Video: Identifying the Purpose of Filtration in a Set of Statements

What is the technique of filtration used for? [A] Separating a solid from a solid [B] Separating a solid from a liquid [C] Separating a gas from a liquid [D] Separating two liquids with different melting points [E] Separating two liquids with different densities

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Video Transcript

What is the technique of filtration used for? A) Separating a solid from a solid, B) separating a solid from a liquid, C) separating a gas from a liquid, D) separating two liquids with different melting points, or E) separating two liquids with different densities.

In everyday life, you might hear the word “filtration” applied most commonly to water. You might see filters that make water softer, making it taste better and making it cleaner. While still a demonstration of the principle of filtration, what we’re actually looking for in this question is the laboratory technique that looks more like this, where we use a filter funnel and perhaps an Erlenmeyer or conical flask.

Filtration is one of the many options in the lab to help purify chemicals. In this setup, you might take a piece of filter paper and turn it into a cone and put it inside the filter funnel. The filter paper doesn’t stop liquid going through. But the filter paper will act as a barrier for any large particles suspended in the liquid. So filtration in the lab is a technique we use to separate solids from liquids. Sometimes the solids what’s important. So we’ll hold on to that and get rid of the liquid. Sometimes, it’s the other way round, and sometimes we want both. Whatever the reason, the technique of filtration is used for separating a solid from a liquid.

Separating a solid from a solid can actually be quite difficult. And there are a range of techniques depending on the material. For instance, a mixture of iron filings and sulphur can be separated using a magnet. The magnet pulls away the iron filings, leaving the sulphur behind. But there are other techniques, like sieving that help to separate solids based on their size. Meanwhile, separating a gas from a liquid is actually not very common. But if you take a liquid close to its boiling point, you’re going to free a lot of any dissolved gas.

Separating two liquids with different melting points is also not very common. Processes like distillation rely on a difference in boiling point. And that allows one liquid to be turned into a vapor and condensed elsewhere. If two liquids have very different melting points, you might be able to freeze one and keep the other as a liquid and pour it off. But generally, if two liquids mix well, having different melting points isn’t going to help you separate them very much.

Then last, we have separating two liquids with different densities. This can be quite easy if, for instance, the liquids don’t mix, like oil and water. We can simply pour off the top layer, leaving the denser layer behind. But if the liquids mixed together, we can have a much more difficult separation. It is possible to use a centrifuge to separate denser materials from less-dense materials. For instance, centrifuges are used to separate components of blood. However, you need reasonably large differences in density for centrifugation to be effective. And you need liquids that won’t simply mix together once the centrifuge stops spinning.

However, the question was about the technique of filtration and which description best applies. And that is separating a solid from a liquid.

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