Chemical analysis

Question 1

What is a pure substance?

Answer 1

A substance is a single element or compound not mixed with any other substance.

Question 2

Is a pure substance melting point and boiling point specific and fixed?

Answer 2

Yes a pure substance has a specific and fixed melting and boiling point.

Question 3

What is the difference in boiling point in an impure substance compared to a pure substance.

Answer 3

Impure substances will boil over a range of temperatures.

Question 4

What is a formulation?

Answer 4

A mixture that has been designed as a useful product.

Example: Medicines, fuels, food

Question 5

Describe how you could test whether water is pure or not?

Answer 5

Heat until it boils [1]

Measure the boiling point / temperature when it boils [1]

Boils at exactly 100 degrees celsius / °C [1]

Question 6

Name a process that could be used to obtain pure water from a sample of sea water.

Answer 6

distillation

Question 7

How you test for hydrogen?

Answer 7

Squeaky pop test.

Using a tube with hydrogen and lighting a splint and if it makes a pop noise then it contains hydrogen.

Question 8

How do you test for oxygen?

Answer 8

Fuels need oxygen to burn.

Using a burnt out splint

Insert it in the tube you collected the oxygen

If it has oxygen the splint is relighted

Question 9

How do you test for carbon dioxide?

Answer 9

Have the reaction going in on one test tube.

Have a delivery tube to another tube.

Inside the other tube is lime water.

If the lime water turns cloudy white then it contains carbon dioxide.

Question 10

How do you test for chlorine?

Answer 10

Using damp blue litmus paper,

Put it into the tube with chlorine

and if it has chlorine the paper turns white (bleaches)

Question 11

What reaction creates carbon dioxide?

Answer 11

Metal carbonate plus acid.

Question 12

What reaction creates hydrogen?

Answer 12

Metal plus acid

Question 13

How do you tell if a substance is pure using chromatography.

Answer 13

If there is only one colour that moves up the paper.

Question 14

How do you tell if a substance is impure using chromatography?

Answer 14

If there are multiple colour that move up the paper.

Question 15

Do more soluble substances move up or down the paper in chromatography?

Answer 15

Down

Question 16

How do you calculate Rf value?

Answer 16

Distance moved by substance / distance moved by solvent

Question 17

Why are Rf values important?

Answer 17

Allows us to compare with other samples.

Allows us to identify unknown substances.

Question 18

How do you do the required practical “Using Chromatography to identify a mixture of food colourings”

Answer 18

Set up a chromatography how you usually would:

Use a beaker that is filled with water 1cm high.

Draw a line on your piece of paper and mark the spots for the substances using pencil.

Hang the paper into the beaker using a glass rod.

Make sure the pencil line doesn’t touch the water.

Let the colours move up the paper.

After draw a line where the water had reached the paper and let the paper dry.

Calculate Rf values of each substance and check if they match the unknown substance.