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 solutedissolves is known as the solvent.

Solution: A solution is formed when a solute dissolves in a solvent.

SOLUTE SOLVENT SOLUTION

Carbon dioxide + water = fizzy water

Salt + Water = Seawater

Test you self!

Water is the ………………Cup of coffee is the ………………..Coffee granules is the ……………..

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.

Dilute and concentrated solutions:

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 solubilityof 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 water2.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 solute7. 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 soluteas 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 indifferent sized crystals.

Conclusion: Crystallisation results when a hot, saturated solution is cooled.