Grade by Grade
E. Earth and Space Systems · Water Systems

Water on Earth: Where It Is, How Much, and in What State

E2.1 A1.5 A3.2About 45 minutes
Learning goals
  • I can identify the three states of water on Earth's surface and where each is found.
  • I can describe the relative amounts of salt water, fresh water, ice and groundwater on Earth.
  • I can explain the conditions (temperature and pressure) under which water is solid, liquid or gas.
  • I can use percentages and a scale model to communicate how little fresh water is easy to reach.

Words to know

hydrosphere
All the water on, under and above Earth's surface: oceans, lakes, rivers, ice, groundwater and water vapour.
fresh water
Water with very little dissolved salt. People, crops and most land animals need it.
salinity
How much salt is dissolved in water. Ocean water has a salinity of about 35 grams of salt per kilogram of water.
glacier
A large, slow-moving mass of ice on land, made from snow that piled up and pressed into ice over many years.
ice sheet
A glacier so huge it covers a large part of a continent. Earth has two: Antarctica and Greenland.
groundwater
Water that fills the tiny spaces between grains of soil and sand and the cracks in rock underground.
permafrost
Ground that stays frozen for at least two years in a row. It covers much of northern Canada.
water vapour
Water in the gas state. It is invisible.

Remember from Grade 7

In Grade 7 science you traced the water cycle (evaporation, transpiration, condensation, precipitation, runoff) as one of the cycles of matter. In Grade 7 geography you learned that about 97% of Earth's water is salty and that Canada has five ocean drainage basins. This unit goes deeper: how much water is where, why it is in each state, how it moves, and how people use, protect and share it.

A planet that should be called "Ocean"

From space, Earth looks blue. Water covers about 71% of its surface. So why do people in many places run short of water to drink?

The answer is all about where the water is and what state it is in. Most of it is too salty, frozen, or deep underground. In this lesson you will find out just how small the easy-to-reach part is.

Three states of water on Earth's surface

Water is the only common substance found naturally on Earth in all three states.

  • Solid (ice and snow): ice sheets in Antarctica and Greenland, mountain glaciers (like those in the Rockies), ice caps on Arctic islands such as Baffin and Ellesmere Island, sea ice on the Arctic Ocean and Hudson Bay, permafrost, winter snow, and lake ice.
  • Liquid: oceans, lakes, rivers, wetlands, groundwater, water in soil, the tiny droplets that make up most clouds, and the water inside living things.
  • Gas (water vapour): in the air everywhere. You can't see it. Clouds, fog and the "steam" above a kettle are not vapour; they are tiny liquid droplets that formed when vapour cooled.
A large white iceberg floats in calm dark sea water, with mist over the water and snowy rocky mountains behind.
Here water is in all three states at once: solid ice, liquid sea water and water vapour that has condensed into mist. AI-generated image

What decides the state? Temperature (and pressure)

The state of water depends mostly on temperature, and a little on pressure.

  • At normal air pressure, pure water freezes and ice melts at 0 °C.
  • Salt lowers the freezing point. Ocean water freezes at about −2 °C (more exactly about −1.9 °C). That is one reason oceans stay liquid when lakes freeze.
  • Water evaporates at any temperature, but faster when it is warm, windy and dry. It boils at 100 °C at sea level, but at a lower temperature high on a mountain, where the air pressure is lower.
  • Ice can turn straight into vapour (sublimation), which is why snowbanks shrink on cold, sunny, dry days even when it never gets above 0 °C.

So ice is found where it is cold for most of the year: near the poles and high in mountains. Liquid water is found where temperatures are above freezing for at least part of the year. Water vapour is found everywhere, but warm air can hold much more of it than cold air.

solidliquidgas−20020406080100°C0 °C: ice melts, pure water freezes100 °C: boils at sea levelocean water freezes at about −2 °Cboils at about 70 °C on top of Mount EverestEvaporation: liquid → gas at ANY temperature (faster when warm).Sublimation: ice → gas without melting, even below 0 °C.
Water's state depends on temperature. At sea-level air pressure, pure water is solid below 0 °C, liquid from 0 to 100 °C, and boils at 100 °C. Salt lowers the freezing point, and lower air pressure (high on a mountain) lowers the boiling point.
All the water on Earthoceans 96.5%fresh water 2.5% (tiny end of the bar)Zoom in on the fresh water onlyice and snow 68.7%groundwater30.1%lakes, rivers, air, soil and more: 1.2%
How Earth's water is shared (bar lengths drawn to match the percentages). Top bar: about 96.5% of all water is in the oceans; a sliver (about 1%) is other salty water such as salty groundwater and salt lakes; only about 2.5% is fresh. Bottom bar: of that fresh water, about 68.7% is ice and snow (the unfilled part), about 30.1% is groundwater, and only about 1.2% is everything else: lakes, rivers, wetlands, soil moisture, the air and living things. Figures from the U.S. Geological Survey.
Where the fresh water isShare of fresh waterEasy to use?
Ice sheets, glaciers, permanent snowabout 68.7%No: frozen and mostly far from people
Groundwaterabout 30.1%Partly: wells reach shallow groundwater; deep groundwater is hard to reach
Ground ice and permafrost, lakes, soil moisture, the air, wetlands, rivers, living thingsabout 1.2% in totalLakes and rivers are the easiest, but they hold less than half of one percent of all fresh water
Source: U.S. Geological Survey estimates of Earth's water. Numbers are rounded, so they may not add to exactly 100%.

What about Canada?

Canada has about 20% of the world's total fresh water, but only about 7% of the world's renewable fresh water (Environment and Climate Change Canada). Renewable water is the part that is refilled each year by rain and snow and flows away in rivers. Much of the rest sits in lakes and glaciers that were filled thousands of years ago, at the end of the last ice age. If we take it out faster than it is refilled, it runs down like a savings account.

The Great Lakes alone hold about one fifth of the world's fresh surface water (lakes and rivers). And many of Canada's biggest rivers flow north to Hudson Bay and the Arctic Ocean, far from where most Canadians live, near the southern border.

Clouds are not water vapour

A cloud is made of tiny liquid droplets (or ice crystals) that formed when invisible water vapour cooled and condensed. If you can see it, it is not vapour.

Worked example

Let's scale it down: Earth's water in a 1 000 L tank

Imagine all of Earth's water fits in a tank that holds 1 000 L. How much of it is fresh water, how much fresh water is ice, and about how much is in lakes and rivers?

  1. Fresh water is about 2.5% of all water. 2.5% of 1 000 L = 0.025 × 1 000 = 25 L.
  2. Ice and snow are about 68.7% of the fresh water. 0.687 × 25 L ≈ 17 L.
  3. Groundwater is about 30.1% of the fresh water. 0.301 × 25 L ≈ 7.5 L.
  4. Lakes and rivers together are about 0.26% of the fresh water (they are a small part of the 1.2%). 0.0026 × 25 L ≈ 0.065 L = about 65 mL.
  5. Compare: 65 mL is about a quarter of a cup, out of a 1 000 L tank.

Out of 1 000 L, about 25 L is fresh, about 17 L of that is frozen, about 7.5 L is underground, and only about 65 mL (a quarter cup) is in lakes and rivers.

Just for fun

If all of Earth's water fit in a 1 000 L hot tub, every lake and river on the planet would fit in about a quarter cup. That's not even enough for a decent water-balloon fight.

Hands-on activity

Earth's water in a measuring cup (A1.5, A3.2)

You need: a 1 L container (or 4 cups measured with a measuring cup), a measuring spoon set (1 tsp = 5 mL), an ice cube tray, a small sponge, a dropper or straw, a pencil and paper.

  1. Fill the container with 1 000 mL of water. This stands for all the water on Earth.
  2. Take out 5 teaspoons (25 mL). That is all the fresh water. The 975 mL left behind stands for salty water.
  3. From the 25 mL, put just under 3½ teaspoons (about 17 mL) into an ice cube tray and freeze it: ice sheets and glaciers.
  4. Pour about 1½ teaspoons (7.5 mL) onto a sponge: groundwater, hidden in the ground.
  5. What's left should be only a few drops (about 6). Spoons aren't exact, so use a dropper to keep just 6 drops. Those drops stand for all the lakes, rivers, wetlands, soil moisture, clouds and living things on Earth.
  6. Communicate (A1.5): make a labelled diagram or a pie chart of your model. Write two sentences telling a younger child why it is important not to waste or pollute fresh water.

What to look for: the child should notice how tiny the "lakes and rivers" part is compared with the whole container.

Tidy up: use the water afterwards to water a plant.

Your turn

1

Name the three states of water and give one place on Earth where each is found.

2

About what percentage of Earth's water is fresh?

3

Where is most of Earth's fresh water stored?

4

Is a cloud made of water vapour? Explain.

5

Why does the Arctic Ocean freeze at a colder temperature than a lake?

6

What does "renewable fresh water" mean?

7

Name two kinds of ice found in Canada.

More practice: warm-up

1

About what percentage of Earth's surface is covered by water?

2

What is groundwater?

3

At what temperature does pure water freeze at normal air pressure?

4

Which state of water is invisible?

5

Name Earth's two ice sheets.

More practice: on your own

1

Canada has about 20% of the world's fresh water but only about 7% of its renewable fresh water. Explain how both can be true.

2

Why are snowbanks sometimes smaller after a week of cold, sunny weather, even though it never went above 0 °C?

3

Water boils at about 100 °C at sea level. Would it boil at a higher or lower temperature on top of a high mountain? Why?

4

If Earth's water were in a 1 000 L tank, about how many litres would be groundwater?

5

Give two reasons why most of Earth's water is hard for people to use.

Challenge

1

Lakes and rivers hold about 0.26% of Earth's fresh water, and fresh water is about 2.5% of all water. What percentage of ALL Earth's water is in lakes and rivers? Show your work.

2

Many of Canada's largest rivers flow north. Explain why this matters for water use in Canada.

3

A classmate says, "Canada has so much water, we never need to save it." Write a respectful reply using at least two facts from this lesson.

Check your understanding

Quiz

1.

About how much of Earth's water is fresh?

2.

Where is most of Earth's fresh water?

3.

A cloud is made mainly of...

4.

Why does ocean water freeze at about −2 °C instead of 0 °C?

5.

Ice turning straight into water vapour without melting is called...

6.

Canada has about what share of the world's renewable fresh water?

For parents and teachers

Big idea (E2.1): water exists on Earth as solid, liquid and gas. Most is salty; most fresh water is frozen or underground; lakes and rivers are a very small fraction. Temperature (and to a lesser extent pressure and salinity) decides which state water is in. The scale model and diagram build communication skills (A1.5) and connect to percent work in math (A3.2).

How to help: do the measuring-cup model together. While cooking, point out that the "steam" above a pot is really tiny droplets; the vapour is the invisible gap just above the water. On a cold sunny day, notice snow shrinking without melting.

Common difficulties:

  • Thinking clouds and steam are water vapour. Vapour is invisible.
  • Mixing up "percent of all water" and "percent of fresh water". Ask: "percent of what?"
  • Thinking Canada can't run short of water. Total supply and renewable supply are different.

Sources: USGS "Where is Earth's water?"; Environment and Climate Change Canada, Water frequently asked questions (20% total, 7% renewable); U.S. EPA Great Lakes facts.