Recrystalisation practical

Question 1

Why use the minimum quantity of hot water

Answer 1

To ensure the hot solution would be saturated

Crystals would form on cooling

Question 2

Why was the flask cooled to room temperature before the crystals were filtered off

Answer 2

Yield lower if warm

Solubility higher if warm

Question 3

Why were the crystals compressed in a funnel

Answer 3

Air passes through the sample not just round it

Question 4

Why is a little cold water poured through the crystals

Answer 4

To wash away soluble impurities

Question 5

Method

Answer 5

1. Dissolve the impure compound in a minimum volume
   of hot (near boiling) solvent.
2. Hot filter solution through (fluted) filter paper quickly
3. Cool the filtered solution by inserting beaker in ice
4. Suction filtrate with a Buchner flask to separate out
   crystals
   5 Wash the crystals with distilled water
5. Dry the crystals between absorbent paper

Question 6

Loss of yield

Answer 6

Crystals lost when filtering or washing
• Some product stays in solution after recrystallisation
• other side reactions occurring

Question 7

% yield > 100

Answer 7

crystals not dried properly

mass is larger than expected

Question 8

Measuring melting point

Answer 8

small amount of the sample is put into a capillary tube. The tube is heated up and is heated slowly near the melting point

Question 9

Heating oil

Answer 9

needs to have boiling point higher than samples melting point and low flammability