الأسبوع 21 · المحور الثالث «حماية كوكبنا»، الوحدة الثالثة «الموارد الموجودة على سطح الأرض»، المفهوم 3.3 «حماية الموارد الموجودة على سطح الأرض» (ب): الهواء والماء النظيف · 3 دروس
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A filter for cloudy water
Discovery Education Science P5: Term 2, Unit 3, pp. 73–78
GoalBuild and judge a layered water filter, say why filtered water is still not safe to drink, and write a claim, evidence and reasoning about protecting resources.
New words
filter · a tool that holds back bits of dirt while the water passes throughcloudy · not clear, because tiny bits of dirt are mixed inlayer · one flat level lying on top of another
Explanation
Clean water is a limited resource, so people look for ways to clean dirty water. A simple filter can hold back bits of dirt. In a cut plastic bottle turned upside down, a grown-up puts cotton at the neck, crushed charcoal on top of it and clean sand on top of that. Muddy water is poured in at the top. The sand catches the biggest bits, the charcoal catches smaller ones, and the cotton stops the rest. The water that drips out looks clearer. But clear is not the same as clean. Tiny living things and dissolved things can still be in it, so nobody tastes or drinks water from a school filter. To test fairly, make the same muddy water each time, pour the same amount, and change only one thing, for example the number of layers.
Worked examples
Layer (top to bottom)
What it catches
sand
bigger bits
charcoal
smaller bits
cotton
the last fine bits
The layers from top to bottom are sand, then charcoal, then cotton. The order matters: the coarse sand catches big bits first so the finer layers do not clog.
Farida’s team pours 200 mL of muddy water through each filter and measures how much comes out. Filter A gives 150 mL and Filter B gives 120 mL, because B’s cotton is packed tighter. 150 − 120 = 30 mL more from A. A is faster, but they also compare how clear the water is.
Part
What it says
Claim
We protect water by using it wisely and cleaning it.
Evidence
Shorter showers saved 80 L; our filter made muddy water clearer.
Reasoning
Less waste leaves more for everyone, and filtering shows cleaning takes work, so we should not pollute.
A scientist can explain the idea in three parts.
Class activity
Filter test (a grown-up helps). A grown-up cuts a plastic bottle in half and turns the top half upside down to make a funnel. Put cotton in the neck, then charcoal, then sand. Make muddy water by mixing a little soil in a jar of tap water. Pour 200 mL slowly through the filter into a clean cup and compare the water before and after. Write what worked and what could work better. Never taste or drink the water. Do not use water from a pond, canal or drain. Wash your hands afterwards.
Practice
1
Why is filtered water from a school filter still not safe to drink?
It only looks clearer. Tiny living things and dissolved things may still be in it.
2
Which beaker holds the water after filtering?
B, because it is clearer than the muddy water in A.
3
Two filters get 250 mL of muddy water. Filter 1 lets 180 mL through and Filter 2 lets 210 mL through. How much more passes through Filter 2?
Filter
Water through (mL)
1
180
2
210
210 − 180 = 30 mL more.
4
Name one thing to keep the same to make a filter test fair.
Any one: the same muddy water, the same amount poured, the same bottle size.
