Matter, Water and Air
Primary 3 · Science and Elementary Technology
Matter, Water and Air
Lesson 2 of 3
15 min
Let's begin
It rained hard in the night. In the morning Uwase walks down to the stream with the jerrycan, and stops. The water is brown — the colour of tea with too much milk in it.
There is no other water today, so she fills the jerrycan and carries it home. Her grandmother looks into it and nods slowly.
"We can use this," she says. "But nobody drinks it yet."
Today you will learn every job that water does for us, and exactly how to turn water like Uwase's into water that is safe to drink.
Follow Uwase's family from morning to night and count.
And there are big jobs happening far away from the house: hospitals, factories, farms, and dams where falling water is used to make the electricity that lights Kigali at night.
All of that fits into five words. Water lets us drink, cook, clean, grow and build.
Dirty water is easy to spot. It is brown or cloudy. Bits of soil and grass float in it. It may smell bad, and flies land on top.
But here is the part that catches people out. Water can be perfectly clear and still make you very sick.
Why? Because water can carry germs. Germs are living things so tiny that a thousand of them could sit on the tip of your pencil and you would still see nothing. Your eyes cannot find them. Not ever.
Germs get into water from toilets, from animal dung, and from rubbish that the rain washes downhill into the stream. Swallow them and they grow inside you. Then come the stomach pains, the diarrhoea, and sometimes a very serious illness like typhoid or cholera.
So carve this into your memory:
Clear is not the same as safe. Clear means you can see through it. Safe means the germs are gone. They are two different things.
Worked example
Uwase pours her brown water into a bucket and puts it in a corner. She does not touch it. She does not shake it. She leaves it standing for a few hours and goes to school.
When she comes back, look! There is a layer of brown soil sitting on the bottom of the bucket, and the water above it is much clearer. Very gently, she pours the clear water off the top into a second clean bucket, and leaves the mud behind.
What settling did: the heavy bits of soil are heavier than water, so if you leave the water alone they sink to the bottom, and you can pour the clearer water off the top.
What settling did NOT do: it did not touch the germs. They are still swimming happily in that clearer water.
A filter is anything that lets water through but holds the bits back.
Worked example
the cloth filter. Uwase takes a clean cloth and folds it in half, then in half again, so it is four layers thick. She ties it over the mouth of a clean jerrycan and pours her water through it slowly. When she unties the cloth, she can see the brown dirt caught on it. The water underneath is clearer than before.
Worked example
a bottle filter you can build yourself.
The water that comes out is far clearer than the water you poured in. The stones catch the leaves and the big bits. The sand catches the fine mud. It really works, and you built it yourself.
What filtering did: it took out the bits you can see.
What filtering did NOT do: a germ is far, far smaller than one grain of sand. Germs slip straight between the grains and go through with the water. Your filtered water is clear. It is not yet safe.
Worked example
Uwase pours her clear water into a clean pot and puts it on the fire. She waits. First small bubbles cling to the sides. Then big bubbles come rolling up from the bottom and the whole surface is moving — that is a real boil. She keeps it boiling for one full minute.
Then she lifts the pot off the fire, puts a lid on it, and leaves it to cool.
What boiling did: the heat killed the germs. That is the whole point of this step. Boiling is what turns clear water into safe water. Nothing you did before it could do that job.
But you must protect what you have made. Pour the cooled water into a clean container and keep it covered, so flies and dust stay out. Take it out with a clean cup or a jug. Never dip dirty hands into it — that puts the germs straight back in, and all that work on the fire is wasted.
(Some families use water-treatment drops or tablets from the shop instead of boiling. Those kill germs too. Only an adult should measure them out.)
Skip step 4 and you have beautiful clear water that is full of germs. Do only step 4 on very muddy water and it is safe but horrible to drink. That is why we do all of them, in that order.
And never forget: somebody carried that water home on their head. Every single drop cost work.
Warning
Boiling water is dangerous. The pot is hot, the steam is hot, and both can burn you badly.
Always ask an adult to do the boiling. You can do the settling and the filtering yourself.
Tip
Four words, always in this order: SETTLE — FILTER — BOIL — COVER.
And keep this one for life: clear is not the same as safe. Only boiling kills the germs you cannot see.
Think it through first, then tap Show answer to check.
Name three jobs your family uses water for.
Answer: many answers are right. Drinking, cooking, washing hands, washing clothes, washing plates, bathing, watering the garden, giving water to the goat, mixing mud for bricks.
Keza pours muddy water through a clean cloth. It comes out looking lovely and clear. Can she drink it now?
Answer: no. The cloth caught the bits she could see, but germs are far too tiny to be caught by cloth — they went straight through with the water. She must still boil it.
Why do we let muddy water stand still for a few hours before we filter it?
Answer: so the heavy soil sinks to the bottom. Then she can pour the clearer water off the top, and there is far less mud left for the filter to deal with.
Settling, filtering, boiling — which one of these actually kills the germs?
Answer: boiling. Heat kills germs. Settling and filtering only remove the bits you can see with your eyes.
Uwase boils the water, lets it cool, then dips her dirty hands into the pot to scoop out a cupful. Was that a good idea?
Answer: no! Her hands put germs straight back into the water she just made safe. Keep the water covered, and pour it out with a clean cup.
Note
What you learned today:
Quick check
Put the steps for safe drinking water in order.
Tap the cards in the right order.
