Separation Techniques, Filtering and Evaporation

Pour sand into a bucket of water and stir, the sand sinks to the bottom while the water stays on top. Those two parts of the mixture already differ in a physical property (particle size vs dissolved), which is exactly what you need to separate them. Every separation method uses one rule: exploit a physical property difference between the parts of the mixture to pull them apart. Four common property differences:

Important: separation methods are physical changes only. They split a mixture, they don't break a compound apart. (Splitting H₂O into H and O needs electricity, that's chemistry.)

Filtration separates an insoluble solid (like sand) from a liquid (like water). The filter paper has tiny pores, water molecules slip through easily, but sand grains are way too big to fit.

Equipment list: filter paper, filter funnel, conical flask (or beaker), retort stand and ring (optional, to hold the funnel), the muddy mixture, a stirring rod.

Procedure:

1. Fold the filter paper into a cone and place it inside the funnel.
2. Sit the funnel on top of the conical flask (or in a stand).
3. Pour the muddy water down a glass stirring rod into the funnel, never directly, or you'll splash.
4. Wait. Water (the filtrate) drips through into the flask. Sand (the residue) stays on the paper.
5. Rinse the residue with a little clean water to wash any trapped filtrate through.

Safety: wear safety glasses (splashes), keep the funnel stable, no flames near the workbench.

Saltwater can't be filtered, the salt has dissolved into individual particles much smaller than the pores in filter paper. To get the salt back, we use evaporation.

How it works: water has a much lower boiling point (100 °C) than salt's melting point (about 800 °C). Heat the saltwater. The water turns to steam and floats away. The salt stays behind in the dish as a white crust.

Equipment: evaporating basin, tripod, gauze mat, Bunsen burner, heatproof mat, tongs, safety glasses.

Procedure:

1. Pour the saltwater into the evaporating basin.
2. Set the basin on a gauze on top of a tripod.
3. Light the Bunsen burner. Use a low (yellow) flame to start, then a gentle blue flame.
4. Heat gently. Stop when only a small amount of liquid is left, the last bit will dry on its own.
5. Let it cool. Use tongs, the basin is hot.

Why not boil it dry hard? A vigorous boil will spit hot salty water onto your skin or the flame, a burn risk and a contamination risk.

Real example: the Coorong salt pans in South Australia produce table salt this way using sun and wind to evaporate seawater, no Bunsen needed.

What if a mixture has THREE parts? Use the methods one after another. Imagine a beaker of salt + sand + water. The plan:

1. Filter the whole mixture. The sand (insoluble) stays on the filter paper. The salt has dissolved, so it passes through with the water as the filtrate.
2. Evaporate the filtrate. The water boils off as steam. The salt stays in the evaporating basin.

That's it, three substances pulled apart by combining two physical separation techniques. The trick is to think about which physical property differs at each step.

You now know four methods. The hard part of a separation question is never carrying out the method, it is deciding which one to reach for, and students who guess usually guess filtration because it is the one they have seen most.

You do not have to guess. Every method in this lesson works by exploiting one physical property difference, so the question is always the same: what is different about the parts of this mixture? Ask it in this order and the method falls out on its own.

1. Is one part magnetic and the other not? If so, a magnet lifts it straight out, and you do it first because a magnet works on a dry mixture and costs you nothing.
2. Is one part a solid sitting in a liquid without dissolving? If so, its particles are far bigger than the pores in filter paper, so filtration will trap it.
3. Is the part you want dissolved in the liquid? If so, filtration is useless, because dissolved particles slip through the pores. Boil the solvent off by evaporation if the solid is what you want to keep.

Notice what the order buys you. Each step removes one part and hands the rest to the next step, so a mixture of four things is not four times harder than a mixture of two, it is the same three questions asked again on what is left over.

The one comparison worth holding onto is between steps 2 and 3, because they look identical from the outside and behave completely differently. Sand in water and salt in water are both "a solid in a liquid". Pour each through filter paper and the sand stops dead while the salt goes straight through with the water. The difference is not the solid, it is whether the solid has dissolved, and that single question decides whether filtration is the right tool or a waste of an afternoon.

Everything in this lesson so far has been a mixture someone made on purpose. A polluted creek is a mixture nobody wanted, and the same three questions decide what can be done about it.

You can model one in a jar. Half fill it with water to stand for the creek, then add four pollutants of four different kinds: a spoonful of soil for the runoff washed off a building site, a few drops of cooking oil for a fuel spill, a few drops of food colouring for a dye leaking from a factory, and a spoonful of salt for the salinity that seeps in from cleared farmland. Stir it and you have something that looks unpleasantly like a real polluted waterway, and, more usefully, a mixture whose parts you can now name.

Now plan the clean-up, part by part. The soil is a solid that has not dissolved, so it settles if you leave the jar still, and filter paper catches whatever is still suspended. The oil does not dissolve either and it is less dense than water, so it floats in a layer you can skim off the top, which is exactly what a floating boom does after a spill in Sydney Harbour. So far, so good.

Then you reach the dye and the salt, and the filter paper does nothing at all. Both have dissolved. Their particles are smaller than the pores, so they pass straight through with the water and the filtrate comes out coloured and still salty. This is not a failure of your method, it is the single most important thing the model has to teach you: the pollutants that are easiest to see are the ones that are easiest to remove, and the ones that have dissolved are neither.

That is why a real water treatment plant does not stop at a filter. To take out something dissolved you have to separate on a different property, and the property left is boiling point, which is distillation, the technique in the next lesson. And it is why John Snow's Broad Street pump water looked perfectly clear and still killed people. Clear water is not clean water. What made that pump lethal was cholera bacteria, which are not dissolved at all — they are far too small to see, so they drifted through the water invisibly and would have gone straight through filter paper as well. Being able to see through a sample tells you only that it holds nothing big enough to cloud it. It tells you nothing about what has dissolved, and nothing about what is simply too small to notice.

Long before laboratory funnels and Bunsen burners existed, Aboriginal and Torres Strait Islander peoples developed precise ways to separate mixtures, refined over tens of thousands of years. They use exactly the same physical-property differences you have just met: particle size, density and solubility.

Leaching is a striking match for the chemistry in this lesson. Nardoo spores and cycad (burrawang) seeds are poisonous if eaten raw, but grinding them and rinsing the paste in running water dissolves the toxin and carries it off, the same idea as separating a soluble substance from an insoluble one. The explorers Burke and Wills ate nardoo that had not been leached properly and became seriously ill, a reminder that this is exact, hard-won knowledge, not guesswork.