Purifying an organic solid

Question 1

how can you purify an organic solid?

Answer 1

by recrystallisation

Question 2

what does recrystallisation work for?

Answer 2

for a solid product that contains small amounts of impurities, it works on the principle that solids are more soluble in hot than cold solvents

Question 3

describe how the solvent is chosen?

Answer 3

it needs to be a solvent that the desired product dissolves in well, when hot, but much less when cold, so crystals form on cooling

Question 4

describe the stages of recrystallisation

Answer 4

• the impure solid is dissolved in the minimum amount of hot solvent (saturated solution)
• any insoluble impurities are removed, whilst the solution is still hot, with filter paper
• leave the filtrate to cool to room temperature, cool even further in an ice bath to crystallise further
• collect the crystals via vacuum filtration, this will remove soluble impurities as they will be in the filtrate rather than crystals on the filter paper
• wash the crystals with a small amount of cold solvent
• dry the crystals

Question 5

how can the purity of a solid be tested?

Answer 5

using melting point determination/analysis, a small amount of solid is added to a capillary tube and its temperature is slowly increased. the melting point is recorded.

Question 6

what does a sharp melting point in melting point determination mean?

Answer 6

it means the product is pure