By Miss Buicke
Solutions and
Crystallisation
What we must know form the syllabus:
OC15
Investigate the solubility of a variety of substances in water
and the effect of temperature on solubility
OC17
Grow crystals using alum or copper sulfate
OC16
Examine the difference between a dilute, concentrated and
saturated solution
Solutes, solvents and solutions
Solute: In a solution the substance which
dissolves is called the solute.
Solvent: The substances in which the solute
dissolves is known as the solvent.
Solution: A solution is formed when a
solute dissolves in a solvent.
Carbon dioxide
+ water
Salt
+ Water
=
=
fizzy water
Seawater
Test you self!
Water is the ………………
Cup of coffee is the ………………..
Coffee granules is the ……………..
Dilute and concentrated solutions:
Dilute solutions:
A small amount of solute in a large amount
of solvent.
Concentrated solutions:
A large amount of solute in a small amount
of solvent.
Making dilute and concentrated solutions:
To make a dilute solution of MiWadi we need more
water (solvent) then MiWadi (solute).
To make a concentrated solution of MiWadi we need
more MiWadi (solute) then water (solvent).
Lets do a demonstration where we will make
a dilute and concentrated solution of MiWadi.
Soluble and insoluble
Soluble: If a substance dissolves in a solvent it is said
to be soluble.
Insoluble: A substance that will not dissolve.
Name a substance which is soluble in
water.
Name a substance which is insoluble
in water.
When a solute dissolves it does not disappear.
It is still present and fills the gaps between the
solvent particles, so there is no change in volume.
Note however that the mass of the solution is equal
to the mass of the solvent and the solute.
For example if 20 grams of salt were added to 100
grams of water the solution of sea water would
have a mass of 120 grams.
Solubility of a substance: The number of grams of
a solute that dissolves in 100g of the solvent.
How many teaspoons of sugar will dissolve in a cup
of tea? The amount depends on two factors:
1) The volume of tea (solvent)
2) The temperature of the tea (solvent)
The solubility of a substance usually increases
with temperature
Experiment:
To investigate the solubility
of a variety of substances
in water.
A variety of substances will be tested to discover if
they are soluble or insoluble in water.
Equipment: test tube, test-tube rack, spatula
Chemicals: water, iodine, potassium permanganate,
sodium chloride, sulfur, wax.
Method:
1.Half fill a test tube with water
2.Add a spatula full of the substance to be tested to
the water.
3.Stopper the test tube and shake for 15-20 seconds.
4.Examine the contents of the test tube and decided
whether the substance being tested is soluble or
insoluble.
Conclusion: If the substance dissolves then it is
soluble in water. If it does disappear it is said to be
insoluble.
How can we make sure that the experiment is a fair
test?
• Use similar amounts of each substance being
tested
• Make sure each test tube has the same amount of
water
• Shake the test tube for the same amount of time.
Experiment: To investigate the effect of temperature
on solubility.
Equipment: Beaker, spatula, thermometer, stirring
rod, pestle and mortar.
Chemicals: Water, copper sulfate crystals.
Method:
1.Place 100cm3 of water in a beaker and note the
temperature.
2.using a pestle and mortar, grind up a sample of
copper sulfate crystals.
3.Slowly and with constant stirring add the powered
copper sulfate to the water. Continue until the
powder will no longer dissolve.
4. Heat the beaker to 50 degrees and note what
happens to the undissolved copper sulfate.
5. Add more copper sulfate until no more will dissolve.
6. Heat the beaker to 80-90 degrees and again try to
dissolve more solute
7. Allow the solution to cool.
Result :
More copper sulfate dissolves in water at higher
temperatures.
Conclusion:
The amount of solvent that will dissolve in a solvent
Depends on the temperature of the solvent.
Solubility curves:
A solubility curve plots the mass of solute dissolved
in a saturated solution at different temperatures.
The graph shows the solubility of copper sulfate
increases as the temperature of the solvent
increases.
A saturated solution:
A solution that contains as much dissolved solute
as possible at that temperature.
Solubility curve
Crystallisation:
Crystals are tiny particles built in a regular manner,
e.g salt, diamonds and snowflakes.
When a hot saturated solution of copper sulfate is
cooled down, some of the copper sulfate will come
out of solution and form copper sulfate crystals.
This is because a cooler solution will not dissolve as
much solute as a warmer one.
Crystallisation: The formation of crystals by
cooling a saturated solution.
Mandatory experiment:
To grow crystals using copper sulfate
Equipment : Hot plate, thermometer, beaker,
stirring rod, evaporating dish, pestle and mortar,
spatula.
Chemicals: Copper sulfate and water.
Method:
• Add 100cm3 of water to a beaker and heat on the
hot plate.
• Slowly and with constant stirring add the powered
copper sulfate to the water
• Heat the water to 70 degrees and add in more
copper sulfate until no more will dissolve
• Pour approx. half of the solution into a pre
warmed evaporating dish and allow to cool.
• The remaining solution in the beaker can be
cooled quickly by standing the beaker in an ice
bath
Result: The different cooling methods results in
different sized crystals.
Conclusion: Crystallisation results when a hot,
saturated solution is cooled.