1.3 Separating Mixtures

Question 1

Mixture def

Answer 1

A mixture is made up of two or more substances (elements or compounds) that are not chemically combined together

Question 2

What does having a fixed composition mean?

Answer 2

The ratio of elements present is always the same in any particular compound

Question 3

What is the difference between compounds and mixtures in terms of fixed composition?

Answer 3

• Compounds have a fixed composition
• Mixtures have no fixed composition (the proportions vary depending on the amount of each substance mixed together)

Question 4

What is the difference between compounds and mixtures in terms of separating the elements?

Answer 4

• Compounds: chemical reactions must be used
• Mixtures: different elements or compounds can be separated more easily (by physical means using the properties of each substance)

Question 5

What are four available techniques for separating mixtures?

Answer 5

• Filtration
• Crystallisation
• Distillation
• Chromatography

Question 6

When is filtration used?

Answer 6

To separate substances that are insoluble in a particular solvent from those that are soluble in the solvent (eg sand and salt)

Question 7

How does filtration work?

Answer 7

• Strain the solvent and its insoluble substance using filter paper
• Sand that is collected on the filter paper can be washed with distilled water to remove any salt solution
• Wet sand dried in a warm oven to evaporate any water left off and leave pure, dry sand

Question 8

When is crystallisation used?

Answer 8

To separate a soluble substance from a solvent (eg salt from water)

Question 9

How does crystallisation work?

Answer 9

• Heat the salt solution on a water bath
• Stop heating at the point of crystallisation (when small crystals appear at the edge of the solution)
• Rest of the water is left to evaporate at room temperature to get a good sample of salt crystals

Question 10

When is distillation used?

Answer 10

To separate a soluble solid from a solvent (but when you need to collect the solvent instead of letting it evaporate)

Question 11

How does simple distillation work?

Answer 11

• Solution is heated and boiled to evaporated the solvent
• Outer glass tube with water flowing through acts as a cooler for the hot vapour in the inner tube. the hot vapour is cooled and condensed back into a liquid for collection in a receiving vessel