A separation step in water treatment removes one kind of unwanted material by using a physical difference, usually particle size or state. You are asked to explain these steps in IGCSE Chemistry when a question gives a flow diagram and wants a reason, not just a name.
This lesson sits inside water, air and environmental chemistry. It also links to separation and chemical analysis, where the same ideas appear with mixtures in the laboratory.
What does each stage of treatment actually do?
A simple treatment sequence for river water has three main stages. Each one deals with a different problem, so the explanation must match the problem.
- Sedimentation: the water stands in a tank and the heavier solid particles sink to the bottom. Sometimes a chemical is added to make tiny particles clump together so they sink faster.
- Filtration: the water passes through beds of sand and gravel. Insoluble particles are larger than the gaps between the grains, so they are trapped while the water passes through.
- Chlorination: a small amount of chlorine is added to kill microbes. This is sterilising, not separating.
The link sentence in every good answer is the same: the thing being removed is a different size or weight from the water particles. Dissolved substances are as small as the water particles themselves, so a filter cannot hold them back.
Worked example
Question (invented scenario): A plant takes in muddy river water. The water leaving the sand filter is clear but still contains dissolved salts. Explain why the filter removes the mud but not the salts.
Step 1, identify what the mud is. Mud is made of insoluble solid particles. They are suspended in the water and are large compared with water particles.
Step 2, identify how the filter works. The sand and gravel leave gaps. Mud particles are bigger than the gaps, so they are trapped. Water particles are much smaller, so they pass through.
Step 3, identify what the salts are. The salts are dissolved. Their particles are spread out among the water particles and are far smaller than the gaps in the filter.
Step 4, write the conclusion. The filter separates by particle size. The mud is too big to pass through, but the dissolved salts are small enough to go through with the water.
Model answer: “Mud is insoluble and its particles are larger than the gaps in the sand, so they are trapped. The salts are dissolved, so their particles are tiny and pass through with the water.”
The mistake to watch for
Mistaken answer: “The filter makes the water pure by removing all the impurities.”
This goes wrong in two ways.
First, “pure” in chemistry means one substance only, and filtered water still holds dissolved substances. Second, it gives no reason. A reason needs a particle-size comparison.
The correction is to name what is removed (“insoluble solids”) and what is not (“dissolved substances pass through”). Then add the size reason.
If dissolved salts really must be removed, a different step is needed. Distillation does this, because the water evaporates and the dissolved solid stays behind.
Check yourself
1. State the purpose of the sedimentation tank and give the reason it works.
Show answer
It lets insoluble solid particles settle out of the water. They are denser than water, so they sink to the bottom of the tank.
2. A student says, “Chlorine is added to separate bacteria from the water.” Correct the sentence.
Show answer
Chlorine does not separate anything. It is added to kill microbes, so the step is sterilising. The microbes are destroyed in the water rather than removed from it.
3. A sample of water after filtering is clear. A drop left to evaporate on a clean glass leaves a white mark. What does this show?
Show answer
The water still contains a dissolved solid. The filtered water was clear, so no insoluble particles remained, but the white residue after evaporation shows something was dissolved.
Where this leads next
Next, practise telling the two kinds of material apart in distinguishing a dissolved substance from a suspension. The equation balance reasoning trainer and the mole and equation-ratio tutor support later lessons in this module where equations appear. When you are ready, test everything in the module practice set.
Many students can recite the stages but lose the marks for the reason. That habit of explaining why is what we work on in online one-to-one Chemistry tuition.