Crystallisation

Crystallisation is the most common method to purify soluble solids. It is preferred over evaporation to dryness as many substances decompose upon strong heating. Also, when all the water is removed during evaporation, any soluble impurities will be left on the crystals (eg. sugar decomposes to become carbon when it is heated strongly). The shape and size of crystals can be controlled by controlling variables such as cooling rate and evaporation rate.

In crystallisation, water is removed by heating the solution. Heating stops when a hot saturated* solution is formed. The resulting solution is left to cool and the dissolved solid will then be formed as pure crystals. This is because the solubility of the solute decreases as the temperature drops. At a lower temperature, less solute can be dissolved in the solution.

*A solution is saturated when it contains the maximum amount of dissolved solute at a given temperature. (When no more solvent can be dissolved in the solute). To test if a solution is saturated, a glass rod is placed into the solution and if there are tiny crystals formed when the rod is removed, the solution is saturated.

Analogy time! (A short short one)
Imagine the solute is your brain and then the solvent is information. When your brain is filled with too much information that whatever remaining information cannot go in, your brain is saturated. :D

For crystallisation to take place, several conditions must be met:
1. The solid must be soluble in water.
2. The solubility of the substance should change with the changing temperature.
3. Solution must be saturated with the solute.

For our practical, we were tasked to purify copper (II) sulphate crystals and investigate the effect of cooling rate on the size of crystals obtained.

My group is making crystals by slow cooling.

Steps:
1. Heat 20cm³ of water in a beaker and stop when the water boils.
2. Add one spatula of copper (II) sulphate into the hot water and mix until all the copper (II) sulphates dissolves.
3. Repeat step 2 until the solution is saturated.
4. Filter the solution to remove solid impurities.
5. Heat the solution in an evaporating dish.
6. Stop heating when about half the solvent has evaporated.
7. Pour the solution into a boiling tube and allow it to cool slowly. Leave overnight if necessary.

Step 1. Heating the water

Step 2 and 3. Adding copper (II) sulphate into the water

Step 4. Filtering the solution.

Step 5. Heating the solution.

Okay there is another step but I'm still currently stuck at Step 7 because crystals don't take that quick to form so we are supposed to review it next week. I will update my observations when I see the crystals that are formed.

My takeaways from this practical:
Crystals formed by rapid cooling are smaller and more irregular than crystals obtained by slow cooling. To obtain more crystals, one can add more water and apply slow cooling instead of rapid cooling. We should stop heating before all the solvent has evaporated as it prevents the solute from decomposing and soluble impurities will be left behind.