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

  2. 2.Sort these substances into the three kinds the chapter names.

    Groups: Element · Compound · Mixture

    • Brass
    • Air
    • Gold
    • Water
    • Sulfur
    • Sodium chloride

  3. 3.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
    • nothing is drawn out, because a new substance has formed
    • a colorless gas with no smell comes off and burns with a pop
    • a colorless gas with a rotten egg-like odor comes off

  4. 4.A packet of ghee is labeled 'pure'. A scientist looks at it and calls it impure. Why the disagreement?

    • a) In science, pure means made of a single substance, while in everyday use it means unadulterated
    • b) In science, any substance sold in a packet counts as impure
    • c) The scientist thinks the ghee has had something cheaper added to it
    • d) In science, pure means safe to eat, and the ghee is not

  5. 5.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

  6. 6.Two substances are stirred together and each keeps its own properties, with no chemical reaction between them. What has been made?

    • a) A pure substance
    • b) An element
    • c) A mixture
    • d) A compound

  7. 7.What does the chapter say a mineral is?

    • a) A mixture of two or more rocks
    • b) A metal that has been extracted from a rock
    • c) Any solid substance that people dig out of the ground
    • d) A natural solid substance with a fixed chemical composition, usually a compound and occasionally a pure element

  8. 8.Elements divide into two families. Sort these into theirs.

    Groups: Metal · Non-metal

    • Oxygen
    • Aluminum
    • Gold
    • Sulfur
    • Iron
    • Carbon

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

  1. 1. d) It is a compound, made of carbon, hydrogen and oxygen
  2. 2. Gold → Element; Sulfur → Element; Water → Compound; Sodium chloride → Compound; Air → Mixture; Brass → Mixture
  3. 3. 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 colorless gas with no smell comes off and burns with a pop; Dilute hydrochloric acid is added to the black mass → a colorless gas with a rotten egg-like odor comes off
  4. 4. a) In science, pure means made of a single substance, while in everyday use it means unadulterated
  5. 5. Air → Uniform mixture; Sugar dissolved in water → Uniform mixture; Stainless steel → Uniform mixture; Sprout salad → Non-uniform mixture
  6. 6. c) A mixture
  7. 7. d) A natural solid substance with a fixed chemical composition, usually a compound and occasionally a pure element
  8. 8. Iron → Metal; Aluminum → Metal; Gold → Metal; Carbon → Non-metal; Sulfur → Non-metal; Oxygen → Non-metal