Chapter 3 - Crystal Chemistry 3b - Water of crystallisation
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Chapter 4 - Paper Chromatography
Experiment 4.1
The analysis of black and green food colourings
• black & green food
colouring
• test tube
• conical flask
• large filter paper
• sellotape
• pair of scissors
• pencil
• small paint brush
or dropping pipette
Here we use filter paper to make a chromatogram. One filter paper
can be used for 4 chromatograms. 1 filter paper can be used for 4
chromatograms.
Cut four 1½ cm wide strips from the widest part of a large (11cm)
filter paper. Stick a second piece cut from the waste filter paper
across the top as in the diagram. Draw a pencil line 2cm from the
bottom end of each strip.
Put 3 strips aside for other experiments. Use the paint brush to paint
a line of black food colouring on the pencil line, or add 2 drops of the
colouring in the middle of the line using the dropping pipette. Put a
test tube full of water in the conical flask and hang the strip in the
flask. The colouring will be just above the water, which will rise up
the paper taking the colouring with it, and separating the different
dyes that make up the black food colouring.
Stop the experiment when the water reaches the top of the filter
paper strip. Dry the chromatogram and label it. Describe what it
shows.
Repeat the experiment using green food colouring.
The two food colourings have listed on their labels the single dyes
which they contain. Do your paper chromatograms agree with what
is given on the labels?
Paper chromatography is a method of separating two or more substances. It is particularly
useful if the substances are coloured.
Experiment 3.9
Does a substance contain water of crystallisation?
• magnesium sulphate
• aluminium potassium
sulphate
• sodium chloride
• sodium sulphate
• test tubes
Put ½ measure of magnesium sulphate into a clean dry test tube.
Heat the solid in the burner flame and look carefully at what
happens. Do you see any water vapour coming off from the
magnesium sulphate and condensing on the cool upper parts
of the test tube? This water is part of the magnesium sulphate
crystal. It is called water of crystallisation. Record in your notebook
that magnesium sulphate contains
water of crystallisation
.
Repeat this experiment with aluminium potassium sulphate, sodium
chloride and sodium sulphate. Do these substances contain water
of crystallisation?
Experiment 3.10
Heating blue copper sulphate crystals
• copper sulphate
• test tube
• dropping pipette
Put ½ measure of blue copper sulphate into a clean dry test tube.
This contains water of crystallisation. Heat the test tube gently and
record what you see happening. Do you see any water condensing
on the upper parts of the test tube? The solid left behind is
anhydrous
copper sulphate. What colour is it? Put the HOT test tube
into an empty beaker to cool.
When the tube is cool add one or two drops of water with the
dropping pipette to the white copper sulphate solid in the test tube.
Does it change colour? What to?
This colour change is a test for water. No other liquid makes
anhydrous copper sulphate change colour.
Sellotape
Pencil
line
Experiment 4.2
The analysis of inks
• black felt-tipped pen
• other inks or felt-tipped
pens
• test tube
• conical flask
• large filter paper
• sellotape
• pair of scissors
• pencil
• small paint brush
or dropping pipette
Use the method given in Experiment 4.1 to analyse black fountain
pen ink. What colour dyes does the ink contain?
Repeat the experiment using a black felt-tipped pen.
Does this contain the same coloured dyes as the black ink?
Try the experiment with other inks or felt-tipped pens. Some colours
will be just a single dye, others will be mixtures. Try several colours,
for example red, green, blue, purple and
brown felt-tipped pens. Do many of those that you try contain
a single dye?