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Class 8 · Science and environment

Is clear water safe to drink?

A cloth filter turns muddy water clear. Is it now safe to drink?

What you will learn: Explain why clear water may still be unsafe, and what tests and treatment check.

Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓

Jump to: Check yourself · For grown-ups · For teachers

Safety first

Never drink or taste it. Wash hands after.

Keep it safe:

  • Never drink or taste any water in this activity.
  • Wash hands with soap afterwards.
  • A grown-up pours the water away, onto soil or down a drain, and washes the cups.

You will need

  • Two clear plastic cups or glasses
  • A clean clothAn old handkerchief, folded twice.
  • A little muddy waterGarden soil stirred into tap water.

Time: About 35 minutes.

The story

On a school trip, the friends find a muddy pond. Dhruv pours some of the water through a folded cloth, and it comes out clear. "Now it's safe to drink!" says Tanmay. Tara stops him. "Clear is not the same as clean." What might still be in water that looks clear?

The idea

Some things in water you can see, such as mud and particles. Others you cannot: germs, dissolved salts and many harmful chemicals.

A cloth filter catches particles, so water looks clearer. Germs are far too small for a cloth to catch, and dissolved things pass straight through.

Clear does not mean safe. Safe drinking water has been boiled, treated (for example by a proper purifier or chlorine at the right dose) or tested.

Testing is done by a water-testing lab or with proper test kits, not by looking, smelling or tasting.

A table. Mud and particles: you can see them, and a cloth filter mostly removes them. Germs: you cannot see them, and a cloth filter does not remove them. Dissolved salts and some harmful chemicals: you cannot see them, and a cloth filter does not remove them.What you can see, and what needs testingIn the waterCan you see it?Cloth removes it?Mud and particlesYesMostlyGermsNoNoDissolved saltsNoNoSome harmful chemicalsNoNo
A table. Mud and particles: you can see them, and a cloth filter mostly removes them. Germs: you cannot see them, and a cloth filter does not remove them. Dissolved salts and some harmful chemicals: you cannot see them, and a cloth filter does not remove them.

Worked example

Asha filters muddy water

  1. She poured a little muddy water through a folded cloth into a clear glass.
  2. The water came out much clearer. Brown bits stayed on the cloth.
  3. She asked: what might still be in the clear water? Germs and dissolved things, which she could not see.
  4. A grown-up poured both glasses away, and everyone washed their hands.

The filter removed what could be seen. Clear water still needs boiling, treatment or testing before anyone drinks it.

Try it

  1. Make the muddy water.

    Stir a spoon of soil into a cup of tap water.

  2. Filter it.

    Hold the folded cloth over the second cup and pour slowly.

  3. Compare.

    Look at both cups. What can you see in each?

  4. Fill the table.

    For each thing that might be in water: can you see it? Did the cloth remove it?

  5. Clear up.

    A grown-up pours the water away. Wash your hands.

What you make (paper or offline)

Your water table

A table of what can be seen in water and what a cloth filter removes.What you can see, and what needs testingIn the waterCan you see it?Cloth removes it?Mud and particlesYesMostlyGermsNoNoDissolved saltsNoNoSome harmful chemicalsNoNo
A table of what can be seen in water and what a cloth filter removes.

Your comparison table.

One sentence explaining why clear water may still be unsafe.

Three ways water is made safe to drink.

Make it on paper. Keep it with you. Nothing to upload.

Check yourself

Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.

  1. Question 1 of 5Filtered water now looks clear. Is it safe to drink?

    See the answer and why
    • A. Yes, because clear water is safe Not this one. Clear water can still hold germs and dissolved chemicals.
    • B. Yes, if it smells fine Not this one. Many germs and chemicals have no smell.
    • C. No, because filtering makes water dirtier Not this one. Filtering makes water cleaner, just not fully safe.
    • D. Not necessarily: germs can still be in it Correct. Germs are too small to see and pass through a cloth.
  2. Question 2 of 5What does a cloth filter remove from muddy water?

    See the answer and why
    • A. Dissolved salt Not this one. Dissolved salt passes through with the water.
    • B. Mud and other particles Correct. The cloth catches bits big enough to see.
    • C. Everything harmful Not this one. It removes only particles. Germs and dissolved chemicals pass through.
    • D. All germs Not this one. Germs are far too small for a cloth to catch.
  3. Question 3 of 5Which of these helps make water safe to drink at home?

    See the answer and why
    • A. Boiling it well Correct. Boiling kills germs.
    • B. Pouring it through a cloth twice Not this one. A cloth still cannot catch germs, however many times you use it.
    • C. Adding ice to it Not this one. Cold does not kill most germs, and ice may carry its own.
    • D. Letting the mud settle Not this one. Settling clears particles but leaves germs.
  4. Question 4 of 5Why must you never taste the water in this activity?

    See the answer and why
    • A. Tasting would change how clear the water looks Not this one. Tasting does not change the water. The reason is safety.
    • B. The cloth will get dirty Not this one. The cloth is not the concern. Your health is.
    • C. It will taste bitter Not this one. Taste is not the reason. Hidden germs are.
    • D. Unseen germs or chemicals may be in it Correct. You cannot tell by looking whether it is safe.
  5. Question 5 of 5Two glasses of water look equally clear. How can you find out which is safe?

    See the answer and why
    • A. Choose the colder one Not this one. Temperature does not show whether water is safe.
    • B. Smell both carefully Not this one. Many harmful things have no smell.
    • C. Have them tested by a water-testing lab Correct. Only testing shows what you cannot see.
    • D. Hold them up to a bright light and compare them Not this one. Germs and dissolved chemicals do not show in light.

Think back

What surprised you about the clear water?

How is drinking water made safe in your home or school?

Investigate further

  • Find out how your area's drinking water is treated.
  • Read the label on a water purifier: what does it say it removes?

For grown-upsTap a heading to open it. The safety notes are at the top of the page.

Before you start
  • Two plastic cups, a cloth, a spoon of garden soil.
  • Pour the water away yourself afterwards.
Questions that help
  • "What might be in the water that we cannot see?"
  • "How do we make our drinking water safe?"
What's really going on

Waterborne diseases are spread by germs invisible in clear water. Boiling, proper treatment and testing protect against them.

Cloth filtration is still useful as a first step to remove particles before boiling or treatment.

Other ways to do it
  • Discuss the table using pictures if soil and water are not available.

For teachersLearning aims, classroom ideas and what to look for.

Learning aims
  • Distinguish visible particles from invisible contaminants.
  • Explain why clarity is not proof of safety.
Subject links
  • Science: water, microorganisms and solutions.
  • Social science: public health.

These links are general. They have not been checked against a specific board syllabus.

In the classroom
  • Compare cloth, sand and paper filters on clarity; then discuss what none of them can show.
What to look for
  • Says clear does not mean safe.
  • Names boiling, treatment or testing.
ProLearners Assist