C8 - Chemical Analysis

Question 1

What is a pure substance in chemistry (2)

Answer 1

• Is a single element or compound, not mixed with any other substance
• Will melt or boil at a specific temperature

Question 2

What is a pure substance in everyday language

Answer 2

Substance that has had nothing added to it, so it is unadulterated and in it natural state e.g pure milk

Question 3

How can you use melting point and boiling point data to distinguish pure from impure substances (3)

Answer 3

• The closer your measured value is to the actual melting or boiling point, the purer your sample is
• Impurities in your sample will lower the melting point and increase the melting range of your substance
• Impurities in your sample will also increase the boiling point and may result in your sample boiling at a range of temperatures

Question 4

Formulations (3)

Answer 4

• Is a mixture which has been designed as a useful product
• Are made by mixing the components in carefully measured quantities to ensure that the product has the required properties
• e,g Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods

Question 5

Chromatography (2)

Answer 5

• Used to separate mixtures and can give information to help identify substances
• Involves a stationary phase and a mobile phase

Question 6

Formula to calculate Rf

Answer 6

Distance moved by substance divided by distance moved by the solvent

Question 7

How does paper chromatography separate mixtures (3)

Answer 7

• The stationary phase is the chromatography paper and the mobile phase is the solvent
• How soluble they are in a solvent and how attracted they are to the paper influences the amount of time the molecules spends in each phase
• Molecules with higher solubility and which are less attracted to the paper will spend more time in the mobile phase

Question 8

Required practical - Calculating Rf values (5)

Answer 8

1) Draw a horizontal across the chromatography paper near the bottom
2) Place a dot each colour on the line using a capillary tube
3) Place the beaker so that the bottom of the paper is in the water
4) Use the ruler to measure the distance of each dot from the starting line
5) Calculate Rf values using the formula

Question 9

Test for Hydrogen

Answer 9

• Use a burning splint held at the open end of a test tube of the gas
• Hydrogen burns rapidly with a pop sound

Question 10

Test for Oxygen (2)

Answer 10

• Uses a glowing splint inserted into a test tube of the gas
• The splint relights in Oxygen

Question 11

Test for the Carbon dioxide (2)

Answer 11

• Use an aqueous solution of calcium hydroxide (limewater)

* When carbon dioxide is shaken with or bubbled through limewater and the limewater turns milky (cloudy)

Question 12

Test for chlorine

Answer 12

Bleaches damp litmus paper, turning it white