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Class 8 Science — Elements, compounds and mixtures

Tell an element from a compound from a mixture, and say which of them can be taken apart by physical means.

Name: ________________________

  1. 1.Match each test done on iron and sulfur to what was seen.

    • A magnet is moved over the stirred mixture of iron filings and sulfur powder
    • A magnet is moved over the black mass left after heating them together
    • Dilute hydrochloric acid is added to the stirred mixture
    • Dilute hydrochloric acid is added to the black mass
    • the iron filings are drawn out, so the two can be separated
    • a colourless gas with no smell comes off and burns with a pop
    • a colourless gas with a rotten egg-like odour comes off
    • nothing is drawn out, because a new substance has formed

  2. 2.These are mixtures. Sort them by whether you could pick out the separate components by eye or with a magnifying glass.

    Groups: Uniform mixture · Non-uniform mixture

    • Stainless steel
    • Air
    • Sprout salad
    • Sugar dissolved in water

  3. 3.Sugar heated in a boiling tube turns brown, then black, and droplets of water appear near the open end. What does that show about sugar?

    • a) It is a compound, made of carbon, hydrogen and oxygen
    • b) It is a mixture of carbon and water stirred together
    • c) It is an element, since carbon is left behind
    • d) It is a metal, since it darkens on heating

  4. 4.Could you separate the hydrogen and oxygen in water by a physical method such as filtering, evaporating or using a magnet?

    • a) Yes — a strong enough magnet pulls the hydrogen out
    • b) No — the elements in a compound are held too tightly for any physical method
    • c) No — because water contains just one element to begin with
    • d) Yes — evaporating the water leaves the oxygen behind

  5. 5.A pure substance is one of two things. Which pair is it?

    • a) An element or a compound
    • b) An element or a mixture
    • c) A compound or a mixture
    • d) A metal or a non-metal

  6. 6.Silicon and boron sit awkwardly between the two families of elements. What are such elements called?

    • a) Compounds
    • b) Alloys
    • c) Minerals
    • d) Metalloids

  7. 7.In water, what is the ratio of the number of hydrogen atoms to the number of oxygen atoms? Write it as a ratio.

  8. 8.Lime water left standing in a petri dish turns milky. What does that show is present in the air?

    • a) Nitrogen
    • b) Carbon dioxide
    • c) Dust particles
    • d) Argon

Answer key — Class 8 Science — Elements, compounds and mixtures

  1. 1. A magnet is moved over the stirred mixture of iron filings and sulfur powder → the iron filings are drawn out, so the two can be separated; A magnet is moved over the black mass left after heating them together → nothing is drawn out, because a new substance has formed; Dilute hydrochloric acid is added to the stirred mixture → a colourless gas with no smell comes off and burns with a pop; Dilute hydrochloric acid is added to the black mass → a colourless gas with a rotten egg-like odour comes off
  2. 2. Air → Uniform mixture; Sugar dissolved in water → Uniform mixture; Stainless steel → Uniform mixture; Sprout salad → Non-uniform mixture
  3. 3. a) It is a compound, made of carbon, hydrogen and oxygen
  4. 4. b) No — the elements in a compound are held too tightly for any physical method
  5. 5. a) An element or a compound
  6. 6. d) Metalloids
  7. 7. 2:1
  8. 8. b) Carbon dioxide