Science in Punggol study guide · Article 0014 of 20
A simple filter can remove some suspended particles from water. It does not automatically remove dissolved substances or microorganisms, and clearer water is not necessarily safe to drink.
Enrichment and practical investigation: this article uses filtration to practise observation and fair comparisons. It does not present filtration as a core Primary or PSLE examination topic.
On this page: Ask what needs to be removed · Explain what ordinary filter paper cannot do · Compare filters using a clear measure · Worked question: sand and dissolved salt
Ask what needs to be removed
Water may contain visible sand, dissolved salt or microorganisms too small to see. These are different substances, so one cleaning method may not remove them all.
Ordinary filter paper allows water to pass through while retaining suitable insoluble particles. Whether particles are retained depends on their size and the filter’s structure.
In a teacher-approved demonstration, a mixture of water and clean sand can be passed through suitable filter paper. Sand left on the paper provides evidence that some particles have been separated. Keep demonstration water for observation rather than drinking.
Clearer collected water is another observation. It supports a conclusion about visible particles under those test conditions, not a claim that the water contains nothing else.
Explain what ordinary filter paper cannot do
Salt dissolved in water does not remain as large grains that ordinary filter paper can catch. It passes through with the water. Filtering a sand-and-salt mixture can therefore remove sand while leaving dissolved salt in the collected water.
Clarity and purity are different properties. A clear liquid can contain dissolved substances. Microorganisms can also be present without being visible to the unaided eye.
A simple classroom filter does not establish drinking-water safety. Specialised treatment may combine processes for different substances, including dissolved-substance removal and disinfection. Their effectiveness requires suitable equipment, operation and testing.
The useful lesson is to match a method to the substance being separated and to recognise the limits of what was observed.
Compare filters using a clear measure
If comparing two classroom filters, use the same starting mixture, water volume and procedure. Keep filter area and other relevant conditions comparable. Decide what will count as a useful result before testing.
Visible cloudiness, amount of retained material and time taken answer different questions. A faster filter may allow more particles through. A clearer sample does not automatically contain fewer dissolved substances.
Record the actual observations. If only appearance was compared, describe the appearance rather than claiming that germs or salt were removed. A further claim needs a suitable test.
Worked question: sand and dissolved salt
Question: Water containing sand and dissolved salt passes through ordinary filter paper. Sand remains on the paper, and the collected water looks clear. Has the method removed both substances?
Worked explanation: The filter paper has retained suitable sand particles, separating them from the water. Dissolved salt passes through with the water, so this method has not removed it. Clear appearance does not show that the collected liquid contains only water.
Common mistake: “The water looks clean, so everything unwanted is gone.” Appearance cannot establish the absence of dissolved substances or microorganisms.
Explain each substance separately. Evidence that sand was removed cannot, by itself, support a conclusion about salt. This is a useful reasoning habit across science investigations.
Previous: The Water Cycle and Changes of State
Next: Forces Change How Objects Move
Related reading: Fair Tests: Change One Variable at a Time
