T3.1.1 Pure substances and identifying them Quiz: KS3 Chemistry, Unit 3

The 20 questions

1. What does a pure substance contain?

   • Two or more substances mixed together in no fixed ratio and not chemically joined
   • Only substances that can be filtered out easily from a liquid by a paper filter
   • Only one type of substance, with no other substances mixed in
   • Only water and salt dissolved together in a fixed ratio of mass to volume

2. Which property can be used to identify a pure substance?

   • A melting point that changes with the sample size
   • A smell that varies from sample to sample
   • A colour that is always different
   • A fixed melting point and boiling point

3. A sample of pure water boils at exactly 100 °C at sea level. What does this show?

   • The sample has a fixed boiling point, as a pure substance does
   • The sample has no boiling point because it is already a liquid at room temperature
   • The sample is impure because it boils at a single number instead of a range
   • The sample is a mixture of water and air that has dissolved in the liquid

4. Why does an impure substance melt over a range of temperatures rather than at one temperature?

   • Its particles become charged when impurities are added, which stops melting
   • Its different substances melt at different temperatures
   • Its particles are much larger than pure particles, so they melt more slowly
   • Its particles stop moving once mixed, so they cannot melt in the normal way

5. Which is the best test to show a sample of water is pure?

   • It boils and freezes at fixed temperatures
   • It turns cloudy when shaken in a jar
   • It leaves no residue when left on a sunny day
   • It has a faint smell of chlorine

6. A chemist finds a sample melts at 0 °C and boils at 100 °C, both sharp. Which is most likely?

   • A pure substance, water
   • A mixture of sand and water
   • A solution of salt dissolved in water
   • A mixture of oil and water

7. Which of these is a pure substance?

   • Coca-Cola, a mixture of water, sugar, flavourings and dissolved carbon dioxide
   • Air, a mixture of nitrogen, oxygen and other gases in the atmosphere
   • Sea water, a mixture of water and dissolved salts and minerals
   • Distilled water, which contains only water molecules

8. Which of these is a pure substance that is a compound?

   • Pure copper
   • Pure iron
   • Pure water
   • Pure oxygen gas

9. A sample melts over a range from 40 °C to 50 °C. What does this suggest?

   • The sample has no melting point
   • The sample is a pure compound
   • The sample is impure
   • The sample is a pure element

10. Which statement about pure elements is correct?

    • A pure element always dissolves in water
    • A pure element is always a liquid at room temperature
    • A pure element contains only one kind of atom
    • A pure element contains atoms of several elements mixed together

11. Why is it important that a pure substance has a sharp melting point?

    • It makes the substance heavier than any impurities that might be mixed in with it
    • It allows scientists to identify the substance by a known property
    • It makes the substance turn into a gas faster when it is heated in the lab
    • It means the substance has no particles at all, so it cannot be weighed

12. Which is an impure substance?

    • Tap water containing dissolved minerals and gases
    • A sample of pure gold that has been polished and checked by a jeweller
    • Distilled water used in a laboratory to rinse glassware and equipment
    • A bar of pure copper that has been cleaned and weighed in the laboratory

13. A metal sample has a boiling point of exactly 2,000 °C. Which conclusion is best supported?

    • The sample must be an impure mixture, because a single boiling point is impossible
    • The sample must be a gas because metals with high boiling points are always gases
    • The sample has no fixed property, so its boiling point changes with the sample size
    • The sample is likely a pure substance

14. How can a melting point be used to show that a substance is pure?

    • Measure its mass on a balance and compare it with the mass of the same sample
    • Measure its size using a ruler and compare it with the size of a known block
    • Measure the temperature at which it melts and check it stays constant
    • Measure its colour under a bright lamp and compare it with a colour chart

15. Which statement describes a mixture rather than a pure substance?

    • It contains only one type of atom, so it can be described as a single element
    • It contains two or more substances that are not chemically joined
    • It has a fixed melting point that does not change as the sample is heated
    • It can be identified by a single boiling point that is measured in the lab

16. Ethanol boils at 78 °C. A second liquid boils between 78 °C and 85 °C. Which sample is impure?

    • The first sample, because it boils at a fixed temperature
    • Both are impure because all liquids boil over a range
    • The second sample, because its boiling point covers a range
    • Neither can be tested because boiling does not depend on purity

17. What is the best definition of an impurity?

    • A substance that is always a gas at room temperature and pressure in the lab
    • A substance that cannot be separated from the sample by any physical method
    • A substance present in a sample that is not the substance being studied
    • A substance made from a single element only, with no other elements present

18. A sample is labelled pure gold. Which test result would most strongly contradict this claim?

    • A shiny yellow colour in good light, which looks like gold to the naked eye
    • A melting range that is wide and not one fixed temperature
    • A sharp melting point at a known temperature that matches the value for pure gold
    • A density that matches gold closely, measured using water displacement in a tank

19. Which statement about pure substances is correct?

    • A pure substance has a fixed melting point and fixed boiling point
    • A pure substance cannot be identified by its properties because they all vary
    • A pure substance always contains two different elements joined together
    • A pure substance melts over a wide range of temperatures as it changes state

20. A student writes that a pure substance must always be an element. Which example shows this is wrong?

    • Pure water is a compound and is a pure substance
    • Pure water contains many different substances mixed
    • Pure water is always a solid at room temperature
    • Pure water is a mixture of hydrogen and oxygen gas

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