietly changed its form.
Water has three forms: solid (ice), liquid (water) and gas (water vapour). Heat makes ice melt into water and water boil or dry into vapour. Cooling makes vapour turn into water drops and water freeze into ice.
In the figure: Heating moves you right: ice to water to vapour. Cooling moves you left.
Key point 1: Heating changes
- Easy : Ice melts to water in a warm room.
- Medium : Wet clothes dry in the sun because the water turns into vapour.
- Exam-type : A boiling pot sends out steam; the water level slowly goes down.
Key point 2: Cooling changes
- Easy : Water in a freezer becomes ice.
- Medium : Drops on the outside of a cold bottle come from vapour in the air turning back to water.
- Exam-type : The mirror in a bathroom turns misty because warm vapour touches the cool glass.
Water is the same substance in all three forms; only its form changes.
Thinking that the drops on a cold glass leak out from inside the glass.
Heat = ice → water → vapour. Cool = vapour → water → ice. Memory clue: "Hot goes up, cool comes down".
Spot the difference
| Form | Example | Can it flow? |
|---|---|---|
| Solid | ice | no, keeps its shape |
| Liquid | rain, tap water | yes |
| Gas | steam, vapour | spreads in the air |
Practice questions
- 1Name the three forms of water.
- 2Which form is steam?
- 3What happens to ice in a warm room?
- 4Why do wet clothes dry in the sun?
- 5Why do drops form outside a glass of cold water?
- 6What is needed to turn water into ice?
- 7Water is boiled in an open pan for a long time. Where does it go?
- 8Is melted ice a new substance? Explain.
Show answers (try them yourself first)
- 1Solid (ice), liquid (water), gas (vapour).
- 2Gas (water vapour).
- 3It melts into water.
- 4The sun's heat turns the water into vapour.
- 5Vapour in the air cools on the cold glass and turns into water.
- 6Cooling (a very cold place).
- 7It changes into vapour and mixes with the air.
- 8No. It is still water, only its form changed.
The water cycle
A wet road dries up an hour after the rain. Where did the water go? It went up into the sky and will come back as rain another day.
The water cycle is the endless journey of water. The Sun heats water in seas, rivers and ponds, which turns into vapour and rises. High up it cools and forms clouds. Clouds give rain, snow or hail, and the water flows back to rivers, lakes and seas.
In the figure: Follow the arrows clockwise: heating, vapour rising, clouds forming, rain falling. Then it starts again.
The water cycle
In the app you can move this figure with your finger.
Open in the appKey point 1: Steps of the cycle
- Easy : The Sun's heat makes water into vapour.
- Medium : Vapour cools high in the air and forms tiny drops; many drops make a cloud.
- Exam-type : Heavy clouds give rain; in very cold places the water falls as snow or hail.
Key point 2: Why it matters
- Easy : Rain refills ponds and rivers.
- Medium : Without the cycle, rivers would dry up and crops would fail.
- Exam-type : No water is lost for ever: the same water is used again and again, only it moves around.
The water cycle is powered by the Sun's heat.
Thinking that clouds are made of smoke, or that rain is "new" water made in the sky.
Heat → Rise → Cool → Fall. Memory clue: "HRCF – Hot Rises, Cool Falls".
Spot the difference
| Step | What happens | Form of water |
|---|---|---|
| Heating | sun warms water | liquid → vapour |
| Cloud making | vapour cools | vapour → drops |
| Rain | drops fall | liquid or ice |
Practice questions
- 1What drives the water cycle?
- 2Name three ways water falls from clouds.
- 3What is a cloud made of?
- 4Where does rainwater finally go?
- 5Why do puddles disappear after rain?
- 6Put in order: cloud forms, sun heats water, rain falls, vapour rises.
- 7If there were no Sun, what would happen to the cycle?
- 8A boy says rain is made new in the sky. Correct him.
Show answers (try them yourself first)
- 1The Sun's heat.
- 2Rain, snow, hail.
- 3Tiny drops of water (and ice crystals).
- 4Into soil, rivers, lakes and seas.
- 5The water turns into vapour in the sun and goes up.
- 6Sun heats water, vapour rises, cloud forms, rain falls.
- 7The water would not turn into vapour, so the cycle would stop.
- 8Rain is old water that went up as vapour and came back.
Groundwater and surface water
Grandfather's village well is full in the rainy season. Where does its water come from? From rain that sank into the ground.
Rain soaks into the soil and sinks through layers of soil and rock. Water stored deep under the ground is groundwater. We draw it with wells, borewells and handpumps. Water on top of the land — rivers, ponds, lakes — is surface water.
In the figure: Rain that soaks in becomes groundwater, and the well takes it out again.
Key point 1: Recharging groundwater
- Easy : Open soil and grass let rain sink in.
- Medium : Ponds, soak pits and more trees help water go underground.
- Exam-type : Cement and concrete cover the soil in cities, so rain runs off and groundwater is not refilled.
Key point 2: How a river flows
- Easy : A river starts on high ground and flows down.
- Medium : Water flows and bends with the shape of the land; it gathers in low places such as lakes.
- Exam-type : Rivers in India end at the Bay of Bengal or the Arabian Sea; the Luni is a rare river that ends in marshy land instead.
Rivers flow from higher ground to lower ground.
Thinking that groundwater is a flowing underground river that never ends.
Rain + open soil = groundwater. Rain + cement = runoff. Memory clue: "Soak, don't soak off".
Spot the difference
| Point | Groundwater | Surface water |
|---|---|---|
| Where | under the ground | on top of the ground |
| Examples | wells, borewells | river, pond, lake |
| Refilled by | rain seeping in | rain and melting snow |
Practice questions
- 1What is groundwater?
- 2Name two ways we draw groundwater.
- 3Name two surface water sources.
- 4Which flows faster to the sea: water on bare rock or on a thick grass field? Why?
- 5Why does a city with many roads have less groundwater?
- 6Name two ways to help rainwater go underground.
- 7A river starts in hills. Which direction does it flow and why?
- 8Your school ground is paved. Suggest a change.
Show answers (try them yourself first)
- 1Water stored deep underground.
- 2Wells, borewells, handpumps (any two).
- 3River, pond, lake (any two).
- 4On bare rock, because grass and soil slow it and soak water in.
- 5Rain cannot soak through the cement, so it runs off.
- 6Ponds, soak pits, trees, open soil (any two).
- 7Downhill, because water always flows from higher to lower ground.
- 8Leave some open soil or grass and plant trees so rain can soak in.
Life in water
At a village pond Asha sees a heron standing still, a fish darting about and a lotus on the water. Each one is built to suit life in or near water.
Animals and plants of water have special features. Fish have fins to swim; some birds have long legs to stand in shallow water; some animals have webbed feet. Water plants may float, be rooted with floating leaves, grow under water or grow along the edge.
In the figure: Water plants are grouped by where their roots and leaves are.
Key point 1: Features that help
- Easy : Fish use fins and a smooth body to swim.
- Medium : A heron's long legs and sharp beak help it catch fish in shallow water.
- Exam-type : A waxy coating makes leaves waterproof, so too much water does not get inside the plant.
Key point 2: Who eats whom
- Easy : Small fish eat tiny plants and insects.
- Medium : A big fish eats small fish; a heron or otter eats the big fish.
- Exam-type : If all the fish vanished, herons and otters would lose food, and water plants might grow too thick.
Every living thing in a pond depends on others for food. This is a food chain.
Thinking all water plants float freely; many are rooted in the bed of the pond.
Match the body part to the job: fins = swim, long legs = wade, webbed feet = paddle. Memory clue: "Fin – swim, Leg – wade, Web – paddle".
Spot the difference
| Plant type | Roots | Example |
|---|---|---|
| Floating | hang free in water | water hyacinth |
| Rooted floating | in the pond bed | lotus, water lily |
| Edge plants | in wet soil at the edge | reeds |
Practice questions
- 1What do fish use to swim?
- 2Name one water bird.
- 3Which type of plant is a lotus?
- 4Why do waxy leaves stay dry?
- 5What may happen to herons if fish die out?
- 6Write a food chain with a small fish, a big fish and a heron.
- 7Why do some birds have long legs?
- 8Why are ponds important for many living things?
Show answers (try them yourself first)
- 1Fins.
- 2Heron (also kingfisher, duck).
- 3Rooted with floating leaves.
- 4The wax stops water from soaking in.
- 5They may go hungry or move away.
- 6Small fish → big fish → heron.
- 7To stand and wade in shallow water to catch food.
- 8They give food, water and shelter to many plants and animals.