C.10

Question 1

What is a pure substance?

Answer 1

A substance made up of only ONE element OR compound and not mixed with any other substances

Question 2

How can we determine if a substance is pure or impure?

Answer 2

Pure substances have a fixed melting point and boiling point. Impure substances have a range for both.

Question 3

What is a formulation?

Answer 3

A complex mixture designed to be a useful product. Each component is measured carefully to the get the desired properties e.g. fuels, medicines, paint.

Question 4

What is the difference in purity in advertising and chem?

Answer 4

In advertising, pure means nothing is added to a product. In chemistry, it means the substance is made of ONE element or compound.

Question 5

Why is paper chromotography useful?

Answer 5

It allows us to identify pure and impure substances, and to seperate substances by their solubilities.

Question 6

How do you do the chromatography practical?

Answer 6

1. Draw a line in pencil 1cm up from the bottom of a sheet of chromatography paper.
2. Use a capillary tube to place different dye spots along the line
3. Attach the top of the paper to a beaker and secure it with tape. Ensure the solvent in the beaker (in this case, water) is below the pencil line and they do not touch.
4. Leave the paper for a while, until the solvent has reached around 3/4 of the way up the paper.
5. Remove and leave to dry.

Question 7

Why must the line be drawn in pencil?

Answer 7

Because if it is drawn in ink, the ink will spread with the solvent.

Question 8

How can we tell if the substances used were pure or impure?

Answer 8

• A pure substance will produce a single spot in any solvent
• An impure substance may seperate into different spots depending on the solvent

More soluble substances will move further up the paper than less soluble substances

Question 9

What is the stationary and mobile phase?

Answer 9

The stationary phase in this experiment is the paper as it does not move. The mobile phase is the solvent as it does move.

Question 10

How do we calculate the substance’s Rf value?

Answer 10

Rf = distance travelled by substance / distance travelled by solvent

• Remember Rf has NO UNIT and is always between 0 and 1

Question 11

How do we use Rf values?

Answer 11

Rf values can be used to identify an unknown substance. If you calculate an Rf value, you can research the substance that corresponds to that value. However, you may want to repeat the experiment with a different solvent as some substances have the same Rf value.

Question 12

What is filtration and how does it work?

Answer 12

Filtration is used to seperate a liquid and insoluble solid.

1. Pour the substance into a filter funnel lined with filter paper
2. The liquid will filter through and into the beaker and the solid will stay in the paper. The liquid part is called the FILTRATE

Question 13

What is crystallisation and how does it work?

Answer 13

Crystallisation is used to seperate a soluble solid and a liquid (aqueous solution)

• Pour the solution into a beaker and leave for a few days. The liquid will evaporate leaving behind solid crystals. This can be sped up using a gentle heat.

Question 14

What is simple distillation and how does it work?

Answer 14

Simple distillation is used to seperate an aqueous solution but keeps the liquid.

1. Set up a flask containing the solution and connect it to a glass tube which leads to a beaker. Ensure the glass tube is surrounded by a cooling jacket containing cold water.
2. Heat the solution until the liquid evaporates and leaves behind the crystallised solid.
3. As the liquid passes through the glass tube, it will condense as it passes through the cooling jacket, where it will turn back into a liquid again.

Question 15

What is one use of simple distillation?

Answer 15

It can be used to seperate seawater into drinking water, however, it is inefficient as it requires a lot of energy.

Question 16

What is fractional distillation and how does it work?

Answer 16

Fractional distillation is used to seperate a mixture of substances by their boiling points

• The equipment set up is the same as fractional distillation except: the flash must contain at least 2 solutions with different boiling points, and the glass tube must be filled with glass beads to make it a fractionating column.
• The substance with a lower boiling point will evaporate and condense first, meaning it can be collected before the substance with the higher boiling point.

Question 17

What are the limits of fractional distillation?

Answer 17

• It is only useful for a small amount of liquid
• It is difficult to seperate liquids with very similar boiling points

Question 18

What is the test for hydrogen?

Answer 18

• Remove the bung from a test tube and hold a burning splint over it. If it produces a pop sound, then hydrogen is present.

Question 19

What is the test for oxygen?

Answer 19

• Hold a glowing splint over a tube and if it re-ignites, then oxygen is present in the reaction.

Question 20

What is the test for carbon dioxide?

Answer 20

• Use a pipette to take carbon dioxide from a test tube and add it to a tube of limewater so it bubbles. If the solution turns cloudy, carbon dioxide is present.

*limewater = aqueous solution of calcium hydroxide

Question 21

What is the test for chlorine?

Answer 21

• Insert a piece of damp litmus paper into a test tube, and if it gets bleached white, chlorine is present