---
title: "Water on Earth"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 3
exercises: 11
source: https://one-course.com/books/earth/1/en/chapter/3-water-on-earth
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 3 — Water on Earth

On the 7th of December 1972, three astronauts on their way to the Moon turned their camera back towards the Earth. Their photograph became one of the most famous pictures ever taken. Look at it. You can see white clouds, a little brown and yellow land, the white ice at the bottom — and blue, blue everywhere. Seen from space, our planet is a blue planet.

![The Earth photographed by the crew of Apollo 17, 7 December 1972. We see Africa, Arabia at the top, and the ice of Antarctica at the bottom. Photograph: NASA (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/img-6ea4b7215b88.jpg)

*The Earth photographed by the crew of Apollo 17, 7 December 1972. We see Africa, Arabia at the top, and the ice of Antarctica at the bottom. Photograph: NASA (public domain).*

**You already know.**

Water can be a solid (ice), a liquid, or a gas that we cannot see (water vapour). Liquid water left in the sun slowly turns into vapour: it evaporates. When the vapour cools, it turns back into tiny drops: it condenses. You have met these changes in the physics books. In a [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape), water flows in [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and lies in [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) ([Chapter 2](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#ch-g1-landscapes-around-us)).

## 3.1 The blue planet

Most of the Earth is covered by water. Imagine the whole surface of the Earth cut into 100 equal squares. If we colour in blue every square that is covered by [sea](#def-g2-water-on-earth-ocean), we colour 71 squares. Only 29 squares are land.

![If the Earth’s surface were cut into 100 equal squares, 71 would be sea (blue) and 29 would be land (green). Computed from a map of the whole Earth’s relief (NOAA ETOPO1).](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/fig-0e15949b040a.svg)

*If the Earth’s surface were cut into 100 equal squares, 71 would be [sea](#def-g2-water-on-earth-ocean) (blue) and 29 would be land (green). Computed from a map of the whole Earth’s relief (NOAA ETOPO1).*

The great stretches of [salt water](#def-g2-water-on-earth-salt-fresh) that cover most of the Earth have names.

**Definition 3.1 (Ocean and sea).**

An *ocean* is one of the very large stretches of [salt water](#def-g2-water-on-earth-salt-fresh) that cover most of the Earth and lie between the continents. A *sea* is a smaller stretch of [salt water](#def-g2-water-on-earth-salt-fresh), usually at the edge of an ocean and partly closed in by land.

All the [oceans](#def-g2-water-on-earth-ocean) are joined to one another: a boat could sail from any [ocean](#def-g2-water-on-earth-ocean) to any other without crossing land. Their names, and where they lie on a globe, are in [Chapter 6](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#ch-g3-continents-and-oceans).

![At the edge of the sea. The water goes on, much farther than the eye can see.](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/img-9bda12f15648.jpg)

*At the edge of the [sea](#def-g2-water-on-earth-ocean). The water goes on, much farther than the eye can see.*

## 3.2 Salt water and fresh water

If you have ever swallowed a mouthful of [sea](#def-g2-water-on-earth-ocean) water while swimming, you know: it tastes very salty. The water of [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) does not.

**Definition 3.2 (Salt water and fresh water).**

*Salt water* has a lot of salt dissolved in it: it is the water of the [oceans](#def-g2-water-on-earth-ocean) and the [seas](#def-g2-water-on-earth-ocean). *Fresh water* has very little salt: it is the water of rain, of most [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), of the ice and the snow, and of springs.

**Example 3.3 (How salty is the sea?).**

In a bottle holding one kilogram of [sea](#def-g2-water-on-earth-ocean) water — about one litre — there are about 35 grams of salt. If we let all the water evaporate, the salt stays behind as white crystals at the bottom. That is how [sea](#def-g2-water-on-earth-ocean) salt is made in salt marshes by the [sea](#def-g2-water-on-earth-ocean), where the sun and the wind slowly dry out shallow pools of [sea](#def-g2-water-on-earth-ocean) water.

![](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/img-ea93e632ac93.jpg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/img-196236d0fb41.jpg)

*[Fresh water](#def-g2-water-on-earth-salt-fresh): a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).*

*[Fresh water](#def-g2-water-on-earth-salt-fresh): a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).*

**Remark 3.4 (We cannot drink sea water).**

People and land animals cannot drink [sea](#def-g2-water-on-earth-ocean) water: there is too much salt in it, and drinking it makes the body lose water instead of gaining it. Sailors far out at [sea](#def-g2-water-on-earth-ocean), surrounded by water, have always had to carry their [fresh water](#def-g2-water-on-earth-salt-fresh) with them.

## 3.3 Where is all the water?

[Fresh water](#def-g2-water-on-earth-salt-fresh) is not only in [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). A lot of it is frozen: in the great white ice of Antarctica and Greenland, and on the high [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). And a lot of it is hidden under our feet, in the ground, where it fills the tiny spaces between grains of sand and cracks in the rock.

![High mountains keep fresh water as snow and ice all year long. In summer some of it melts and feeds the streams.](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/img-838aa7e14170.jpg)

*High [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) keep [fresh water](#def-g2-water-on-earth-salt-fresh) as snow and ice all year long. In summer some of it melts and feeds the streams.*

Imagine that we could pour all the water of the Earth into 100 buckets of the same size. How many buckets would each kind of water fill?

![All the water of the Earth in 100 buckets. The rivers, the lakes, the clouds and the water of all living things together would not fill even one bucket. Numbers from the U.S. Geological Survey.](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/fig-b64e2e9265bf.svg)

*All the water of the Earth in 100 buckets. The [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the clouds and the water of all living things together would not fill even one bucket. Numbers from the U.S. Geological Survey.*

**Proposition 3.5 (Most of the Earth’s water is salty).**

Out of 100 buckets of all the water on Earth, 96 are the [salt water](#def-g2-water-on-earth-salt-fresh) of the [oceans](#def-g2-water-on-earth-ocean). The [fresh water](#def-g2-water-on-earth-salt-fresh) we drink is only a very small part of all the water: most of it is frozen in the ice, or hidden under the ground.

## 3.4 Where does the rain go?

When it rains, the water does not stay where it falls. Watch the yard of the school after a shower: some water runs over the ground into the gutters, some soaks into the soil of the flower beds, and puddles slowly disappear as the sun comes back.

![Where the rain goes. The water that rises from the sea and from the land as vapour makes new clouds, and the clouds bring new rain.](https://one-course.com/images/onecourse/chapters/earth-1/g2-water-on-earth/fig-8038109cf703.svg)

*Where the rain goes. The water that rises from the [sea](#def-g2-water-on-earth-ocean) and from the land as vapour makes new clouds, and the clouds bring new rain.*

**Method 3.6 (Following a drop of rain).**

To follow a drop of rain, ask three questions in turn:

1. Where does it fall: on the [sea](#def-g2-water-on-earth-ocean) , on a roof, on the soil, on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) ?
2. Does it soak into the ground, run off over the surface, or freeze into ice?
3. Where does it go next: into a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) , a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) , the [sea](#def-g2-water-on-earth-ocean) , the ground, or back into the air?

In the end, every drop comes back to the [sea](#def-g2-water-on-earth-ocean) or to the air, and the journey starts again.

**Example 3.7 (A drop on the mountain).**

A drop of rain falls as snow on a high [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) in winter. It stays there, frozen, until the warm days of summer. Then it melts, runs down into a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) stream, joins a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), and the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carries it to the [sea](#def-g2-water-on-earth-ocean). One sunny day the drop evaporates from the [sea](#def-g2-water-on-earth-ocean) and rises into the air, where it becomes part of a new cloud.

## 3.5 Exercises

**Exercise 3.1 ★.**

[Salt water](#def-g2-water-on-earth-salt-fresh) or [fresh water](#def-g2-water-on-earth-salt-fresh)? The [sea](#def-g2-water-on-earth-ocean); a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); rain; a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform); an [ocean](#def-g2-water-on-earth-ocean); the ice on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

**Solution of Exercise 3.1.**

[Salt water](#def-g2-water-on-earth-salt-fresh): the [sea](#def-g2-water-on-earth-ocean), the [ocean](#def-g2-water-on-earth-ocean). [Fresh water](#def-g2-water-on-earth-salt-fresh): the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the rain, the [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the ice on the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

**Exercise 3.2 ★.**

Why do we call the Earth the “blue planet”?

**Solution of Exercise 3.2.**

Because most of its surface is covered by the blue water of the [oceans](#def-g2-water-on-earth-ocean): seen from space, it looks blue.

**Exercise 3.3 ★.**

On the grid of 100 squares, 71 squares are blue. How many squares are land?

**Solution of Exercise 3.3.**

$100 - 71 = 29$ squares are land.

**Exercise 3.4 ★.**

Which is bigger, an [ocean](#def-g2-water-on-earth-ocean) or a [sea](#def-g2-water-on-earth-ocean)?

**Solution of Exercise 3.4.**

An [ocean](#def-g2-water-on-earth-ocean) is bigger: a [sea](#def-g2-water-on-earth-ocean) is a smaller stretch of [salt water](#def-g2-water-on-earth-salt-fresh), often at the edge of an [ocean](#def-g2-water-on-earth-ocean).

**Exercise 3.5 ★.**

Can a sailor in the middle of the [ocean](#def-g2-water-on-earth-ocean) drink the water around the boat? Why?

**Solution of Exercise 3.5.**

No: the water of the [ocean](#def-g2-water-on-earth-ocean) is [salt water](#def-g2-water-on-earth-salt-fresh), and drinking it makes the body lose water. The sailor has to carry [fresh water](#def-g2-water-on-earth-salt-fresh).

**Exercise 3.6 ★.**

Name three places where [fresh water](#def-g2-water-on-earth-salt-fresh) is found.

**Solution of Exercise 3.6.**

For example: in [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), in [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), in the ice and snow of the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and of Antarctica, under the ground, in the rain.

**Exercise 3.7 ★★.**

In the picture of the 100 buckets, how many buckets are [salt water](#def-g2-water-on-earth-salt-fresh)? How many are [fresh water](#def-g2-water-on-earth-salt-fresh) (ice and water under the ground together)?

**Solution of Exercise 3.7.**

[Salt water](#def-g2-water-on-earth-salt-fresh): 96 buckets. [Fresh water](#def-g2-water-on-earth-salt-fresh): $2 + 2 = 4$ buckets. (Strictly, part of the water under the ground is salty too; the picture counts it with the [fresh water](#def-g2-water-on-earth-salt-fresh).)

**Exercise 3.8 ★★.**

Put these in order, from the one that holds the most water to the one that holds the least: the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); the [oceans](#def-g2-water-on-earth-ocean); the ice and the snow.

**Solution of Exercise 3.8.**

The [oceans](#def-g2-water-on-earth-ocean) (96 buckets), then the ice and the snow (2 buckets), then the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) (much less than one bucket).

**Exercise 3.9 ★★.**

It rains on a field. Give two things that can happen to the water.

**Solution of Exercise 3.9.**

It can soak into the ground; it can run over the ground into a stream or a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); it can evaporate back into the air when the sun comes out.

**Exercise 3.10 ★★.**

A pan of [sea](#def-g2-water-on-earth-ocean) water is left in the sun for many days, until the water has gone. What is left at the bottom of the pan? Where did the water go?

**Solution of Exercise 3.10.**

Salt is left at the bottom of the pan. The water has evaporated: it has gone into the air as water vapour.

**Exercise 3.11 ★★.**

Tell the journey of a drop of rain that falls on the roof of the school, until it is back in a cloud.

**Solution of Exercise 3.11.**

For example: the drop runs down the roof into the gutter and the drain, then into a stream and a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carries it to the [sea](#def-g2-water-on-earth-ocean); one sunny day it evaporates from the [sea](#def-g2-water-on-earth-ocean), rises into the air and becomes part of a cloud.