Separating Salt From Rock Salt
Topic
Separating mixtures
Introduction
A mixture consists of two or more substances mixed together but not chemically
combined. We can separate mixtures by physical processes. Mixtures that occur
in nature, such as rock salt, have to be separated in order to produce pure
products – in this case salt. Rock salt contains a mixture of salt (sodium
chloride) and other minerals. In this experiment you will use the processes of
filtration and evaporation to obtain pure salt.
Time required
45 minutes
Materials
rock salt
100 ml beaker
50 ml measuring cylinder
water
spatula
glass stirring rod
funnel
filter paper
evaporating dish (large enough to
hold 30 ml of liquid)
Bunsen burner
tripod and gauze
safety glasses
Safety note
Teacher supervision is necessary. Ensure that students wear safety glasses at
all times and do not attempt to peer closely as the salt crystals appear in the
evaporating dish. Long hair must be pulled back.
Procedure
1. Measure out 6 heaped spatulas of rock salt and place in the beaker.
2. Measure out 30 ml of water and pour this into the beaker with the rock salt.
3. Stir the mixture of rock salt and water with the glass rod for about 5 minutes.
4. Fold a filter paper in half and then in half again; fit the cone of filter paper
inside the funnel.
5. Pour the rock salt and water mixture through the filter paper and allow the
liquid filtrate to drip into the evaporating dish. Take care when pouring that
you don’t fill above the level of the filter paper.
6. Wait until all the liquid has dripped into the evaporating dish.
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1
rock salt and
water mixture
filter
paper
2
filtrate
evaporating
dish
funnel
gauze
tripod
Bunsen
burner
evaporating
dish
7. Set up the Bunsen burner and position the tripod and gauze over it.
8. Carefully place the evaporating dish of filtrate on top of the tripod and gauze.
9. Put on your safety glasses, light the Bunsen burner, and begin gently heating
the filtrate in the evaporating dish.
10. Keep heating until all the liquid has gone. Keep your face well clear, and keep
your safety glasses on at all times. The salt crystals that appear in the dish can
sometimes start to spit, and the salt is hot.
11. Record your observations.
Analysis
1. What did the original mixture look like?
2. What did the water that dripped through the filter funnel contain? What is the
reasoning behind your answer?
3. Why is it important not to over-fill the filter paper cone?
4. What was left on the filter paper? What is the reasoning behind your answer?
5. What happened as you started to heat the filtrate?
6. What was left behind in the evaporating dish?
7. Think of other examples of mixtures that can be separated using filtration
and/or evaporation.
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Published by Facts On File, Inc. All electronic storage, reproduction,
or transmittal is copyright protected by the publisher.
3.07 Separating Salt From Rock Salt
1. The original mixture was a cloudy brown color, containing some large insoluble pieces of
material.
2. The water that dripped through the filter funnel contained dissolved sodium chloride (salt).
Sodium chloride is a very soluble compound.
3. Over-filling would allow unfiltered liquid to trickle down between the glass funnel and the filter paper and contaminate the filtrate in the dish below.
4. Material such as rock fragments and insoluble minerals remained on the filter paper. None of
this material is soluble in water. Also, the particles of these materials are too large to pass
through the tiny holes in the filter paper.
5. As the filtrate is heated, the level of the water goes down and steam can be seen coming from
the dish. Gradually white salt can be observed around the edge of the dish. Eventually all the
water evaporates, leaving white salt crystals in the dish.
6. Pure sodium chloride is left behind in the evaporating dish.
7. Filtration is one of the key stages in sewage treatment and water purification, both of which
are essential to public health. Filters are used to remove dust and pollen from the air in airconditioning systems, and from the air entering the carburetors of automobile engines.
Evaporation is used to separate gasoline, kerosene, and other fuels and oils from crude petroleum oil. It is
also used to extract crystalline sugar from the sugary juice squeezed from sugarcane, and to
make juice concentrates from fresh fruit juices.