Topic 8: Chemical Analysis

Question 1

What is a chemically pure substance?

Answer 1

A substance made up of only a single element or compound that has exact and specific melting and boiling points

Question 2

How can you test the purity of a substance? (4)

Answer 2

1) Chemically pure substances melt or boil at specific temperatures
2) Measure the melting and boiling points of a sample
3) Compare the results to the values of the pure substance from a data book
4) The closer the values the purer the substance

Question 3

What can impurities in a substance cause? (2)

Answer 3

1) Increases the melting range by lowering the melting point

2) Increases the boiling range by increasing the boiling point

Question 4

What are formulations?

Answer 4

Mixtures of chemicals that are designed to create useful products

Question 5

To meet its required function, what must each component be? (2)

Answer 5

1) Each component must be a precisely measured quantity
2) Each component must contribute to the properties of the formulation

So that the formulation meets its required function

Question 6

Give examples of formulations? (3)

Answer 6

1) Paint
2) Fuels
3) Medicines

Question 7

What is the formulation of paint? (4)

Answer 7

1) Pigment - gives paint colour
2) Solvent - dissolves other components, alters viscosity
3) Binder - forms a film that holds pigment in place
4) Additives - further changes any properties of paint

Question 8

What is chromatography?

Answer 8

The separation of a mixture by passing it in solution through a medium in which the components move at different rates

Question 9

What are the “phases” of chromatography? (2)

Answer 9

Mobile phase - where molecules can move or the solvent

Stationary phase - where molecules can’t move or the paper

Question 10

What are the two stages of chromatography?

Answer 10

1) The substance is picked up by the mobile phase

2) The mobile phase moves through the stationary phase

Question 11

What affects the extent of the separation of substances? (3)

Answer 11

1) A substance doesn’t move far if it is more attracted to the stationary phase
2) A substance moves further if it is more attracted to the mobile phase
3) A substance moves further if it is more soluble
4) The number of spots and how far away they are will change depending on the solvent

Question 12

Describe the process of chromatography (5)

Answer 12

1) Use a pencil to draw a horizontal line near the bottom of the chromatography paper
2) On the line put samples of known food colourings alongside an unknown substance
3) Place the paper in a beaker containing a small volume of solvent (deionised water)
4) Wait for the solvent to travel to near the top of the paper
5) Compare spots produced by unknown substance the known food colourings to identify the unknown substance.

Question 13

What is the result of a chromatography analysis called?

Answer 13

Chromatogram

Question 14

What is the Rf value of a chemical?

Answer 14

The ratio between the distance travelled by the substance and the distance travelled by the solvent

Question 15

What can we use Rf values to determine?

Answer 15

To determine if a certain substance is present in a mixture

Question 16

How can we use Rf values to determine the presence of a certain substance?

Answer 16

If the Rf values for a pure sample of the substance (or reference) match the unknown mixture then the substance may be present in the mixture