Welcome back to freesciencelessons.co.uk. By the end of this four part video, you should be
able to describe physical separation techniques. These include filtration, crystallisation,
distillation and chromatography. In this video, we are going to focus
on filtration and crystallisation. Now the first thing that I need
to point out is that physical separation techniques such as these
are used to separate mixtures only. They cannot be used to separate
the elements in a compound. In later videos, we are going to look at
how we separate the elements in compounds, for example by chemical reactions
such as reduction or by electrolysis but in these videos, we are focusing on physical separation techniques and how they
are used to separate mixtures. We are going to start by looking at one
of the simplest separation techniques, which is filtration. Filtration is used to separate
an insoluble solid from a liquid. Now remember that the word insoluble means
that a solid will not dissolve in a liquid. So I have got here two chemicals. On the left we have silver chloride
and on the right we have water. These small letters are called state symbols
and they tell us the state of a chemical for example whether it is a solid, a
liquid, a gas or dissolved in water. This little “s” tells us that
silver chloride is a solid. The little “l” here tells
us that water is a liquid. Silver chloride is actually insoluble
in water, so it will not dissolve and you would be given that
information in the exam. So I have a solid material suspended in a
liquid and I am showing you that here. Now obviously, this diagram is not
to scale as you would not be able to see individual particles of silver
chloride, but you should get the idea. Because the silver chloride is insoluble in water, we can use filtration to
separate these two substances. To do this we use a filter funnel
and a filter paper like this. We start by pouring our
mixture into the filter paper. The liquid, in this case the water, simply passes
through the tiny pores in the filter paper…… However, the solid material, in this case the silver chloride cannot pass through
the filter paper so it is trapped. So in the end, we have got our
liquid separated from our solid. Filtration is a really useful separation
technique and is used a lot in Chemistry. Another important separation technique is
called crystallisation so let us take a look at that now. Crystallisation is used to separate
a soluble solid from a liquid so you can see that it is different from filtration. In this example, we have
sodium chloride and water. Now the little aq here means that the
sodium chloride is dissolved in water. Scientists call this an aqueous solution. We want to separate the sodium
chloride from the water. So here is our sodium chloride solution. If we leave this solution for a few
days, then the water will evaporate. This will leave behind crystals of
solid sodium chloride like this. Now can you see that I have changed the state
symbol for the sodium chloride from aq to s Because now the sodium chloride is not
dissolved in water, it is now a solid. If we want to make crystallisation happen faster, then we can gently heat our
solution to evaporate the water. However, we have to be careful that heating does not affect the chemical that
we are trying to crystallise. For example, certain chemicals
will break down if we heat them. So sometimes it is better to just leave
the water to evaporate on its own. OK so hopefully now you should be able to describe the physical separation techniques
of filtration and crystallisation.