Separation and purification

Question 1

what does a pure substance consist of?

Answer 1

one element or one compound

Question 2

what does a mixture consist of?

Answer 2

different substances, not chemically joined together

Question 3

what does an element contain?

Answer 3

just one type of atom

Question 4

what does a compound contain?

Answer 4

two or more types of atom joined together

Question 5

how could you distinguish between pure substances and mixtures? (2)

Answer 5

Pure substances have a sharp melting point

mixtures melt over a range of temperatures

Question 6

what would an impure substance produce in relation to temperature?

Answer 6

gradual decrease over a range of temperatures as it freezes.

Question 7

what does the horizontal part of a temperature-time graph show?

Answer 7

that the substance has a sharp melting point

Question 8

what is the purpose of filtration?

Answer 8

to separate an insoluble solid from a liquid

Question 9

what is filtration particularly useful for?

Answer 9

separating excess reactant from a solution.

Question 10

why does filtration work? (2)

Answer 10

filter paper has tiny holes, or pores, in it.

large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.

Question 11

what is the purpose of crystallisation?

what happens during the process?

Answer 11

• to produce solid crystals from a solution.

- when the solution is warmed, some of the solvent evaporates leaving behind a more concentrated solution.

Question 12

describe the process used to obtain large, regularly shaped crystals: (4)

Answer 12

1-put the solution in an evaporating basin
2-warm the solution by placing the evaporating basin over a boiling water bath
3-stop heating before all the solvent has evaporated

-After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or in air.

Question 13

what is the purpose of simple distillation?

Answer 13

to separate a solvent from a solution

Question 14

what is simple distillation particularly useful for?

Answer 14

producing water from salt solution.

Question 15

why does simple distillation work?

Answer 15

the dissolved solute has a much higher boiling point than the solvent

Question 16

describe the process of simple distillation: (3)

Answer 16

• When the solution is heated, solvent vapour evaporates from the solution.
• The gas moves away and is cooled and condensed.
• The remaining solution becomes more concentrated in solute as the amount of solvent in it decreases.

Question 17

what is the purpose of fractional distillation?

Answer 17

to separate different liquids from a mixture of liquids.

Question 18

what is fractional distillation particularly useful for?

Answer 18

separating ethanol from a mixture of ethanol and water, and for separating different fractions from crude oil.

Question 19

why does fractional distillation work?

Answer 19

different liquids have different boiling points

Question 20

what happens when a mixture is heated in fractional distillation? (3)

Answer 20

• vapours rise through a column which is hot at the bottom, and cold at the top
• vapours condense when they reach a part of the column that is below the temperature of their boiling point
• the liquid flows out of the column

Question 21

describe the two ways of obtaining different liquids from the column in fractional distillation: (2)

Answer 21

• By collecting different liquids from different parts of the column. The substance with the lowest boiling point is collected at the top of the column.
• By continuing to heat the mixture to increase the temperatures in the column. The substance with the lowest boiling point is collected first.

Question 22

what is the purpose of chromatography?

Answer 22

to separate mixtures of soluble substances

Question 23

describe the chromatography process:

Answer 23

1. Water and ethanol solution is heated
2. As the paper is lowered into the solvent, some of the dye spreads up the paper
3. The paper has absorbed the solvent, and the dye has spread further up the paper

Question 24

what does chromatography rely on? and what are they called? (2)

Answer 24

two different phases

- stationary and mobile phase

Question 25

what is the stationary phase?

Answer 25

very uniform, absorbent paper

Question 26

what is the mobile phase?

Answer 26

the solvent that moves through the paper, carrying different substances with it

Question 27

describe the attractions of the different substances in a mixture? (2)

Answer 27

• the different dissolved substances in a mixture are attracted to the two phases in different proportions.
• this causes them to move at different rates through the paper.

Question 28

what does Separation by chromatography produce?

Answer 28

chromatogram

Question 29

what can a paper chromatogram be used to distinguish between?

Answer 29

pure and impure substances

Question 30

results of pure and impure substances on a chromatogram: (2)

Answer 30

a pure substance produces one spot on the chromatogram

an impure substance produces two or more spots