Crystals from a cooling solution
Crystallization gives pure solids.
A hot, very sweet syrup
A hot solution holds a lot of dissolved solid. It is saturated. You let it cool slowly. Cold water holds less of the solid.
What happens to the extra solid?
What this lesson covers
The idea
Crystallization forms pure crystals from a saturated solution: on cooling, solute in excess of its solubility at the lower temperature separates as pure solid; based on solubility differences with temperature, it purifies solids and separates them from impurities.
A hot, very sweet syrup
A hot solution holds a lot of dissolved solid. It is saturated. You let it cool slowly. Cold water holds less of the solid.
What happens to the extra solid?
- It separates out as pure solid, often crystals
- It vanishes
- It changes into water
Cool the solution
Look at a saturated solution of compound B at 60 °C. Then cool it to 40 °C.
Crystallization
Crystallization is the process of forming crystals from a saturated solution. When it cools, the solute in excess of its solubility at the lower temperature separates out as a pure solid. It is based on the difference in solubility at different temperatures.
Uses. It purifies solids and separates them from impurities. It can separate two solids if one is present in a small quantity and both dissolve in the same solvent.
Example. 287 g of compound B in 100 g of water at 60 °C is saturated. At 40 °C the solubility is 241 g per 100 g of water, so 46 g separates out.
Salt in India. Coastal communities made panga salt by boiling concentrated sea brine, and karkatch salt by evaporating sea water.
Notes
Crystallization forms pure crystals from a saturated solution: on cooling, the solute in excess of its solubility separates out. It purifies solids.
Check yourself
Compound B: 287 g dissolves in 100 g of water at 60 °C and 241 g at 40 °C. How many grams separate out on cooling?
Answer: 46 g
287 g − 241 g = 46 g separates out.
On cooling a saturated solution, what separates out?
Crystallization works because…
What is crystallization used for?
- All the water. The water stays as the solvent.
- The solute in excess of its solubility at the lower temperature, as pure solid — correct. Yes.
- The impurities, as crystals. The pure substance crystallizes; the impurities stay dissolved.
- solutes are heavier than water. It is about solubility, not weight.
- water boils at 100 °C. Boiling is not the principle here.
- solubility differs at different temperatures — correct. Yes.
- To purify solids and separate them from impurities — correct. Yes.
- To separate two miscible liquids. That is distillation.
- To separate colours in ink. That is chromatography.