5.3 Filtration & Crystalisation

Question 1

What is the difference between a mixture and a solution?

Answer 1

• A mixture consists of an insoluble solid and a liquid
• A solution consists of a soluble solid (solute) and a liquid (solvent)

Question 2

What is filtration used for?

Answer 2

Separating an insoluble solid from a liquid

Question 3

Describe how to do the filtration practical

Answer 3

• Put some filter paper folded into a cone shape into a filter funnel and pour your mixture into it. The liquid part of the mixture runs through the paper, leaving behind a solid residue.

Question 4

What is crystallisation used for?

Answer 4

Separating a solute (dissolved solid) from a solution

Question 5

Describe how to the crystallisation practical (4)

Answer 5

• Pour the solution into an evaporating dish and gently heat the solution. This technique is called evaporation. Some of the solvent (which will usually be water) will evaporate and the solution will get more concentrated. You could just evaporate all the solvent, but you’ll end up with tiny crystals and your substance could decompose (thermal decomposition).
• Instead, once some of the solvent has evaporated, or when you see crystals start to form (the point of crystallisation), you should remove the dish from the heat and leave the solution to cool.
• Filter the crystals out of the solution using filter paper and a filter funnel, and leave them in a warm place to dry.
• You could also use a drying oven or a desiccator (a desiccator contains chemicals that remove water from the surroundings).

Question 6

Solid substances that can dissolve in liquids are described as ______.

Answer 6

soluble

Question 7

Solids that cannot dissolve are described as _________.

Answer 7

insoluble

Question 8

Why is filter paper used for filtration?

Answer 8

Filter paper is full of small holes that are large enough to allow liquid particles to move through, but too small to allow solids through. This separates the solids from the liquids in filtration.