C2.1

Question 1

What is meant by the term “a pure substance”?

Answer 1

A substance is pure if it is made up completely of one element or compound

Question 2

How would you test a liquid to see if it was water? (assume the water is pure)

Answer 2

If it freezes at exactly 0 degrees and boils at exactly 100 degrees then it is pure water

Question 3

Does adding an impurity to a substance increase of decrease its boiling point?

Answer 3

Increases its boiling point. Eg: seawater boils at around 100.6 degrees

Question 4

Does adding an impurity to a substance increase its melting point?

Answer 4

It will be lower, for example sea water will freeze/melt at -2 degrees

Question 5

What is simple distillation used for?

Answer 5

Separating a liquid from a solution

Question 6

Describe the process of simple distillation (4-5 steps)

Answer 6

Simple distillation of seawater:

1) pour your sample into a distillation flask
2) set up your apparatus (diagram in textbook because i dont have Brainscape pro)
3) gradually heat the distillation flask and the part of the solution that has the lowest boiling point will boil first (in our case it will be water)
4) the water vapour will pass through the condenser where it cools and condenses and then will flow into the beaker where it is collected
5) you will be left with salt in the distillation flask

Question 7

Why is fractional distillation used?

Answer 7

To separate a mixture with many different liquids in it, all of which have different boiling points

Question 8

Describe the process of fractional distillation (4 steps)

Answer 8

1) put mixture in a flask and connect fractionating column
2) gradually heat the flask, the liquids all have different boiling points and so will all boil at different times
3) the liquid with the lowest boiling point will boil first and then condense in the condenser
4) the liquid will collect in a test tube, switch this test tube with a new one for the next liquid and repeat until the whole mixture is separated

Question 9

Propan-1-ol, methanol and ethanol have boiling points of 97 °C, 65 °C and 78 °C respectively.
A student uses fractional distillation to separate a mixture of these compounds.
State which liquid will be collected in the second fraction and explain why. (2 marks)

Answer 9

Methanol because it has the second highest boiling point and so it will be the second liquid to boil and condense into a test tube

Question 10

What is filtration used for?

Answer 10

To separate an insoluble solid from a liquid

Question 11

Why do we use crystallisation?

Answer 11

To separate a soluble solid from a solution

Question 12

Describe a method to separate pure copper sulfate crystals from water (4 marks)

Answer 12

• slowly heat the solution to evaporate off some of the water
• once copper crystals begin to form, stop heating the solution
• allow the solution to cool down until copper sulfate crystals form
• filter the crystals out of the solution

Question 13

What is the mobile phase in paper chromatography? (2 marks)

Answer 13

It is the phase where molecules can move, in paper chromatography this is the solvent

Question 14

What is the stationary phase in paper chromatography? (2 marks)

Answer 14

The stationary phase is the phase where molecules cannot move, for paper chromatography this is the chromatography paper

Question 15

What is the formula for the Rf value of a substance

Answer 15

Distance travelled by solute
Rf= —————————————
Distance travelled by solvent (baseline to top pencil line)

Question 16

What is the mobile phase is gas chromatography? (1 mark)

Answer 16

An unreactive gas like nitrogen

Question 17

What is the stationary phase in gas chromatography? (1 mark)

Answer 17

A viscous liquid like oil

Question 18

Describe the process of gas chromatography (3 steps)

Answer 18

1) The unknown mixture is injected into a long tube coated on the inside with the stationary phase
2) the mixture moves along the tube with the mobile phase until it comes out of the others end. Each substance in the mixture spends different amounts of time dissolved in the mobile phase and stuck to the stationary phase
3) the time it takes for a chemical to pass through is called retention time and the retention time for each chemical is different, allowing us to identify them

Question 19

What does each peak on a gas chromatography represent?

Answer 19

It represents a different chemical in the mixture

Question 20

What do the relative areas under each peak show?

Answer 20

They show the amount of the chemical in the mixture

Question 21

What is the relative atomic mass of a substance?

Answer 21

It is the average mass of one atom of an element, compared to 1/12 of the mass of one atom of carbon-12

Question 22

What is the relative formula mass of NaCl?

Answer 22

58.5

Question 23

What is a molecule?

Answer 23

Something made from 2 or more atoms, and those atoms are made up of chemical bonds

Question 24

What is a formulation?

Answer 24

Formulations are mixtures that have been prepared using a specific formula