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Separation of mixtures

Separation of mixtures

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You have salt water in a beaker, and water is boiled away. What happens to the salt?

Separation of mixtures

Jenny has given Michael a challenge. To separate the grains of salt... From this this beach sand. The salt particles are way too many, and too small, to pick out by hand. What, you're giving up already?

Okey then, let Jenny show you how to do it. Start by dissolving the salt in water. This causes the salt crystals to break up into ions. But hang on, doesn't that make it even more difficult to separate it from the sand? No, look.

The sand sinks to the bottom. While the salt is dispersed in the liquid. One way to separate the salty water from the sand is to carefully pour out the water. This is called decanting. That method works well when you have a solid substance with large particles that have sunk to the bottom.

But in this case the decanted water is a little muddy, and the sand still has some water in it. To separate sand and water, this method wasn't perfect. Let's pour the salt water back again. This time Jenny uses a funnel with a cone of special paper - filter paper. The holes in the filter paper are big enough for the water and the salt ions to pass through... ...

But the sand stays behind. And... we have the sand on its own. This method of separation is called filtration. But we're not done yet.

The next step is to separate the salt - from the water. As you know, water boils at one hundred degrees Celsius. Salt, or sodium chloride, has a boiling point well over one thousand degrees Celsius. That means we can just boil the water to get rid of it, and the salt will stay behind! This method of separation is called evaporation.

It's great for separating a dissolved solid - in this case the salt - from a liquid. Now we have the salt in the bottom of the beaker. The sand is in the filter paper. The separation is complete! No, wait a minute!

I have a question: What if it's the water you're after, not the salt? Then it's no good if it just evaporates? Ah, good point, Michael. Ehrm. I think Jenny has a way to solve that as well.

Let's mix some more salt water. To collect the liquid that boils away, you need to add one thing to your setup: A long tube that has cold water passing around it -- a condenser tube. Now when the water is evaporated, it passes up into the condenser which cools it down... ... turning it back into a liquid. The water then drips into another beaker.

This method of separation is called distillation. We are left with salt crystals as before, and completely pure - distilled - water. Distillation is also useful if you have a solution of different liquids, and you want to extract the liquid with the lowest boiling point. Each method of separation is useful for a different purpose. Decanting to separate liquids from solids-with-large-particles.

Filtration to separate more fine-grained solids from liquids. Evaporation to separate dissolved solids from liquids. And distillation if it's the evaporated liquid you're after. (Psst, don't forget to turn off the heater)