Elements, Compounds, Mixtures, Purity and Experimental Methods

1. A student heats a sample containing iron and sulfur. After heating strongly, the black solid formed cannot be separated into iron and sulfur by using a magnet.

Which statement is correct?

A The black solid is a mixture because it contains two elements.
B The black solid is a compound because the elements are chemically combined.
C The black solid is an element because it has only one colour.
D The black solid is a mixture because it was made by heating two solids.

2. Which row correctly compares a compound and a mixture?

	compound	mixture
A	fixed composition	fixed composition
B	fixed composition	variable composition
C	variable composition	fixed composition
D	variable composition	variable composition

3. Substance X contains only copper atoms. Substance Y contains copper and zinc atoms mixed together but not chemically combined.

Which row is correct?

	X	Y
A	element	compound
B	element	mixture
C	compound	alloy
D	mixture	alloy

4. Which substance is a compound?

A air
B brass
C carbon dioxide
D sea water

5. A sample contains calcium carbonate and silicon(IV) oxide only.

Which statement is correct?

A The sample is a compound because both substances contain oxygen.
B The sample is a mixture because it contains two chemically different substances.
C The sample is an element because it contains calcium, carbon, silicon and oxygen.
D The sample is pure because all particles contain oxygen.

6. Which statement about pure substances is correct?

A A pure substance always contains only one element.
B A pure substance always melts over a range of temperatures.
C A pure substance contains only one chemical substance.
D A pure substance cannot be a compound.

7. A colourless liquid boils sharply at 78°C and leaves no residue when evaporated.

Which conclusion is safest?

A It is definitely ethanol.
B It is pure, but its identity is not confirmed by boiling point alone.
C It is a mixture because it evaporates completely.
D It is impure because all liquids leave no residue.

8. Which sample is definitely a mixture?

A oxygen gas
B sodium chloride
C aqueous sodium chloride
D magnesium oxide

9. A student says, “All compounds can be separated into elements by filtration.”

Which correction is best?

A Compounds cannot be separated by physical methods such as filtration.
B Compounds are always separated by crystallisation.
C Compounds contain substances in variable proportions.
D Compounds are always soluble in water.

10. Which statement is correct about air?

A It is a compound because nitrogen and oxygen are chemically bonded.
B It is an element because nitrogen is the main gas.
C It is a mixture because its gases are not chemically combined.
D It is pure because it is colourless.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. A solid melts from 114°C to 119°C.

What does this suggest?

A The solid is pure because it melts near one temperature.
B The solid is impure because it melts over a range.
C The solid is a compound because compounds melt over a range.
D The solid is an element because only elements have melting points.

12. A student wants to check the purity of a solid. Which result gives the best evidence that the solid is pure?

A It dissolves completely in water.
B It has a sharp melting point.
C It is white.
D It reacts with dilute acid.

13. Which observation best suggests that a liquid contains dissolved impurities?

A It boils at exactly 100°C.
B It freezes at exactly 0°C.
C It leaves a residue after evaporation.
D It is colourless.

14. A liquid boils between 102°C and 106°C.

Which statement is most likely?

A The liquid is pure water.
B The liquid is impure.
C The liquid is an element.
D The liquid contains no dissolved solids.

15. A student tests a food dye by chromatography. Only one spot is seen.

Which conclusion is safest?

A The dye is definitely pure.
B The dye contains one coloured substance in that solvent.
C The dye contains one element.
D The dye contains no soluble substances.

16. Which experimental mistake would make Rf values unreliable in paper chromatography?

A drawing the baseline in pencil
B keeping the solvent level below the baseline
C allowing the solvent front to reach the top edge of the paper without marking it
D using a small concentrated spot on the baseline

17. In chromatography, why must the baseline be drawn in pencil?

A Pencil graphite is insoluble in the solvent.
B Pencil graphite reacts with the dyes.
C Pencil graphite dissolves and moves with the solvent.
D Pencil graphite lowers the Rf value of all dyes.

18. A student places the chromatography baseline below the solvent level.

What is the main problem?

A The paper will not absorb solvent.
B The sample may dissolve directly into the solvent instead of moving up the paper.
C The solvent front will move downwards.
D The Rf value will always be greater than 1.

19. Which mixture is most suitable for separation by paper chromatography?

A sand and salt
B ethanol and water
C coloured dyes in ink
D oil and water

20. Which statement about chromatography is correct?

A A substance with a larger Rf value has moved further relative to the solvent front.
B A substance with a smaller Rf value must be insoluble in all solvents.
C Rf values are always the same in every solvent.
D Pure substances always produce no spots.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. A student wants to separate an insoluble solid from a liquid.

Which apparatus is essential?

A burette and pipette
B filter funnel and filter paper
C separating funnel and beaker
D condenser and round-bottom flask

22. In filtration, which statement is correct?

A The filtrate remains on the filter paper.
B The residue passes through the filter paper.
C The filtrate is the liquid collected below the filter paper.
D The residue must always be soluble in water.

23. A mixture contains sand and sodium chloride. Which sequence obtains dry sodium chloride crystals?

A filter the mixture, wash the residue, dry the residue
B add water, filter, evaporate the filtrate and crystallise
C add water, filter, evaporate the residue
D heat the mixture strongly, cool, then filter

24. A student evaporates a salt solution to complete dryness using a Bunsen burner.

Why may this be unsuitable for preparing pure crystals?

A The salt may decompose or spit out of the dish.
B The water cannot evaporate at high temperature.
C The salt will become insoluble.
D The salt will change into an element.

25. Which method is best for obtaining pure dry crystals from a salt solution?

A evaporate to dryness rapidly
B filter the solution without heating
C gently evaporate to saturation, cool, filter and dry the crystals
D distil the solution and collect the distillate

26. A mixture contains water and ethanol. They are miscible liquids with different boiling points.

Which method should be used to separate them?

A filtration
B separating funnel
C fractional distillation
D crystallisation

27. Which pair can be separated using a separating funnel?

A ethanol and water
B copper(II) sulfate solution and water
C kerosene and water
D sodium chloride and water

28. A liquid X is collected during simple distillation of seawater.

What is X?

A salt
B pure water
C seawater
D concentrated salt solution

29. Which statement explains why simple distillation can obtain water from copper(II) sulfate solution?

A Copper(II) sulfate has a lower boiling point than water.
B Water evaporates and condenses, leaving copper(II) sulfate behind.
C Copper(II) sulfate passes through filter paper.
D Water crystallises before copper(II) sulfate.

30. Which apparatus is used to measure a fixed volume of liquid most accurately during titration preparation?

A measuring cylinder
B beaker
C pipette
D evaporating basin

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. Which apparatus is used to deliver variable measured volumes of acid in a titration?

A burette
B pipette
C volumetric flask
D thermometer

32. A student measures 25.0 cm³ of sodium hydroxide solution for titration.

Which apparatus is most suitable?

A 25.0 cm³ pipette
B 100 cm³ beaker
C 50 cm³ measuring cylinder
D gas syringe

33. Which apparatus is most suitable for measuring the volume of gas produced in a reaction?

A burette only
B gas syringe
C filter funnel
D volumetric flask

34. A student collects carbon dioxide over water.

Why may this give an inaccurate gas volume?

A Carbon dioxide is slightly soluble in water.
B Carbon dioxide is denser than air.
C Carbon dioxide is colourless.
D Carbon dioxide reacts with glass.

35. Which gas should not be collected over water if an accurate volume is required?

A hydrogen
B oxygen
C ammonia
D nitrogen

36. Which method is suitable for collecting a gas that is less dense than air and very soluble in water?

A downward delivery
B upward delivery
C collection over water
D filtration through cotton wool

37. Which drying agent is suitable for drying neutral gases such as oxygen but not ammonia?

A concentrated sulfuric acid
B water
C aqueous sodium hydroxide
D ethanol

38. Why is concentrated sulfuric acid not used to dry ammonia gas?

A It reacts with ammonia.
B It freezes ammonia.
C It makes ammonia insoluble.
D It turns ammonia into oxygen.

39. A student wants to test whether a sample of water is pure.

Which method is most reliable?

A check whether it is colourless
B check whether it conducts electricity
C measure its boiling point and compare with 100°C at standard pressure
D smell it carefully

40. Which sample is most likely to conduct electricity?

A pure water
B sugar crystals
C sodium chloride solution
D solid sodium chloride

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. Which row correctly classifies the substances?

	element	compound	mixture
A	carbon	carbon dioxide	air
B	carbon dioxide	carbon	air
C	air	carbon dioxide	carbon
D	carbon	air	carbon dioxide

42. Which formula represents a compound?

A O2
B N2
C H2O
D Ar

43. Which statement about alloys is correct?

A Alloys are pure compounds with fixed formulae.
B Alloys are mixtures containing at least one metal.
C Alloys are elements because they contain metal atoms.
D Alloys are always separated by filtration.

44. Which statement about brass is correct?

A It is a compound of copper and zinc.
B It is a mixture of copper and zinc.
C It is an element because it conducts electricity.
D It is a pure metal because it contains copper.

45. A student compares a mixture of iron and sulfur with iron sulfide.

Which property would distinguish the mixture from the compound?

A The mixture can be separated using a magnet.
B The compound contains sulfur atoms.
C The mixture has mass.
D The compound is solid at room temperature.

46. Which statement about a mixture is always correct?

A It contains only elements.
B Its substances are chemically bonded together.
C Its composition can vary.
D It has a sharp melting point.

47. Which statement about a compound is always correct?

A It contains two or more substances physically mixed.
B It contains two or more elements chemically combined.
C It can be separated by filtration.
D It has the same properties as the elements that form it.

48. A student mixes hydrogen and oxygen gases in a sealed container. No spark is applied.

Which statement is correct?

A A compound must form because two elements are present.
B A mixture is present because the gases are not chemically combined.
C Water is present because hydrogen and oxygen always react instantly.
D An alloy is present because two elements are mixed.

49. Which observation gives the strongest evidence that a chemical compound has formed?

A The substances can be separated by dissolving one component in water.
B The final material has properties different from the starting elements and cannot be separated physically.
C The substances are the same colour after mixing.
D The final material contains the same total mass as the starting materials.

50. Which statement correctly links purity and separation?

A Pure substances are separated from mixtures using suitable physical methods.
B Compounds are separated into elements by chromatography.
C Elements can be separated into simpler substances by distillation.
D Mixtures always have sharp melting points.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. B

A wrong: A compound can contain two elements; that does not make it a mixture.
B right: Iron and sulfur chemically combine to form iron sulfide, a compound.
C wrong: It is not an element because it contains iron and sulfur chemically combined.
D wrong: Heating can cause a chemical reaction, not just mixing.

2. B

A wrong: Mixtures do not have fixed composition.
B right: Compounds have fixed composition; mixtures can have variable composition.
C wrong: Compounds do not have variable composition.
D wrong: Compounds have fixed composition.

3. B

A wrong: Y is not a compound because copper and zinc are not chemically combined.
B right: X is an element; Y is a mixture because it contains copper and zinc physically mixed.
C wrong: X is not a compound.
D wrong: X is not a mixture.

4. C

A wrong: Air is a mixture of gases.
B wrong: Brass is an alloy, so it is a mixture.
C right: Carbon dioxide contains carbon and oxygen chemically combined.
D wrong: Sea water is a mixture.

5. B

A wrong: Containing oxygen does not make the whole sample one compound.
B right: Calcium carbonate and silicon(IV) oxide are two different substances mixed together.
C wrong: A sample containing several elements is not automatically an element.
D wrong: “All particles contain oxygen” does not prove purity.

6. C

A wrong: Pure substances can be elements or compounds.
B wrong: Pure substances usually have sharp melting points.
C right: A pure substance contains only one chemical substance.
D wrong: A compound can be pure.

7. B

A wrong: A boiling point of 78°C suggests ethanol, but does not prove identity alone.
B right: Sharp boiling point and no residue suggest purity, but more tests are needed to identify it.
C wrong: Evaporating completely does not make it a mixture.
D wrong: Leaving no residue suggests no dissolved solid impurity.

8. C

A wrong: Oxygen gas is an element.
B wrong: Sodium chloride is a compound.
C right: Aqueous sodium chloride is sodium chloride dissolved in water, so it is a mixture.
D wrong: Magnesium oxide is a compound.

9. A

A right: Compounds require chemical methods to break them into elements; filtration is physical.
B wrong: Crystallisation separates dissolved solids from solutions.
C wrong: Mixtures, not compounds, have variable proportions.
D wrong: Compounds are not always soluble.

10. C

A wrong: Nitrogen and oxygen are not chemically bonded in air.
B wrong: Air contains more than one substance.
C right: Air is a mixture because its gases are physically mixed.
D wrong: Colourless does not mean pure.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. B

A wrong: A pure solid melts sharply at one temperature.
B right: Melting over a range suggests impurity.
C wrong: Compounds can be pure and have sharp melting points.
D wrong: Both elements and compounds can have melting points.

12. B

A wrong: Dissolving completely does not prove purity.
B right: A sharp melting point is strong evidence of purity.
C wrong: Colour does not prove purity.
D wrong: Reacting with acid does not prove purity.

13. C

A wrong: Boiling at exactly 100°C suggests pure water at standard pressure.
B wrong: Freezing at exactly 0°C suggests pure water at standard pressure.
C right: Residue after evaporation suggests dissolved impurities.
D wrong: Colourless substances can still be impure.

14. B

A wrong: Pure water boils sharply at 100°C at standard pressure.
B right: Boiling over a range suggests impurity.
C wrong: Elements do not generally boil over a range if pure.
D wrong: A boiling range suggests impurities may be present.

15. B

A wrong: One spot does not definitely prove purity; another solvent may separate more components.
B right: It shows one coloured substance in that solvent system.
C wrong: Chromatography does not prove the dye is an element.
D wrong: A spot shows there is a soluble coloured substance.

16. C

A wrong: Pencil baseline is correct.
B wrong: Solvent level should be below the baseline.
C right: If the solvent front reaches the top and is not marked, the distance moved by solvent cannot be measured accurately.
D wrong: A small concentrated spot is usually acceptable if it does not spread too much.

17. A

A right: Pencil graphite is insoluble in the solvent, so it does not move up the paper.
B wrong: Pencil does not react with dyes.
C wrong: If pencil dissolved, it would be unsuitable.
D wrong: Pencil does not lower all Rf values.

18. B

A wrong: The paper will still absorb solvent.
B right: The sample may dissolve directly into the solvent instead of travelling up the paper.
C wrong: The solvent front still moves up by capillary action.
D wrong: Rf values should not be greater than 1.

19. C

A wrong: Sand and salt are separated by dissolving, filtering and crystallising.
B wrong: Ethanol and water are separated by fractional distillation.
C right: Paper chromatography separates coloured dissolved substances such as dyes.
D wrong: Oil and water are separated by separating funnel.

20. A

A right: Higher Rf means the substance moved further compared with the solvent front.
B wrong: A small Rf does not mean insoluble in all solvents.
C wrong: Rf values depend on the solvent and paper used.
D wrong: Pure coloured substances usually produce one spot.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. B

A wrong: Burette and pipette are used for titration.
B right: Filtration needs filter funnel and filter paper.
C wrong: Separating funnel separates immiscible liquids.
D wrong: Condenser and flask are used in distillation.

22. C

A wrong: The residue remains on the filter paper.
B wrong: The filtrate passes through the filter paper.
C right: The filtrate is the liquid collected below the filter paper.
D wrong: The residue is usually insoluble.

23. B

A wrong: Filtering the dry mixture first does not separate salt from sand.
B right: Salt dissolves in water, sand is filtered off, and salt crystals are obtained from the filtrate.
C wrong: The salt is in the filtrate, not the residue.
D wrong: Strong heating is unnecessary and may cause problems.

24. A

A right: Strong heating to dryness can cause spitting or decomposition, giving impure/lost crystals.
B wrong: Water evaporates faster at high temperature.
C wrong: Heating does not make the salt insoluble.
D wrong: A salt does not become an element by heating normally.

25. C

A wrong: Rapid evaporation to dryness may produce powdery impure solid or cause spitting.
B wrong: Filtering alone does not obtain crystals from a solution.
C right: Gentle evaporation to saturation, cooling, filtering and drying gives pure dry crystals.
D wrong: Distillation obtains the solvent, not the salt crystals.

26. C

A wrong: Filtration cannot separate miscible liquids.
B wrong: Separating funnel works for immiscible liquids only.
C right: Fractional distillation separates miscible liquids with different boiling points.
D wrong: Crystallisation separates dissolved solids from solutions.

27. C

A wrong: Ethanol and water are miscible.
B wrong: Copper(II) sulfate solution and water mix completely.
C right: Kerosene and water are immiscible liquids.
D wrong: Sodium chloride dissolves in water.

28. B

A wrong: Salt remains in the flask.
B right: Water evaporates and condenses as pure water.
C wrong: Seawater is the original mixture, not the distillate.
D wrong: Concentrated salt solution remains behind.

29. B

A wrong: Copper(II) sulfate does not boil off before water.
B right: Water vaporises and condenses, leaving copper(II) sulfate behind.
C wrong: Filtration is not involved in simple distillation.
D wrong: Water does not crystallise before copper(II) sulfate.

30. C

A wrong: Measuring cylinders are less accurate.
B wrong: Beakers are not accurate for fixed volumes.
C right: A pipette measures a fixed volume accurately.
D wrong: An evaporating basin is for evaporation.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A

A right: A burette delivers variable measured volumes accurately.
B wrong: A pipette delivers a fixed measured volume.
C wrong: A volumetric flask prepares a fixed volume of solution.
D wrong: A thermometer measures temperature.

32. A

A right: A 25.0 cm³ pipette accurately measures 25.0 cm³.
B wrong: A beaker is not accurate.
C wrong: A measuring cylinder is less accurate than a pipette.
D wrong: A gas syringe measures gas volume.

33. B

A wrong: A burette is mainly used in titration, not normally for collecting reaction gas.
B right: A gas syringe measures the volume of gas produced.
C wrong: A filter funnel is for filtration.
D wrong: A volumetric flask prepares solutions.

34. A

A right: Some carbon dioxide dissolves in water, so the measured gas volume may be too low.
B wrong: Density does not cause this inaccuracy.
C wrong: Colourlessness does not affect measured volume.
D wrong: Carbon dioxide does not react with glass.

35. C

A wrong: Hydrogen is only slightly soluble in water.
B wrong: Oxygen is only slightly soluble in water.
C right: Ammonia is very soluble in water, so it should not be collected over water.
D wrong: Nitrogen is only slightly soluble in water.

36. B

A wrong: Downward delivery is used for gases denser than air.
B right: A gas less dense than air and very soluble in water is collected by upward delivery.
C wrong: It is very soluble, so collection over water is unsuitable.
D wrong: Filtration through cotton wool does not collect gas.

37. A

A right: Concentrated sulfuric acid dries many neutral gases such as oxygen, but reacts with ammonia.
B wrong: Water cannot dry gases.
C wrong: Aqueous sodium hydroxide is not a general drying agent for oxygen.
D wrong: Ethanol is not a drying agent.

38. A

A right: Ammonia is basic and reacts with concentrated sulfuric acid.
B wrong: It does not freeze ammonia.
C wrong: It does not simply make ammonia insoluble.
D wrong: It does not turn ammonia into oxygen.

39. C

A wrong: Colourless water may still contain dissolved impurities.
B wrong: Conductivity can suggest ions, but boiling point is a better purity test here.
C right: Pure water boils at 100°C at standard pressure.
D wrong: Smelling is unsafe and unreliable.

40. C

A wrong: Pure water conducts very poorly.
B wrong: Sugar crystals do not have mobile ions/electrons.
C right: Sodium chloride solution contains mobile ions, so it conducts electricity.
D wrong: Solid sodium chloride has ions fixed in place, so it does not conduct.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. A

A right: Carbon is an element, carbon dioxide is a compound, and air is a mixture.
B wrong: Carbon dioxide is not an element; carbon is not a compound.
C wrong: Air is not an element; carbon is not a mixture.
D wrong: Air is not a compound; carbon dioxide is not a mixture.

42. C

A wrong: O2 is an element because it contains only oxygen atoms.
B wrong: N2 is an element because it contains only nitrogen atoms.
C right: H2O contains hydrogen and oxygen chemically combined.
D wrong: Ar is an element.

43. B

A wrong: Alloys are mixtures, not pure compounds with fixed formulae.
B right: Alloys are mixtures containing at least one metal.
C wrong: Mixtures of metals are not elements.
D wrong: Alloys are not separated by filtration.

44. B

A wrong: Brass does not have copper and zinc chemically combined in a fixed formula.
B right: Brass is an alloy, so it is a mixture of copper and zinc.
C wrong: Conducting electricity does not make something an element.
D wrong: Brass contains copper but is not pure copper.

45. A

A right: Iron in the mixture can be removed by a magnet; iron sulfide cannot be separated this way.
B wrong: Both the mixture and compound contain sulfur atoms.
C wrong: Both have mass.
D wrong: Both are solid at room temperature.

46. C

A wrong: Mixtures can contain elements, compounds, or both.
B wrong: Substances in mixtures are not chemically bonded together.
C right: Mixtures can have variable composition.
D wrong: Mixtures usually melt/boil over a range.

47. B

A wrong: That describes a mixture.
B right: A compound contains two or more elements chemically combined.
C wrong: Compounds cannot be separated by filtration.
D wrong: Compounds usually have properties different from their elements.

48. B

A wrong: Elements do not form compounds merely by being placed together.
B right: Without a spark, hydrogen and oxygen remain physically mixed.
C wrong: Hydrogen and oxygen need activation energy to react.
D wrong: An alloy requires at least one metal.

49. B

A wrong: Physical separation suggests a mixture, not a compound.
B right: New properties and inability to separate physically strongly suggest compound formation.
C wrong: Colour alone is weak evidence.
D wrong: Conservation of mass happens in both physical and chemical changes.

50. A

A right: Physical methods can separate pure substances from mixtures.
B wrong: Chromatography does not split compounds into elements.
C wrong: Elements cannot be separated into simpler substances chemically or physically.
D wrong: Mixtures usually do not have sharp melting points.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.