Clean Water: Treatment and a Water-Filter Project – Pure Substances and Mixtures (Senior 1 Chemistry) | Ganzaa

Pure Substances and Mixtures

Lessons

Senior 1 · Chemistry

Pure Substances and Mixtures

5 lessons

Lesson 5 of 5

Clean Water: Treatment and a Water-Filter Project

25 min

From a brown river to a clear tap

The water that comes out of a tap in Kigali may have started its journey as the brown water of a river such as the Nyabarongo. Between the river and the tap, a treatment plant separates out leaves, mud and germs, using the same separation methods you have studied in this unit, but on an enormous scale. In this lesson you will follow that journey, and then design and build a water filter of your own.

Definition

Potable water: water that is safe to drink: clear, with no harmful germs and no harmful amounts of dissolved substances. Potable water is not pure water; it still contains small amounts of dissolved minerals.

How a treatment plant cleans water

Each stage in Figure 5.1 removes a different part of the mixture.

Figure 5.1: The stages of water treatment. Stages 1, 3 and 4 are physical separations; stages 2 and 5 are chemical treatments.

Screening works like sieving: metal bars and mesh catch sticks, leaves and plastic.
Coagulation: the mud particles in river water are too small to settle or be filtered easily. Aluminium sulfate (alum), $\mathrm{Al_2(SO_4)_3}$ , is added. It forms a sticky jelly of aluminium hydroxide, $\mathrm{Al(OH)_3}$ , which gathers the fine particles into larger clumps called flocs.
Sedimentation: the water flows slowly through large tanks; the heavy flocs sink to the bottom and the clearer water flows off the top, like decanting.
Filtration: the water trickles down through beds of sand and gravel, which trap the particles that are left.
Disinfection: chlorine is added. It reacts with water to form hypochlorous acid, which kills bacteria:

$\mathrm{Cl_2(g) + H_2O(l) \rightarrow HCl(aq) + HOCl(aq)}$

Definition

Coagulation: adding a chemical such as alum to water so that tiny suspended particles stick together into larger clumps (flocs) that can settle or be filtered out.

Definition

Sedimentation: letting solid particles in a liquid sink to the bottom under their own weight, so the clearer liquid above can be poured or run off.

Definition

Disinfection: killing the harmful germs in water, for example by adding chlorine or by boiling.

Worked example

Problem. A family treats water at home with a chlorine solution bought at a pharmacy. The label says: 5 cm³ (one capful) for every 20 L of water, then wait 30 minutes before drinking. How much solution do they need for 60 L of water?

Solution. $60\,\mathrm{L}$ is $\frac{60}{20} = 3$ times as much water, so they need three times as much solution: $V = 3 \times 5\,\mathrm{cm^3} = 15\,\mathrm{cm^3}$

Answer: 15 cm³ (three capfuls), then wait 30 minutes.

Warning

Chlorine solution must be used exactly as the label says, stored in its own labelled bottle and kept away from children. Too much makes the water taste bad and can irritate the stomach; too little does not kill the germs.

Choosing a separation method

Table 5.1: Choosing a separation method

Mixture	Method	Why it works
Leaves and sticks in river water	screening (sieving)	the objects are bigger than the holes
Mud in water	coagulation, sedimentation, filtration	the solid is insoluble
Salt in water, to get the salt	evaporation	water turns to vapour, salt does not
Salt in water, to get the water	simple distillation	water boils and is condensed; salt stays behind
Ethanol and water	fractional distillation	the liquids have different boiling points
Dyes in ink or food colouring	paper chromatography	each dye moves a different distance on the paper
Oil and water	separating funnel	immiscible liquids of different density
Iron filings and sand	magnet	only iron is magnetic

Worked example

Problem. A spilled mixture contains sand, salt and iron filings. Plan how to obtain all three separately.

Solution.

Pass a magnet wrapped in paper over the dry mixture to remove the iron filings.
Add water to the rest and stir, so the salt dissolves.
Filter: the sand stays in the filter paper; rinse it with a little water and dry it.
Evaporate the filtrate until crystals appear, then leave it to dry to obtain the salt.

The order matters: if water were added first, the iron filings would get wet and start to rust.

Project: build a water filter from local materials

Aim: design, build and test a filter that makes muddy water as clear as possible, using materials you can find at home or in the market.

You need: a clear 1.5 L plastic bottle, a knife or scissors (with an adult's help), a clean jar, cotton wool or a clean cloth, crushed charcoal (washed), fine sand, coarse sand and small stones (all washed), and muddy water.

Cut the base off the bottle and stand it upside down in the jar.
Push a plug of cotton wool or cloth into the neck.
Add the layers in this order: charcoal, fine sand, coarse sand, small stones (Figure 5.2). Each layer should be about 4 cm deep.
Pour clean water through first to rinse the layers, and throw that water away.
Pour in 500 cm³ of muddy water. Time how long it takes to pass through, and compare the clearness of the water before and after.
Change one thing, such as the order or depth of the layers, and test again. Record your results in a table.

Figure 5.2: A bottle water filter. The coarse layers catch big particles first, so the fine sand does not clog quickly.

Warning

Your filtered water is NOT safe to drink. The filter removes mud, but germs and dissolved substances can pass through. Water for drinking must also be boiled or treated with chlorine. Ask an adult to cut the bottle, and cut away from your body.

Worked example

Problem. Three groups test their filters with the same muddy water. Clearness is judged by how many centimetres down through the water a black mark can still be seen (more is clearer).

Group	Layers from top	Time for 500 cm³	Mark visible through
A	stones, coarse sand, fine sand, charcoal	4 min	18 cm
B	fine sand only	11 min	15 cm
C	charcoal, fine sand, coarse sand, stones	3 min	9 cm

Which design is best? Explain the results.

Solution.

A gives the clearest water (18 cm) in a short time: the coarse layers on top trap big particles first, and the fine sand and charcoal below catch the small ones.
B filters well but slowly, because the fine sand quickly clogs with mud when it has no coarse layers above it.
C is in the wrong order: the big spaces between the stones at the bottom let fine particles through, so the water is least clear.

Answer: design A, with the coarsest layer on top and the finest at the bottom.

Table 5.2: How your project will be assessed

Criterion	Excellent (3)	Good (2)	Needs work (1)
Design	layers in a sensible order, with reasons	layers in order, few reasons	no clear plan
Testing	fair test: same muddy water and volume each time	tested once	not tested
Results	table with time and clearness for each design	some results recorded	no results
Explanation	uses particle size to explain the results	partly explained	no explanation
Safety	states clearly that the water is not safe to drink and why	mentions safety	no safety note

Tip

A filter works from coarse to fine: big spaces at the top catch big particles, small spaces at the bottom catch small ones. A treatment-plant sand filter is built the other way up: the fine sand is on top and the gravel below only supports it, so the sand clogs and must be washed often.

MEL Science: How to make a water filter with sand and charcoal (1 min). Compare their layers with Figure 5.2.

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Practice

Name the stage of water treatment that (a) removes floating leaves, (b) kills germs, (c) lets flocs settle.
What is the job of alum in water treatment?
Which stages of water treatment are physical changes, and which is chemical?
How much chlorine solution is needed for 100 L of water if the dose is 5 cm³ per 20 L?
In a bottle filter, why are the stones placed above the fine sand and not below it?
Plan how to obtain pure water, and dry salt, from salty muddy water.
A neighbour says: "Our filtered water looks perfectly clear, so it is safe to drink." Evaluate this claim.

Show answersHide answers

(a) screening (b) disinfection with chlorine (c) sedimentation.
Alum forms sticky aluminium hydroxide, which makes the tiny mud particles clump together into flocs that can settle or be filtered.
Screening, sedimentation and filtration are physical separations; disinfection with chlorine is a chemical process (coagulation also involves a reaction).
$\frac{100}{20} \times 5\,\mathrm{cm^3} = 25\,\mathrm{cm^3}$ .
The stones catch the big particles first; otherwise the fine sand would clog quickly and the filter would become very slow.
Filter to remove the mud; distil part of the filtrate to collect pure water; evaporate the rest (or the liquid left in the flask) to obtain the salt.
The claim is wrong: clearness shows only that mud has been removed. Germs and dissolved harmful substances can still be present, so the water must be boiled or chlorinated before drinking.

1From a brown river to a clear tap

2How a treatment plant cleans water

3Choosing a separation method

4Project: build a water filter from local materials

From a brown river to a clear tap

How a treatment plant cleans water

Choosing a separation method

Project: build a water filter from local materials