seperating mixtures

Question 1

What is a mixtures?

Answer 1

A mixture is made up of two or more elements or compounds which are not chemically combined together

Question 2

What is the composition like in a compound?

Answer 2

They have a fixed composition, which means that the ratio of the elements present is always the same in any particular compound

Question 3

How can the elements in a compound be separated?

Answer 3

A chemical reaction is needed

Question 4

What are the type of bonds present in a compound?

Answer 4

There are chemical bonds between atoms of the different elements in the compound

Question 5

What is the composition like in a mixture?

Answer 5

There is no fixed compositions, and so the proportions vary depending on the amount of each substance mixed together

Question 6

How can the elements or compounds in a mixture be separated?

Answer 6

These can be separated more easily, which can be done by physical means using the differences in properties of each substance in the mixture

Question 7

What are the type of bonds present in a mixture?

Answer 7

There are no chemical bonds between atoms of the different substances in a mixture

Question 8

What techniques are there available to separate various mixtures?

Answer 8

• filtration
• distillation
• crystallization
• chromotography

Question 9

When is filtration used to separate a mixture?

Answer 9

To separate substances that are insoluble in a particular solvent from those that are soluble

Question 10

How is filtration carried out?

Answer 10

• a conical flask or beaker is used to collect the filtrate
• a funnel, with filter paper inside it, is placed on top of the flask or beaker
• the substance to be separated is poured into the funnel
• the residue is left on top of the filter paper

Question 11

How can pure sand be obtained from a mixture of sand, salt and water?

Answer 11

1) filtration would be used to separate the sand from the salt solution
2) then the sand would be washed with distilled water to remove any salt left in solution on it
3) finally it would be dried in a warm oven

Question 12

When is crystallization used to separate a mixture?

Answer 12

To obtain a sample of pure salt from the sand / salt mixture, and after filtration has happened

Question 13

How would the salt and water be separated?

Answer 13

The sodium chloride would need to be got from the solution in water through crystallization

Question 14

How could a pure salt sample be collected from the salt solution?

Answer 14

• the water is evaporated from the sodium chloride solution by heating it in an evaporating dish on a water bath
• heating should stop when the solution is at the point of crystallization
• the rest of the water is then left to evaporate off the saturated solution at room temperature to get a good sample of crystals

Question 15

Why might a water bath be used for crystallization rather than directing on a tripod and gauze?

Answer 15

The water bath is a gentler way of heating the substance

Question 16

How would the salt and water be separated?

Answer 16

The sodium chloride would need to be got from the solution in water through crystallization

Question 17

What is distillation used for?

Answer 17

To collect the solvent that was mixed with a soluble solid instead of just letting it evaporate off into the air

Question 18

What happens in distillation?

Answer 18

• the solution is heated and boiled to evaporate the solvent
• the vapour given off is then cooled and condensed back into a liquid for collection
• the dissolved solids remain in the heated flask

Question 19

Why might a flat bottomed crystallization dish or Petri dish be used for the final step?

Answer 19

To give a large surface area for the water to evaporate from

Question 20

What is distillation used for?

Answer 20

To collect the solvent that was mixed with a soluble solid instead of just letting it evaporate off into the air

Question 21

What happens in distillation?

Answer 21

• the solution is heated and boiled to evaporate the solvent
• the vapour given off is then cooled and condensed back into a liquid for collection
• the dissolved solids remain in the heated flask

Question 22

What other mixtures can distillation separate?

Answer 22

Miscible liquids, such as ethanol and water

Question 23

What are miscible liquids?

Answer 23

Liquids that dissolve in each other, and so they do not form separate layers as in immiscible liquids. The liquids also have different boiling points

Question 24

How do the different boiling points of miscible liquids affect distillation?

Answer 24

These can be used to distill off and collect the liquid with the lowest boiling point first

Question 25

Why is it difficult to get pure liquids from mixtures of liquids by simple distillation?

Answer 25

Because vapour is given off from the liquids before they actually reach their boiling point. This is especially the case when the liquids in the mixture have similar boiling points

Question 26

How can the problems of simple distillation be overcome?

Answer 26

To aid separation, a fractioning column can be added to the apparatus

Question 27

What is the fractioning column?

Answer 27

This is usually a tall glass column filled with glass beads, fitted vertically on top of the flask being heated

Question 28

How does the fractioning column help the process of distillation?

Answer 28

The vapours must pass over and between the glass beads before they reach the condenser. Therefore the substance with the higher boiling point will condense more readily on the glass beads, and drop back down into the flask beneath