1.
State what is meant by a pure substance.
[1 mark]
- single element or compound / sharp melting point
JD SCIENCE
Unit 7 answers — Chemical analysis
Indicative marking points. Award marks for equivalent scientific or mathematical wording. Do not issue this sheet with the student worksheet.
2.
State what is meant by a formulation.
[1 mark]
- mixture designed as a useful product
3.
State what is meant by Rf value.
[1 mark]
- distance spot ÷ distance solvent front
4.
State what is meant by chromatography.
[1 mark]
- separating dissolved substances
5.
State what is meant by a flame test.
[1 mark]
- identifies some metal ions by colour
6.
State what is meant by a cation test.
[1 mark]
- sodium hydroxide gives coloured precipitates
7.
State what is meant by an anion test.
[1 mark]
- carbonate, sulfate, halide tests
8.
State what is meant by instrumental analysis.
[1 mark]
- machines such as flame photometers
9.
State what is meant by a melting point.
[1 mark]
- temperature at which a solid becomes liquid
10.
State what is meant by limewater.
[1 mark]
- calcium hydroxide solution
11.
Owain is asked about a pure substance in a WJEC Chemistry paper.
(a) [2 marks]
- not a mixture
(b) [3 marks]
- used to check purity
12.
Owain is asked about a formulation in a WJEC Chemistry paper.
(a) [2 marks]
- paints, medicines, alloys
(b) [3 marks]
- measured quantities
13.
Owain is asked about Rf value in a WJEC Chemistry paper.
(a) [2 marks]
- identifies substances
(b) [3 marks]
- must be 0 to 1
14.
Owain is asked about chromatography in a WJEC Chemistry paper.
(a) [2 marks]
- stationary and mobile phases
(b) [3 marks]
- different solubilities
15.
Owain is asked about a flame test in a WJEC Chemistry paper.
(a) [2 marks]
- lithium red, sodium yellow, potassium lilac, calcium orange-red, copper green
(b) [3 marks]
- clean wire needed
16.
Owain is asked about a cation test in a WJEC Chemistry paper.
(a) [2 marks]
- Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown
(b) [3 marks]
- add NaOH
17.
Rf = 4.2 / 6.0
[3 marks]
- Final answer: 0.70
18.
A solvent front is 8.0 cm and Rf is 0.25. How far did the spot travel?
[3 marks]
- Final answer: 2.0 cm
19.
How many ions are tested if five flame colours are recorded?
[3 marks]
- Final answer: five metal ions
20.
A solution gives a white ppt with acidified BaCl₂. Which ion?
[3 marks]
- Final answer: sulfate
21.
A solution gives a cream ppt with AgNO₃ that dissolves in concentrated NH₃. Which halide?
[3 marks]
- Final answer: bromide
22.
Percentage purity if 9.6 g of product contains 9.12 g of the pure compound
[3 marks]
- Final answer: 95%
23.
Owain carries out a WJEC-style required practical linked to chemical analysis. This is a typical WJEC required-practical style question.
(a) [2 marks]
- Independent: the independent variable for this topic
- Dependent: the measured outcome
(b) [2 marks]
- keep all other variables constant
- so the test is fair / only one variable is changed
(c) [2 marks]
- repeat and calculate a mean / use a tighter method
24.
Owain carries out collecting valid data for chemical analysis. This is a typical WJEC required-practical style question.
(a) [2 marks]
- Independent: a chosen factor
- Dependent: a quantitative measurement
(b) [2 marks]
- calibrated instrument
- so the test is fair / only one variable is changed
(c) [2 marks]
- systematic error from zero error
25.
Owain carries out drawing a graph for chemical analysis. This is a typical WJEC required-practical style question.
(a) [2 marks]
- Independent: x-variable
- Dependent: y-variable
(b) [2 marks]
- same scale / same apparatus
- so the test is fair / only one variable is changed
(c) [2 marks]
- not drawing a line of best fit through the trend
26.
Owain carries out evaluating a method about chemical analysis. This is a typical WJEC required-practical style question.
(a) [2 marks]
- Independent: method change
- Dependent: precision / accuracy
(b) [2 marks]
- same sample size
- so the test is fair / only one variable is changed
(c) [2 marks]
- too few repeats
27.
Compare a pure substance with a formulation.
[4 marks]
- single element or compound / sharp melting point
- mixture designed as a useful product
- give one linked difference
28.
Compare a formulation with Rf value.
[4 marks]
- mixture designed as a useful product
- distance spot ÷ distance solvent front
- give one linked difference
29.
Compare Rf value with chromatography.
[4 marks]
- distance spot ÷ distance solvent front
- separating dissolved substances
- give one linked difference
30.
Compare chromatography with a flame test.
[4 marks]
- separating dissolved substances
- identifies some metal ions by colour
- give one linked difference
31.
Apply your knowledge of an anion test to this situation: HCl + limewater; BaCl₂; AgNO₃
[3 marks]
- HCl + limewater; BaCl₂; AgNO₃
32.
Apply your knowledge of instrumental analysis to this situation: faster, more accurate, sensitive
[3 marks]
- faster, more accurate, sensitive