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Separating an insoluble mixture is a little bit easier than separating a soluble one. Some of the processes that work for soluble mixtures (such as distillation) can also work for insoluble mixtures. However, there are much easier ways to separate the parts of suspensions and colloids.

Decanting, sieving and filtration are all methods to separate suspensions. Decanting is the simplest. As discussed in Topic 1, Chapter 2, a suspension eventually settles and separates over time. If the suspension is moved (for example, to pour it from one container to another), the sediment can become remixed into the liquid it was once suspended in. Decanting is when liquid is carefully poured off the top of a mixture into another container, leaving the sediment behind in the original container. Decanting is often used for older wines, as some can contain sediments that make the wine taste strange. It is also used in the production of some chemicals, including biodiesel.

See video 'Sieving'

Sieving is used to separate large particles from suspensions, usually suspensions involving solid materials. A sieve is a utensil that is a mesh placed over a frame. An example of a sieve is a tea strainer, which is used to separate tea leaves from water. Sieves work because large particles cannot fit through the holes in a sieve, but small particles can. See Image 1. Sieving is often used in preparing food, but it also has other uses. For example, very small sieves can separate viruses from bacteria, while a fishing net could be considered a very large sieve.

Filters are very fine sieves. Filters can be made of many different substances. A common example is a coffee filter in a coffee machine, which is used to separate coffee grounds from water. See Image 2 Other examples include filter paper (which is used in laboratory experiments), tea bags and swimming pool filters. Common filters are made of paper, while some others (such as swimming pool filters) are made of fine sand or other materials. A suspension is passed through the filter, and the larger, suspended particles are strained out of the mixture as the suspension medium is allowed to pass through. Filtration works because filters have small holes through which particles of the suspension medium can fit, but through which large suspended particles cannot fit.

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Question 1/5

1. Which of these is decanting used for?

Separating coffee grounds from water

Separating tea leaves from water

Separating viruses from bacteria

Separating old wine from sediment


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