← Separation Techniques

Lesson 1 of 7 · Textbook 4.1–4.2

Why separate, and how to choose

There are only five techniques in this chapter. The skill worth having is knowing which one to reach for — and that always comes down to a single question.

Why bother separating mixtures?

Mixtures are everywhere — "from the food we eat to the waste we discard". Being able to pull a mixture apart into its constituents is what makes it possible to reduce, reuse and recycle, and so to live sustainably.

Water is the clearest case. Millions of people worldwide lack access to clean water, and climate change has left fewer sources of drinking (potable) water. Separation techniques are how we boost the supply — and in Singapore they are the reason NEWater exists.

Where it is usedExample from your textbook
Waste managementMagnetic attraction pulls iron out of scrap metal so it can be reused or recycled (Figure 4.1).
Food safetyEvaporation dries ikan bilis to extend shelf life and cut food waste (4.2). Chromatography tests food for harmful substances (4.3).
Water purificationFiltration and reverse osmosis recycle used water into potable water (4.4).

The decision that runs the chapter

Here is the sentence everything rests on:

The principle The different constituents of a mixture retain their physical properties. The differences between those properties are what allow us to choose a separation technique.

So you never have to guess. Find the property that differs, and the technique is decided for you:

What is different about the constituents? Technique
One is magnetic, the other is notmagnetic attraction
They have different particle sizes (one is an insoluble solid in a liquid)filtration
A solid is dissolved in a liquid — and you want the solidevaporation
They have different boiling points — and you want the liquiddistillation
They dissolve to different extents in the same solventchromatography
Evaporation and distillation are the pair people confuse Both separate a dissolved solid from a liquid. The difference is which one you end up holding. Evaporation keeps the solid and the liquid escapes as vapour. Distillation keeps the liquid and the solid stays behind in the flask. So the question to ask is not "what is in the mixture?" but "what do I want to collect?"

Three more things that decide the choice

Physical properties tell you what would work. In the real world three other factors decide what is actually used:

The textbook's own example: reverse osmosis gives Singapore potable water, but it is costly. That is why it cannot be the only method we use — and it is why the Four National Taps exist rather than one national tap.

Plan a separation yourself

Pick a mixture, then choose your steps in order. The planner tells you what happens each time — including when a step is the wrong one, or the right one done too early.

Separation planner

In the beaker
Collected
Try iron + salt + sawdust in the wrong order — add water before using the magnet — and see what goes wrong. That mixture is a real exam question, and the order is the whole answer.

Check yourself