Title: Separation of salt from crushed marble
Introduction:
Students will be given the following paragraph prior to the experiment. They will also
be given a safety sheet. They will not have access to their science books initially nor
will they have the method.
Smugglers!!!!
A school had been on a day trip to France. On the way home a smuggler
slipped some crushed ‘rough’ diamonds into a packet of salt which one of
the girls was bringing back form Calais. Customs officers had been
watching the smuggler for some time and stopped the students as they
re-entered the U.K. They seized the smuggled diamonds but now need to
recover them from the salt, and for the purposes of the Crown Court
prosecution they now need to know the precise composition of the
mixture. Since the government has already analysed through the Home
Office they are required by law to have an independent analysis. This is
the job for the teams.
Aim:
 To get students thinking about how they can use techniques they already know in
everyday life.
 To get students to link modern forensics to aspects they already are familiar with.
 For students to enjoy chemistry applications
 To learn and revise separation techniques.
Target group:
This experiment will be completed by a T.Y. class. They will be working in groups of
2. The experiment itself should last approximately 60 minutes allowing ten minutes
for instruction and safety talk and five minutes for clean up this will easily be
completed within a double class period.
Apparatus per group:
Chemicals:
Filter paper
Silver nitrate
Glass beaker x 3
Distilled water
Conicle flask x 1
3g of table salt
Glass stirring rod
3g of sand
Evaporating dish (ceramic)
Filter funnel
Bunsen burner
Natural gas supply
Tripod, gauze, clamp stand
Small test tubes wash bottle
Safety analysis:
 Wear safety glasses
 Wear laboratory coats
 Tie back long hair
 Wear safety gloves
 Silver nitrate is a carcinogen and so very careful handling of this is required. Only
small amounts should be distributed and silver nitrate dangers should be
discussed before the commencement of the experiment
 Silver nitrate must not be poured down the sink. Is has to be disposed of in
organic waste
 Silver nitrate stains and so gloves must be worn at all times and all lab coats
should have their sleeves rolled down.
 Use the silver nitrate in a well ventilated area.
 An open flame is being used so all necessary gas precautions need to be adhered
to.
Method:
 Place the 6g mixed sample in a beaker
 Add 10cm3 of water
 Dissolve the salt by swirling
 Allow the marble to settle and decant
 Pour into an evaporating dish
 Put the dish on a tri pod and heat using a Bunsen
 Reduce heat once crystallisation occurs
 Wash the marble on filter paper
 Test the filtrate for the presence of chloride ions
 Add the filtrate to the salt solution
 Dry the filter paper in an oven @ 700C
 Allow to cool and weigh the marble
 Allow the salt to cool and weigh
Set up diagrams:
On the left the salt has been dissolved by the
water. The sand has settled to the bottom.
This can now be decanted off.
On the right we have a mixture of salt and
sand.
Here we can see stages t this experiment. At 1 the salt is being dissolved. The sand
settles out. At 2 we are heating the solution a little to ensure all the salt dissolves. At 3
we are filtering the sand from the salt water solution. At 4 the salt has been dried
through evaporation and the salt has crystallized back out of solution. The salt is dried
in an oven.
Here we can see the water evaporating to leave the salt:
Sample results:
Mass of salt originally:
3g
Mass of sand originally:
3g
Mass of salt recovered:
2.75 g
Mass of sand recovered:
2.98 g
% of diamonds in mixture: 2.98 / 5.73 = 52%
% error 2.98 / 3 = 0.07%
Observations:
 The sample must be homogenous and granular
 Sand can be got in a garden centre.
 Sea salt would be coarser than normal salt and so would be more similar to sand.
 Make sure that a ceramic evaporating dish is used and not a glass one as the clock
glass will crack in the heat of the flame as the solute is added through out the expt.
 For speediness the solubility of salt could be given to each team. Eg 1g salt
dissolves in 2.8 cm3 of water at 250C.
 Care must be given when drying the filter paper in the oven. If the sand is
gathered in a small heap in one spot, the wet paper will fray and tear on placing
into the oven. This will lead to a loss in sand.
Conclusions:
This experiment is relatively easily to undertake. Students will enjoy the forensic
theme to it. Care must be taken when using the silver nitrate and when using the gas
flames. This experiment requires students to apply all their previous knowledge for
this experiment. The worksheet to be completed for homework serves as a good
revision aid for the experiment.
Links to Irish Curriculum:
This experiment draws on the experiences of Junior Certificate Science. This is not a
mandatory experiment for Junior Certificate Science. However some of the methods
used in this experiment will have been mandatory for the students to have completed,
e.g. distillation of sea water. It serves as a good revision tool for leaving cert
chemistry as a lot of fundamental principles are covered.
These topics are covered in detail in the Exploring Science textbook for Junior
Certificate Science. The rapid revision book for chemistry also deals with the areas of
separation comprehensively.
Possible questions to ask:
 Which method of separation was more accurate? Why? Does it depend on what
you are separating?
 Possible worksheet:
Challenge: Separating salt and sand
What problem did we have to solve?
We had to try and see if we could separate ______ and _____ after they had been ________
together.
What apparatus did we use to solve our problem?
We decided to use...
(Draw a diagram of the apparatus)
What did we actually do to solve our problem?
First of all we put the salt and sand in a ________. We added ______ to the beaker to
__________ the salt.
We poured the mixture through a ________ _______ into another beaker. The _______ got
trapped by the _______ ________ and the _____ solution went through into the beaker.
We poured the salt solution into a ______ and put that over a pan of _________ _________. The
heat _________ the water and the ___________ was left behind.
What Science did we use? Why did our solution work?
We knew that our solution would work because we knew that salt _________ dissolved in water.
We knew that we could _______ sand from water by ________. We knew that if we _______
the salt water solution then it would ________ and leave the ______ behind.
You may need some of these words to help you write up your experiment.
funnel
heated
filtering
filter
paper
bowl
boiling
evaporated
separate
salt
sand
beaker
water
Dissolve