3. 6 SUSPENSIONS AND FILTRATION
Separating solids from liquids. Water from
rivers, ponds and wells is sometimes dirty. If we
look carefully, we may see tiny solid particles
suspended in it. When we leave dirty water to
stand, sand and mud slowly sink to the bottom.
After some time the water may become
completely clear. Then we can pour it carefully
into a clean container without disturbing the
sediment. Separating clear water from a
sediment in this way is called decanting (or
decantation).
A quicker way to obtain a small volume of clear
water is to filter it. Scientists use circular filter
papers for this. The filter paper is folded into
quarters, opened into a cone, and placed in a
filter funnel. The dirty water is then poured into
the cone of paper. The water passes through the
filter paper, but the suspended particles are
trapped. Filtering can be used to separate any
liquid from solid particles suspended in it. The
liquid that passes through the filter paper is
called the filtrate and the solid that remains in the
filter paper is called the residue. You should note
that filtration does not remove any solute that is
dissolved in the liquid. Solute and solvent
together pass straight through
filter paper.
Filer paper
under microscope
Decantation
Filtration
Separating salt and sand
How would you separate a mixture of salt and
sand? Easy! First stir up the mixture with
water. The salt dissolves. Then filter. The
sand remains as a residue on the filter paper.
The salt is in the filtrate. So evaporate the
water from the filtrate to obtain the salt.
To see how filter paper works, tear off a small piece and
look at it under a microscope. Like any paper, it is made of
tiny fibres jammed together. The clear liquid can find its
way between the fibres, but the solid particles get stuck.
Suspensions and colloids. When particles of a solid are suspended in a liquid, we call the
mixture a suspension. Muddy water is a suspension of sand and clay in water. When the water is
still, most of the particles settle to the bottom quite quickly, but the tiniest particles of clay may
take a long time. If they are very small they may never settle, so the water always looks a bit
cloudy. Even when we filter the water, it still remains cloudy. The solid particles are so small that
they pass through the filter paper. A suspension that never settles, and cannot be separated by
filtration, is called a colloid. You can make a cloudy colloidal suspension of starch at home by
boiling a little flour with water. This colloid makes a good glue for sticking paper. Another wellknown colloid is black ink.
Suspensions are quite common and may involve different combinations of solids, liquids and
gases. Milk is a colloidal suspension of solids and immiscible liquids in water. Colloidal
suspensions of immiscible liquids are called emulsions. Toothpaste and many household paints
are emulsions. They contain millions of tiny droplets of oily liquids suspended in water. Smoke is
a suspension of solid particles in the air. Mist, fog and clouds are suspensions of tiny droplets of
water in the air. We use aerosol cans to spray tiny droplets of insecticides or pleasant smelling
scents which remain suspended in the air for a long time. The foam we get when we use soap is
a suspension of air bubbles in a watery solution. Ice cream consists of very tiny air bubbles
suspended in a very cold flavoured cream. Coloured plastics are suspensions of solids in solids the plastic has tiny particles of colour embedded in it.

1. What are (i) decantation, (ii) filtration, (iii) a
suspension (iv) a colloid, and (v) an emulsion?

3. How would you prepare a suspension of chalk in
water?

2. Name six colloids that are mentioned on this page.

4. Give examples of suspensions of (i) a liquid in a
gas, (ii) a gas in a liquid, and (iii) a solid in a gas.
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