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F3 Chemistry Recap: Elements, Compounds, Mixtures and Air | F3 (2026)

F3 Chemistry — Elements, Compounds, Mixtures and Air

Subject: Chemistry

F3 (2026)

Issue date: Wednesday, 14 October 2026

Total: 100 marks  |  Suggested time: 110–120 minutes  |  Topic: elements, compounds and mixtures, the Bunsen burner, simple laboratory apparatus, air and its uses, fractional distillation of liquid air

Instructions

  1. Question 1 consists of 20 multiple-choice questions. Each has one correct answer. Write your answers in the answer boxes at the end of Question 1.
  2. In Question 2, state whether each statement is True or False. If a statement is false, rewrite it correctly.
  3. Questions 3 to 12 are structured questions. Write your answers in the spaces provided. Show your working clearly in any calculation.
  4. Where an equation is asked for, a word equation is sufficient.
  5. The letters used for gases in the questions are not the actual symbols, unless stated otherwise.
  6. Some data are given below.

Paper structure

Question Type Marks Score
1 Multiple choice (20 questions) 20
2 True or false (20 statements) 20
3 Comprehensive structured question 1 6
4 Comprehensive structured question 2 6
5 Comprehensive structured question 3 6
6 Comprehensive structured question 4 6
7 Comprehensive structured question 5 6
8 Comprehensive structured question 6 6
9 Comprehensive structured question 7 6
10 Comprehensive structured question 8 6
11 Comprehensive structured question 9 6
12 Comprehensive structured question 10 6
Total 100
Data — boiling points at atmospheric pressure: nitrogen = −196 °C, argon = −186 °C, oxygen = −183 °C.

Total score: / 100

01

Section A — Elements, Compounds, Mixtures and Air

Multiple Choice

Each question has one correct answer. Choose the best option. (1 mark each)

  1. Which of the following is an element?
    1. Sea water
    2. Sulphur
    3. Carbon dioxide
    4. Magnesium oxide
  2. Which of the following is a compound?
    1. Air
    2. Brass
    3. Mercury
    4. Magnesium oxide
  3. Which of the following statements about a mixture is correct?
    1. Its components are chemically combined.
    2. It has a fixed composition.
    3. Its components keep their own properties.
    4. Its components can be separated only by chemical methods.
  4. Iron powder and sulphur powder are mixed and heated strongly. The mixture glows and a black solid, iron(II) sulphide, is formed. Which of the following statements is correct?
    1. Iron(II) sulphide is a compound and cannot be separated into iron and sulphur using a magnet.
    2. Iron(II) sulphide is a mixture of iron and sulphur.
    3. Iron(II) sulphide still has the properties of both iron and sulphur.
    4. Iron and sulphur combine to form iron(II) sulphide without any heating.
  5. Which of the following pairs consists of one element and one mixture?
    1. Oxygen and carbon dioxide
    2. Water and sea water
    3. Sodium chloride and magnesium oxide
    4. Copper and brass
  6. Which of the following is the correct way to light a Bunsen burner?
    1. Open the air hole, turn on the gas, then bring a lighted match to the chimney.
    2. Close the air hole, bring a lighted match to the top of the chimney, then turn on the gas.
    3. Turn on the gas, then close the air hole and strike a match.
    4. Open the air hole fully, strike a match, then turn on the gas.
  7. Which of the following describes the flame of a Bunsen burner when the air hole is closed?
    1. Blue and non-luminous
    2. Blue and very hot
    3. Pale and almost invisible
    4. Yellow and luminous
  8. A student wants to heat a beaker of water quickly using a Bunsen burner. Which flame is the most suitable, and why?
    1. The yellow flame, because it is hotter.
    2. The yellow flame, because it does not produce soot.
    3. The blue flame, because it is hotter and does not leave soot on the beaker.
    4. The blue flame, because it is easier to see.
  9. Which of the following is a safe practice when heating a liquid in a test tube over a Bunsen burner?
    1. Hold the test tube with a test tube holder and tilt it slightly, with its mouth pointing away from people.
    2. Fill the test tube to the top so that the liquid heats evenly.
    3. Point the mouth of the test tube towards a neighbour so that he or she can see the liquid.
    4. Heat the bottom of the test tube strongly without moving it.
  10. Which apparatus should be used to measure 25.0 cm3 of water most accurately?
    1. Beaker
    2. Conical flask
    3. Test tube
    4. Measuring cylinder
  11. Which pair of apparatus is used to support a beaker above the flame of a Bunsen burner?
    1. Test tube rack and heat-proof mat
    2. Tripod and wire gauze
    3. Retort stand and spatula
    4. Tongs and glass rod
  12. Which of the following apparatus–use pairs is correct?
    1. Spatula — transferring a small amount of a solid chemical
    2. Tongs — adding a liquid drop by drop
    3. Dropper — holding a hot crucible
    4. Thermometer — measuring the mass of a solid
  13. Which of the following shows the three most abundant gases in dry air, in order of decreasing percentage by volume?
    1. oxygen > nitrogen > argon
    2. nitrogen > oxygen > carbon dioxide
    3. nitrogen > oxygen > argon
    4. nitrogen > argon > oxygen
  14. In an experiment, 100 cm3 of air was passed repeatedly over heated excess copper powder using two gas syringes. After cooling to room temperature, 79 cm3 of gas remained. What is the percentage by volume of oxygen in the air, according to this result?
    1. 12%
    2. 21%
    3. 29%
    4. 79%
  15. Which of the following statements best shows that air is a mixture and not a compound?
    1. Air is a colourless gas.
    2. Air has a definite chemical formula.
    3. Air cannot be turned into a liquid.
    4. The oxygen in air still supports burning, and the gases in air can be separated by physical methods.
  16. Which of the following gives a correct use of the gas?
    1. Oxygen — filling snack packets to keep the food fresh
    2. Neon — filling weather balloons
    3. Nitrogen — liquid nitrogen is used as a coolant to freeze food quickly
    4. Argon — breathing gas for hospital patients
  17. Why is argon used to fill filament light bulbs?
    1. It is unreactive, so the hot filament does not burn away.
    2. It supports the burning of the filament.
    3. It is a good fuel.
    4. It is lighter than air.
  18. Liquid air is warmed slowly in a fractionating column. The boiling points of nitrogen, argon and oxygen are −196 °C, −186 °C and −183 °C respectively. Which gas boils off first?
    1. Oxygen
    2. Argon
    3. Nitrogen
    4. Carbon dioxide
  19. Water vapour and carbon dioxide are removed from air before the air is cooled to very low temperatures. Why is this necessary?
    1. They would freeze into solids and block the pipes.
    2. They would react violently with liquid nitrogen.
    3. They are too light to be turned into liquids.
    4. They would make the liquid air coloured.
  20. The steps below are some of the steps in obtaining gases from air by the fractional distillation of liquid air.
    • (1) The liquid air is warmed slowly in a fractionating column.
    • (2) The air is filtered to remove dust.
    • (3) Water vapour and carbon dioxide are removed.
    • (4) The remaining air is cooled to about −200 °C to form liquid air.
    Which of the following shows the correct order of the steps?
    1. (2), (4), (3), (1)
    2. (2), (3), (4), (1)
    3. (3), (2), (4), (1)
    4. (2), (3), (1), (4)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
20 marks
02

Section B — Elements, Compounds, Mixtures and Air

True or False

For each statement, state whether it is True or False. If it is false, rewrite the statement correctly. (1 mark each)

  1. An element is a pure substance that cannot be broken down into simpler substances by chemical methods.
  2. Water is an element because it is a clear liquid that cannot be separated by filtration.
  3. The properties of a compound are the same as the properties of the elements it contains.
  4. A mixture can contain its components in any proportion, and each component keeps its own properties.
  5. After iron powder and sulphur powder have been heated together strongly to form iron(II) sulphide, the iron can still be separated from the product using a magnet.
  6. Before a Bunsen burner is lit, the air hole should be closed.
  7. A yellow flame is produced when the air hole of a Bunsen burner is fully open.
  8. The blue flame of a Bunsen burner is hotter than the yellow flame and does not leave soot on the apparatus.
  9. When a Bunsen burner is not being used for heating, the air hole should be opened fully so that the flame is easy to see.
  10. A beaker is more suitable than a measuring cylinder for measuring exactly 10.0 cm3 of a liquid.
  11. When a beaker is heated over a Bunsen burner, a wire gauze is placed on a tripod to spread out the heat.
  12. A spatula is used to transfer a small amount of a solid chemical.
  13. Air is a compound because it consists of nitrogen, oxygen and other gases.
  14. Nitrogen makes up about 78% of air by volume, while oxygen makes up about 21%.
  15. Carbon dioxide is the third most abundant gas in dry air.
  16. Helium is used to fill weather balloons because it is less dense than air and does not burn.
  17. Oxygen is used to fill food packets because it stops the food from reacting with the air.
  18. The fractional distillation of liquid air separates the gases because they have different boiling points.
  19. When liquid air is warmed slowly in a fractionating column, oxygen boils off first because it has the lowest boiling point of the three main gases.
  20. Water vapour and carbon dioxide are removed from air before it is liquefied so that they do not freeze and block the pipes.
20 marks
03

Section C — Comprehensive Structured Question 1

Elements, Compounds and Mixtures

The table below lists six substances.

(a) Complete the table by writing element, compound or mixture for each substance. (3 marks)

Substance Element, compound or mixture?
Sulphur
Sea water
Carbon dioxide
Copper
Air
Magnesium oxide

(b) Write the chemical symbols of the two elements in the table. (1 mark)

(c) State two differences between a compound and a mixture. (2 marks)

6 marks
04

Section C — Comprehensive Structured Question 2

Iron and Sulphur: Mixture or Compound?

A student mixes some grey iron powder with yellow sulphur powder in a boiling tube. The student then heats the mixture strongly in a Bunsen burner flame. The mixture glows red. After the boiling tube has cooled down, a black solid is formed.

(a) Before the mixture is heated, a magnet is held close to it. Describe what is observed. (1 mark)

(b)

  1. Name the black solid formed. (1 mark)
  2. Write the word equation for the reaction that takes place when the mixture is heated. (1 mark)

(c) The magnet is now held close to the black solid. No solid is attracted to the magnet. Explain why. (1 mark)

(d) State two differences between the mixture of iron and sulphur before heating and the black solid formed after heating. (2 marks)

6 marks
05

Section C — Comprehensive Structured Question 3

The Bunsen Burner and Its Flames

Fig. 1 shows a Bunsen burner. Four parts of the burner are labelled P, Q, R and S.

P Q R S
Fig. 1 — A Bunsen burner

(a) Name the parts P, Q, R and S. (2 marks)

(b) The burner is giving a yellow flame. What should be done to change it into a blue flame? (1 mark)

(c) State two differences between a blue flame and a yellow flame, other than their colours. (2 marks)

(d) When a Bunsen burner is not being used for heating but is left burning, the air hole should be closed. Explain why. (1 mark)

6 marks
06

Section C — Comprehensive Structured Question 4

Using a Bunsen Burner Safely

A student wants to heat 5 cm3 of water in a test tube using a Bunsen burner. The steps for lighting the Bunsen burner are listed below, but they are not in the correct order.

  • A Turn on the gas at the gas tap.
  • B Strike a match.
  • C Close the air hole of the burner.
  • D Hold the lighted match at the top of the chimney.
  • E Connect the rubber tubing from the gas tap to the burner.

(a) Write the letters A to E in the correct order for lighting the Bunsen burner. (2 marks)

(b) State three safety precautions the student should take when heating the water in the test tube. (3 marks)

(c) After the experiment, how should the student turn off the Bunsen burner? (1 mark)

6 marks
07

Section C — Comprehensive Structured Question 5

Simple Laboratory Apparatus

(a) The table below describes five uses of laboratory apparatus. Write the name of the apparatus that is the most suitable for each use. (5 marks)

Use Apparatus
Measuring 25.0 cm3 of water accurately
Transferring a small amount of a solid powder from a reagent bottle into a test tube
Heating a salt solution until all the water has gone and only the solid is left
Holding a hot test tube while it is being heated
Adding a liquid to a test tube drop by drop

(b) A beaker of water is heated using a Bunsen burner, a tripod and a wire gauze. State the purpose of the wire gauze. (1 mark)

6 marks
08

Section C — Comprehensive Structured Question 6

Air as a Mixture

The table below shows the composition of dry air by volume.

Gas Percentage by volume in dry air
Nitrogen
Oxygen
Noble gases (mainly argon) about 0.9
Carbon dioxide about 0.03

(a) Fill in the approximate percentages by volume of nitrogen and oxygen in the table. (2 marks)

(b) Name two noble gases found in air. (1 mark)

(c) Water vapour is not included in the table. Explain why. (1 mark)

(d) Give two reasons why air is a mixture and not a compound. (2 marks)

6 marks
09

Section C — Comprehensive Structured Question 7

Finding the Percentage of Oxygen in Air

A student uses a hard glass tube containing an excess of copper powder, connected to two gas syringes. At the start, one syringe contains 90 cm3 of air and the other syringe is empty. The student heats the copper strongly with a Bunsen burner and pushes the air slowly back and forth over the copper until the volume of gas no longer changes. After the apparatus has cooled down to room temperature, the total volume of gas in the syringes is 71 cm3.

(a) State the colour change of the copper powder, and write the word equation for the reaction that takes place. (2 marks)

(b) Why must the apparatus be allowed to cool down to room temperature before the final volume of gas is read? (1 mark)

(c) Why is an excess of copper used? (1 mark)

(d) Calculate the volume of oxygen used up and the percentage by volume of oxygen in the air. Give your answer to the nearest whole number. (2 marks)

6 marks
10

Section C — Comprehensive Structured Question 8

Uses of the Gases in Air

Some gases that can be obtained from air are listed below.

nitrogen     oxygen     helium     argon     neon

(a) From the list, choose the gas that is used for each purpose below. Each gas is used once only. (5 marks)

  1. Breathing apparatus for patients in hospitals
  2. Filling food packets to stop the food from going bad
  3. Filling weather balloons
  4. Filling filament light bulbs
  5. Making advertising signs that glow with an orange-red light when electricity passes through

(b) Explain why the gas used in (a)(ii) is suitable for this purpose. (1 mark)

6 marks
11

Section C — Comprehensive Structured Question 9

Obtaining Gases from Liquid Air

Nitrogen, oxygen and argon are obtained from air in the following stages.

  • Stage 1 The air is passed through a filter.
  • Stage 2 The air is compressed and cooled. Water vapour and carbon dioxide are removed.
  • Stage 3 The remaining air is cooled to about −200 °C and becomes liquid air.
  • Stage 4 The liquid air is warmed slowly in a fractionating column (Fig. 2). Nitrogen, argon and oxygen leave the column at X, Y and Z, not necessarily in this order.
Liquid air X Y Z (from the top) (from the middle) (from the bottom) Cooler at the top Warmer at the bottom
Fig. 2 — A fractionating column for liquid air

(a) What is removed from the air in Stage 1? (1 mark)

(b) Explain why water vapour and carbon dioxide must be removed before Stage 3. (1 mark)

(c) Describe the change of physical state that the air undergoes in Stage 3. (1 mark)

(d) Name the process used in Stage 4 to separate the liquid air. (1 mark)

(e) Using the data on the cover page, identify X, Y and Z. (2 marks)

6 marks
12

Section C — Comprehensive Structured Question 10

Fractional Distillation and Boiling Points

Liquid air at −200 °C is placed in a fractionating column and warmed slowly. The table shows the boiling points of the three main gases in liquid air.

Gas Boiling point / °C
Nitrogen −196
Argon −186
Oxygen −183

(a) Name the gas that starts to boil off first and state the temperature at which this happens. (1 mark)

(b) Liquid air is a mixture. Explain why. (1 mark)

(c) At −190 °C, nitrogen is a gas but oxygen is a liquid. Explain why, using the boiling points in the table. (2 marks)

(d) State the physical state of argon at −190 °C. (1 mark)

(e) Oxygen is collected at the bottom of the column. Explain why. (1 mark)

6 marks

Question 1 — Multiple Choice

1. B
2. D
3. C
4. A
5. D
6. B
7. D
8. C
9. A
10. D
11. B
12. A
13. C
14. B
15. D
16. C
17. A
18. C
19. A
20. B
  1. B — Sulphur. Sulphur is a single element. Sea water is a mixture, and carbon dioxide and magnesium oxide are compounds.
  2. D — Magnesium oxide. It is formed when magnesium and oxygen combine chemically. Air is a mixture, brass is a mixture (an alloy of copper and zinc) and mercury is an element.
  3. C — Its components keep their own properties. A and B describe a compound. D is wrong because the components of a mixture can be separated by physical methods.
  4. A. Iron(II) sulphide is a compound with new properties. It is not attracted by a magnet, so the iron cannot be separated out in this way. B and C describe a mixture. D is wrong because heating is needed for the reaction.
  5. D — Copper and brass. Copper is an element and brass is a mixture. A is an element and a compound, B is a compound and a mixture, and C is two compounds.
  6. B. The air hole is closed first, so that the burner gives a yellow flame that is easy to see. The match is lit and held at the top of the chimney, and then the gas is turned on.
  7. D — Yellow and luminous. A closed air hole gives a yellow, luminous flame that leaves soot. A and B describe the blue flame, and the flame is clearly visible, so C is wrong.
  8. C. The blue flame is hotter and does not leave soot on the glassware. The yellow flame is cooler and deposits soot.
  9. A. The test tube is held with a test tube holder, tilted slightly, with its mouth pointing away from people. It should be no more than about one-third full and heated gently while being shaken.
  10. D — Measuring cylinder. A measuring cylinder has a scale that allows volumes of liquid to be measured accurately. Beakers, conical flasks and test tubes are not suitable for accurate measurement.
  11. B — Tripod and wire gauze. The tripod supports the beaker over the flame and the wire gauze spreads out the heat.
  12. A. A spatula is used to transfer a small amount of a solid. Tongs hold hot objects, a dropper adds liquid drop by drop, and a thermometer measures temperature.
  13. C — nitrogen > oxygen > argon. Dry air contains about 78% nitrogen, 21% oxygen, 0.9% noble gases (mainly argon) and 0.03% carbon dioxide.
  14. B — 21%. Volume of oxygen used up = 100 − 79 = 21 cm3. Percentage of oxygen = 21 ÷ 100 × 100% = 21%.
  15. D. In a mixture, the components keep their own properties (oxygen in air still supports burning) and can be separated by physical methods such as fractional distillation. A compound has a definite formula, so B is wrong.
  16. C. Liquid nitrogen is very cold and is used as a coolant to freeze food quickly. Nitrogen, not oxygen, is used in food packets. Weather balloons use helium, and patients breathe oxygen, not argon.
  17. A. Argon is unreactive, so it does not react with the hot filament, which therefore does not burn away. The other statements are wrong: argon does not support burning and is not a fuel.
  18. C — Nitrogen. Nitrogen has the lowest boiling point (−196 °C), so it boils off first.
  19. A. At the very low temperatures used to liquefy air, water vapour and carbon dioxide would freeze into solids and block the pipes.
  20. B — (2), (3), (4), (1). Dust is filtered out first, then water vapour and carbon dioxide are removed, then the remaining air is cooled to form liquid air, and finally the liquid air is warmed slowly in the fractionating column.

Question 2 — True or False

  1. True.
  2. False. Water is a compound. It is made of hydrogen and oxygen chemically combined, and it can be broken down into simpler substances by chemical methods.
  3. False. The properties of a compound are usually very different from those of the elements it contains (for example, iron(II) sulphide compared with iron and sulphur).
  4. True.
  5. False. Iron(II) sulphide is a compound and is not attracted by a magnet. The iron cannot be separated from it using a magnet.
  6. True. The burner then gives a yellow flame that is easy to see.
  7. False. A yellow flame is produced when the air hole is closed. A blue flame is produced when the air hole is open.
  8. True.
  9. False. When the burner is not used for heating, the air hole should be closed, to give a yellow flame that is easy to see.
  10. False. A measuring cylinder is more suitable than a beaker for measuring a volume of liquid accurately.
  11. True.
  12. True.
  13. False. Air is a mixture of nitrogen, oxygen and other gases. They are not chemically combined.
  14. True.
  15. False. The third most abundant gas in dry air is argon (about 0.9% noble gases). Carbon dioxide is only about 0.03%.
  16. True.
  17. False. Nitrogen is used to fill food packets because it is unreactive. Oxygen would make the food go bad.
  18. True.
  19. False. Nitrogen boils off first because it has the lowest boiling point (−196 °C). Oxygen has the highest boiling point of the three (−183 °C).
  20. True.

Question 3 — Elements, Compounds and Mixtures

(a) Classification

Substance Element, compound or mixture?
Sulphur Element
Sea water Mixture
Carbon dioxide Compound
Copper Element
Air Mixture
Magnesium oxide Compound
6 correct (3); 4 or 5 correct (2); 2 or 3 correct (1); 0 or 1 correct (0).

(b) Chemical symbols

Sulphur: S; copper: Cu (both needed) (1)

(c) Differences between a compound and a mixture

Any two of the following, 1 mark each:
• In a compound the elements are chemically combined; in a mixture the components are not chemically combined.
• A compound has a fixed composition; a mixture can have any composition.
• A compound has properties different from those of its elements; the components of a mixture keep their own properties.
• A compound can only be broken down by chemical methods; a mixture can be separated by physical methods.
Common errors: (1) calling air or sea water a compound; (2) writing the difference for one side only (for example, “a mixture can be separated” without saying what happens to a compound); (3) writing the symbol for sulphur as “Su”.

Question 4 — Iron and Sulphur: Mixture or Compound?

(a) Magnet and the mixture

The iron powder is attracted to the magnet (and sticks to it), but the sulphur powder is not attracted. (1)

(b) The black solid

(i) Iron(II) sulphide (accept iron sulphide) (1)
(ii) iron + sulphur → iron(II) sulphide (1)

(c) Magnet and the black solid

The iron has combined chemically with sulphur to form a new substance (a compound) whose properties are different from those of iron, so it is no longer attracted by a magnet. (1)

(d) Differences between the mixture and the black solid

Any two, 1 mark each (both sides must be stated):
• The mixture contains iron and sulphur that keep their own properties; the black solid has new properties.
• The iron can be separated from the mixture with a magnet; the iron cannot be separated from the black solid with a magnet.
• The mixture can have any proportion of iron and sulphur; the black solid has a fixed composition.
• The mixture is grey and yellow; the black solid is black (accept differences in appearance).
Common errors: (1) writing “iron oxide” instead of iron(II) sulphide; (2) writing the equation as a mixture “iron + sulphur” without an arrow; (3) saying that the iron has “disappeared” instead of saying that it has chemically combined with sulphur.

Question 5 — The Bunsen Burner and Its Flames

(a) Parts of the burner

P: chimney (barrel); Q: air hole (accept collar); R: gas inlet (where the rubber tubing is connected); S: base.
4 correct (2); 2 or 3 correct (1).

(b) Yellow flame to blue flame

Open the air hole (turn the collar so that air can enter). (1)

(c) Differences between the flames

Any two, 1 mark each:
• The blue flame is hotter; the yellow flame is cooler.
• The blue flame does not leave soot on the apparatus; the yellow flame deposits soot.
• The blue flame is non-luminous (not very bright); the yellow flame is luminous (bright).

(d) Closing the air hole when not heating

The yellow flame is easy to see, so people will not accidentally burn themselves or other things. (It is called the safety flame.) (1)
Common errors: (1) naming R as “gas tap” or “tube”; (2) saying that the blue flame is “brighter”; (3) saying that the yellow flame is used because it is hotter.

Question 6 — Using a Bunsen Burner Safely

(a) Order of steps

E → C → B → D → A (accept C and E interchanged)
All correct (2); one pair of adjacent steps interchanged (1).

(b) Safety precautions

Any three, 1 mark each:
• Wear safety goggles.
• Tie back long hair / keep loose clothing away from the flame.
• Hold the test tube with a test tube holder.
• Tilt the test tube slightly, with its mouth pointing away from people.
• Fill the test tube with no more than about one-third liquid.
• Heat gently and keep shaking the test tube.
• Keep flammable substances away from the flame.

(c) Turning off the burner

Turn off the gas at the gas tap. (1) The burner should be left to cool before it is touched.
Common errors: (1) striking the match after the gas has been turned on; (2) giving the same precaution twice in (b) in different words (for example “tilt the test tube” and “point the mouth away from people” are accepted as separate points only if both are clearly stated); (3) saying “blow it out”.

Question 7 — Simple Laboratory Apparatus

(a) Apparatus for each use

Use Apparatus
Measuring 25.0 cm3 of water accurately Measuring cylinder
Transferring a small amount of a solid powder Spatula
Heating a salt solution until only the solid is left Evaporating dish
Holding a hot test tube Test tube holder
Adding a liquid drop by drop Dropper
1 mark for each correct answer. (5)

(b) Wire gauze

The wire gauze spreads out the heat evenly (so that the glass does not crack because of uneven heating). (1)
Common errors: (1) naming a “beaker” for the measuring cylinder; (2) naming “tongs” for the test tube holder; (3) saying that the wire gauze “holds the beaker”, which is the job of the tripod.

Question 8 — Air as a Mixture

(a) Percentages of nitrogen and oxygen

Nitrogen: about 78% (accept 78–79%) (1)
Oxygen: about 21% (accept 20–21%) (1)

(b) Noble gases in air

Any two of argon, neon, helium (accept krypton, xenon). (1)

(c) Water vapour

The amount of water vapour in air varies from place to place and from time to time (for example, between a humid day and a dry day), so it has no fixed percentage. (1)

(d) Why air is a mixture

Any two, 1 mark each:
• The gases in air are not chemically combined.
• Each gas keeps its own properties (for example, the oxygen in air still supports burning).
• The composition of air can vary (for example, the amount of water vapour).
• The gases in air can be separated by physical methods (fractional distillation of liquid air).
Common errors: (1) giving the percentages the wrong way round; (2) giving “air is a gas” as a reason in (d); (3) writing “carbon” instead of “carbon dioxide”.

Question 9 — Finding the Percentage of Oxygen in Air

(a) Colour change and word equation

The copper changes from brown (pinkish-brown) to black. (1)
copper + oxygen → copper(II) oxide (accept copper oxide) (1)

(b) Cooling to room temperature

A hot gas expands and would have a larger volume. The final volume must be measured at the same temperature as the starting volume (room temperature) to make a fair comparison. (1)

(c) Excess copper

To make sure that all the oxygen in the air is used up (reacts with the copper). (1)

(d) Calculation

Volume of oxygen used up = 90 − 71 = 19 cm3 (1)
Percentage of oxygen = 1990 × 100% = 21.1% ≈ 21% (1)
Check: This is close to the accepted value of about 21% oxygen in air.
Common errors: (1) dividing 19 by 71 instead of by the original 90 cm3; (2) giving “the copper reacts with nitrogen”; (3) forgetting the unit cm3 for the volume of oxygen.

Question 10 — Uses of the Gases in Air

(a) Choosing the gases

(i) oxygen
(ii) nitrogen
(iii) helium
(iv) argon
(v) neon
1 mark for each correct answer. (5)

(b) Why nitrogen is suitable

Nitrogen is unreactive, so it does not react with the food and stops oxygen in the air from making the food go bad. (1)
Common errors: (1) answering “oxygen” for (ii); (2) answering “neon” for (iv) or “argon” for (v); (3) saying in (b) that nitrogen is used because it is “a gas in air” without referring to its being unreactive.

Question 11 — Obtaining Gases from Liquid Air

(a) Stage 1

Dust is removed. (1)

(b) Removing water vapour and carbon dioxide

At the very low temperatures in Stage 3 they would freeze (turn into solids) and block the pipes. (1)

(c) Stage 3

The air changes from a gas to a liquid (it condenses / is liquefied). (1)

(d) Stage 4

Fractional distillation (1)

(e) Identifying X, Y and Z

X: nitrogen; Y: argon; Z: oxygen
3 correct (2); 1 or 2 correct (1).
Reasoning: Nitrogen has the lowest boiling point (−196 °C), so it boils off first and leaves at the cooler top. Oxygen has the highest boiling point (−183 °C), so it stays as a liquid and leaves at the warmer bottom. Argon (−186 °C) is in between.
Common errors: (1) writing “distillation” only or “filtration” for Stage 4; (2) saying in (b) that water vapour and carbon dioxide are removed because they are “harmful”; (3) giving the order of X and Z the wrong way round.

Question 12 — Fractional Distillation and Boiling Points

(a) First gas to boil off

Nitrogen, at −196 °C (1)

(b) Liquid air is a mixture

It contains different liquids (liquid nitrogen, argon and oxygen) which are not chemically combined. (1)

(c) Nitrogen and oxygen at −190 °C

−190 °C is higher than the boiling point of nitrogen (−196 °C), so nitrogen is a gas. (1)
−190 °C is lower than the boiling point of oxygen (−183 °C), so oxygen is still a liquid. (1)

(d) Argon at −190 °C

Liquid, because −190 °C is lower than the boiling point of argon (−186 °C). (1)

(e) Oxygen at the bottom

Oxygen has the highest boiling point of the three, so it is the last to boil off and stays as a liquid at the warmer bottom of the column. (1)
Common errors: (1) thinking that a lower number such as −196 is a “higher” temperature than −183; (2) stating that argon is a gas at −190 °C; (3) saying that the gases are separated because of their different densities or colours.
Mark weighting: Section A (20) + Section B (20) + Questions 3 to 12 (10 × 6 = 60) = 100 marks.
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