Separate Chemistry - C8 Chemical Analysis

Question 1

What is the chemical meaning of a ‘pure’ substance?

What is the every day meaning of a ‘pure’ substance?

Answer 1

In Chemistry: A single element or compound that is not mixed with any other substance.

In everyday language: A substance that hasn’t had anything added to it, e.g. pure milk.

Question 2

What data can be used to prove a substance is ‘pure’ and not a mixture?

Answer 2

Melting and Boiling points (because pure substance melt and boil at specific temperatures)

Question 3

What is a formulation?

Answer 3

A mixture that has been designed as a useful product.

Question 4

Give three examples of formulations

Answer 4

Any three from:

Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Question 5

Name the two phases in chromatography.

Answer 5

• Stationary Phase
• Mobile Phase

Question 6

In Chromatography, how do you calculate the R_f value for a substance?

Answer 6

R_f = distance moved by substance / distance moved by solvent

Question 7

What will the chromatogram of a pure substance look like?

Answer 7

A pure substance will only show a single spot on the chromatogram.

Question 8

Explain how paper chromatography can be used to separate a mixture.

Answer 8

• A substance is added to the chromatography paper (stationary phase).
• Molecules within a substance dissolve in the solvent and move up the paper (mobile phase).
• The molecules within the substance separate because different chemicals spend different amounts of time in the mobile and stationary phases and therefore move different distances up the paper.

Question 9

Describe how you would test for Hydrogen.

Answer 9

• Collect gas in a test tube
• Hold a burning splint at the open end of a test tube.
• You will hear a ‘pop’ sound if Hydrogen is present

Question 10

Describe how you would test for Oxygen.

Answer 10

• Collect gas in a test tube
• Insert a glowing splint into the test tube
• The splint will relight if oxygen is present

Question 11

Describe the test for Carbon dioxide.

Answer 11

• Bubble the gas through limewater (calcium hydroxide solution)
• If the limewater turns milky (cloudy) then carbon dioxide is present

Question 12

Describe the test for Chlorine.

Answer 12

• Collect gas in a test tube
• Place damp litmus paper in the test tube
• If chlorine is present, the litmus paper is bleached and turns white

Question 13

Separate Q. What type of ions are metal ions?

Answer 13

Cations

Question 14

Separate Q. What would the flame test results be for the following metal compounds?

a) Lithium
b) Sodium
c) Potassium
d) Calcium
e) Copper

Answer 14

a) Crimson flame
b) Yellow flame
c) Lilac flame
d) Orange-red flame
e) Green flame

Question 15

Separate Q. If a sample contains a mixture of metals, why might it be difficult to identify the metals using a flame test?

Answer 15

Some flame colours can be masked by others

Question 16

Separate Q. Describe the results you would see when adding sodium hydroxide to compounds containing the following metals:

a) Aluminium
b) Calcium
c) Magnesium
d) Copper (II)
e) Iron (II)
f) Iron (III)

Answer 16

a) Aluminium: White precipitate
b) Calcium: White precipitate
c) Magnesium: White precipitate
d) Copper (II): Blue Precipitate
e) Iron (II): Green Precipitate
f) Iron (III): Brown Precipitate

Question 17

Separate Q. Name the solution added to metal compounds to test for metal ions.

Answer 17

Sodium hydroxide solution

Question 18

Separate Q. Aluminium, calcium and magnesium all form white precipitates when sodium hydroxide is added. If sodium hydroxide is added in excess, which precipitate will dissolve?

Answer 18

Aluminium hydroxide

Question 19

Separate Q. Complete the word equations for the reaction of sodium hydroxide with:

a) Calcium nitrate
b) Copper sulfate

Answer 19

a) calcium nitrate + sodium hydroxide → calcium hydroxide + sodium nitrate
b) copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfate

Question 20

Separate Q. Describe the test for a carbonate.

Answer 20

• Add dilute acid to the unknown compound.
• Bubble gas produced through limewater.
• If limewater turns milky (cloudy) then the compound is a carbonate.

Question 21

Separate Q. Describe the test for Halide ions.

Answer 21

• Add silver nitrate and dilute nitric acid to your unknown solution.
• If a precipitate is produced, the unknown solution contains Halide ions.
• Silver chloride is white, silver bromide is cream, silver iodide is yellow.

Question 22

Separate Q. Complete the word equations for the following reactions:

Silver nitrate + Barium chloride →

Silver nitrate + Barium bromide →

Silver nitrate + Barium iodide →

Answer 22

Silver nitrate + Barium chloride → Silver chloride + Barium nitrate

Silver nitrate + Barium bromide → Silver bromide + Barium nitrate

Silver nitrate + Barium iodide → Silver iodide + Barium nitrate

Question 23

Separate Q. Describe the test for sulfate ions.

Answer 23

• Add barium chloride and dilute hydrochloric acid to your unknown solution.
• A white precipitate will form if sulfate ions are present.

Question 24

Separate Q. Give three advantages of using instrumental methods to identify substances.

Answer 24

• Fast
• Accurate
• Sensitive

Question 25

Separate Q. Name the instrumental method that can be used to identify metal ions in solutions.

Answer 25

Flame emission spectroscopy

Question 26

Separate Q. Describe how flame emission spectroscopy works.

Answer 26

• Sample is put in a flame
• Light emitted by flame is passed through a spectroscope
• Line spectrum output is produced by the machine
• This line spectrum is used to identify the metal ions and measure their concentration in the solution.

Question 27

Separate Q. Identify the metal ion present in the solution using the flame emission spectrometry data below.

Answer 27

The unknown metal ion is Ion Y because the line spectrum for the unknown metal ion matches the reference spectrum for Ion Y.