Student Worksheet

Activity Instructions

NASA is always on the lookout for earth-like planets that we might be able to colonise in the future. 71% of the Earth’s surface is covered in water. That sounds like a lot, but how much can we actually drink? Use your mathematics skills to determine the actual amount of potable water on Earth, so NASA has a clearer idea of what to be looking for.

Part A: Calculating

Step 1.

Earth has in total 233.2 billion litres of water. Perform the following calculations to get an idea of Earth’s water stores. Remember to list your answer in litres.

		% of Earth’s total water
All of Earth’s water	233,200,000,000 Litres	100%
Only the fresh water	233,200,000,000 x 0.03 =	3%
Frozen fresh water	233,200,000,000 x 0.024 =	2.4%
Water not frozen but polluted or trapped underground	233,200,000,000 x 0.00597 =	0.597%
Usable fresh water	233,200,000,000 x 0.00003 =	0.003%

Step 2.

Round up the usable fresh water to the closest million.

In words, write how much fresh water is available on Earth?

out of 233,200,000,000 Litres

Step 3.

This number can be hard to visualise. The table below has 100 squares, representing 100% of Earth’s water. Colour the squares as suggested below to help visualise the amounts of water.

EARTH’S WATER SUPPLIES

Saltwater = 97 squares

Water frozen in the polar ice caps = 2 ½ squares

Polluted water or water trapped underground = ½ a square

Fresh, drinkable water = A tiny dot or 1/1000 of a square

Part B: Visualising

Step 1.

We can get an idea how much 7 million litres looks like compared to 233.2 billion litres by using a much more manageable scale. Fill your measuring jug with one litre of water from the tap.

Step 2.

From the jug, pour 30 ml into the measuring cup. This represents Earth’s fresh water.

Step 3.

Using your finger, get a single drop of water and place it on the plate. This tiny amount is all the fresh, usable drinking water Earth contains.

Step 4.

Create a letter, containing your diagram, to send to NASA scientists to help them assess suitable planets for human colonisation.

Taking It Further

Using the inspiration of the table you coloured above titled Earth’s Water Supplies, create a poster educating people about how little water we have, and how we should try to reduce usage and recycle our water as much as possible.

Investigate desalination plants on the web. Many countries are looking at building them to turn saltwater into usable water to relieve bad droughts. Do you think they’re a good or bad approach?

Clean Water Calculations

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