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The five steps of crystallisation of copper (II) sulfate are stated as follows:
Heat (gently) the copper (II) sulfate solution to obtain a saturated solution.
Cool the saturated solution to allow solid copper (II) sulfate crystals to form.
Filter the mixture to obtain the copper (II) sulfate crystals as the residue
Wash the copper (II) sulfate crystals with cold distilled water to remove impurities
Dry the copper (II) sulfate crystals between a few sheets of filter paper
Out of these five steps of crystallisation, the concept behind Steps 1, 2 and 4 is centred on solubility concepts.
What is a saturated solution?
A saturated solution is obtained when the maximum mass of solute is dissolved in a given volume of solvent at a particular temperature.
E.g., The solubility of copper (II) sulfate is 35 g for every 100 mL of water at room temperature of 25°C.
If 10 g of copper (II) sulfate is dissolved in 100 mL, the solution is unsaturated, and more solute can be dissolved
If 35 g of copper (II) sulfate is dissolved in 100 mL, the solution is saturated, and no more solute can be dissolved
If 40 g of copper (II) sulfate is dissolved in 100 mL, the solution is super-saturated, and the excess solute will not dissolve. More crystals may be formed if the solubility drops.
Effect of Temperature on Solubility
Temperature is directly proportional to solubility
An increase in temperature will cause an increase in solubility of a solute
A decrease in temperature will cause a decrease in solubility of a solute
Purpose of Heating and Cooling Steps of Crystallisation (Steps 1 and 2)
The purpose of Steps 1 and 2 of crystallisation is to form a saturated solution, and to allow crystals to form from the saturated solution after cooling.
Read through the following example below using values of solubility to understand:
Preparing the Solution
The solubility of copper (II) sulfate at 100°C is roughly 200 g per 100 mL of water.
A student dissolves 150 g of copper (II) sulfate in 100 mL of water.
The student heats up the solution to 100°C and the water in the solution starts boiling. This causes the volume of water in the solution to start decreasing.
Producing Saturated Solution from Heating (Step 1)
With heating, the volume of water decreases from 100 mL to 50 mL due to boiling.
Based on the solubility of copper (II) sulfate at 100°C, only a maximum of 100 g of the solute is able to dissolve in 50 mL of water
(solubility at 100°C = 200 g per 100 mL of water)As 150 g of copper (II) sulfate is present, the solution becomes saturated due to the decrease in volume of water
100 g of copper (II) sulfate is dissolved in 50 g of water
50 g of copper (II) sulfate crystals are formed
Allowing More Crystals to Form by Cooling Saturated Solution (Step 2)
When the saturated solution is allowed to cool to 25°C, the solubility of copper (II) sulfate decreases to 30 g per 100 mL of water.
As there is only 50 mL of water, only a maximum of 15 g of the solute is able to dissolve
(solubility at 25°C = 30 g per 100 mL of water)As 150 g of copper (II) sulfate is present in total
15 g of copper (II) sulfate is dissolved in 50 g of water
135 g of copper (II) sulfate crystals are formed
(even more crystals are formed from the initial 50 g at 100°C)
Washing of Crystals should be done with Cold Distilled Water
After the copper(II) sulfate crystals are filtered and obtained as residue, the crystals needs to be rinsed with cold distilled water to remove impurities.
Cold distilled water reduces the mass of copper (II) sulfate crystals that will be dissolved in the washings (lower temperature, lower solubility), therefore increasing the yield of pure crystals formed
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