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Elements, Compounds and Mixtures — Answers

Answer sheet with model answers and success criteria for the Year 8 mixtures & chemical reactions worksheet.

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Success TutoringWorksheetsAnswer sheetYear 8 · Science · Mixtures & Chemical Reactions

Elements, Compounds and Mixtures — Answers

Part A: Classify the substance

An atom is the smallest particle of an element. An element contains only one type of atom and is shown by a symbol, such as O for oxygen or Fe for iron.

A molecule is two or more atoms chemically joined. A compound contains two or more different elements chemically joined in a fixed ratio, such as water, H₂O.

A mixture contains two or more substances that are not chemically joined. Their amounts can vary, and they can be separated by physical methods such as filtering or evaporation.

1.Oxygen gas (O₂)
  1. AElement
  2. BCompound
  3. CMixture

Answer: Element

O₂ molecules contain only oxygen atoms, so oxygen is an element.

2.Water (H₂O)
  1. AElement
  2. BCompound
  3. CMixture

Answer: Compound

Water contains hydrogen and oxygen atoms chemically joined in a fixed ratio of 2 : 1.

3.Air
  1. AElement
  2. BCompound
  3. CMixture

Answer: Mixture

Air contains nitrogen, oxygen, argon, carbon dioxide and other gases that are not chemically joined.

4.Salt water
  1. AElement
  2. BCompound
  3. CMixture

Answer: Mixture

Salt dissolved in water is a mixture: the salt can be recovered by evaporating the water.

5.Copper (Cu)
  1. AElement
  2. BCompound
  3. CMixture

Answer: Element

Copper contains only one type of atom.

6.Carbon dioxide (CO₂)
  1. AElement
  2. BCompound
  3. CMixture

Answer: Compound

Carbon and oxygen atoms are chemically bonded together.

7.All molecules are compounds.

Answer: False

A molecule is two or more atoms joined together. In O₂ or N₂ the atoms are all the same element, so these molecules are elements, not compounds.

Part B: Symbols and formulas

A chemical formula shows which elements are in a substance and how many atoms of each. In NH₃ (ammonia), there is 1 nitrogen atom and 3 hydrogen atoms. A small number (subscript) applies to the symbol just before it. A large number in front (2H₂O) tells you how many molecules there are.

8.Match each symbol to its element.
  • Na
  • Fe
  • K
  • Cl
  • Ag
  • Chlorine
  • Potassium
  • Sodium
  • Silver
  • Iron

Answer: Na → Sodium; Fe → Iron; K → Potassium; Cl → Chlorine; Ag → Silver

9.How many atoms of each element are in one molecule of methane, CH₄? How many atoms in total?

Answer: C 1, H 4; 5 atoms in total

  1. Carbon: 1 (no number means 1)
  2. Hydrogen: 4
  3. Total = 1 + 4 = 5
10.How many atoms of each element are in one molecule of glucose, C₆H₁₂O₆? How many atoms in total?

Answer: C 6, H 12, O 6; 24 atoms in total

  1. Total = 6 + 12 + 6 = 24
11.How many atoms of each element are in sulfuric acid, H₂SO₄? How many atoms in total?

Answer: H 2, S 1, O 4; 7 atoms in total

  1. Total = 2 + 1 + 4 = 7
12.What does 2H₂O mean? How many hydrogen and oxygen atoms are there altogether?

Answer: Two water molecules; 4 hydrogen atoms and 2 oxygen atoms

  1. The large 2 means two molecules
  2. Hydrogen: 2 × 2 = 4
  3. Oxygen: 2 × 1 = 2

Part C: Mixture or compound?

13.A student mixes iron filings and sulfur powder. How could she separate them again? Why does this work?

Answer: Use a magnet to pull out the iron filings. It works because in a mixture the iron keeps its own properties, and iron is magnetic while sulfur is not.

14.When the iron and sulfur mixture is heated strongly, it glows and forms a black solid, iron sulfide (FeS). Why can a magnet no longer separate the iron?

Answer: The iron and sulfur have chemically reacted to form a new compound with different properties. The iron atoms are now bonded to sulfur atoms, and iron sulfide is not magnetic like iron.

15.Which statement about compounds is correct?
  1. AThe elements in a compound can be separated with a magnet or by filtering
  2. BA compound always has the same properties as the elements in it
  3. CA compound has a fixed ratio of elements and usually very different properties from those elements
  4. DA compound can contain any amount of each element

Answer: A compound has a fixed ratio of elements and usually very different properties from those elements

For example, sodium is a reactive metal and chlorine is a toxic gas, but the compound sodium chloride is table salt.

Part D: Compare

16.Compare a mixture of hydrogen and oxygen gases with the compound water. In your answer refer to how the atoms are arranged, the ratio of elements, properties, and how each could be separated.[4 marks]

Model answer: In the mixture, hydrogen molecules (H₂) and oxygen molecules (O₂) move around separately and are not chemically joined; they can be present in any ratio. Each gas keeps its own properties, so the mixture is a colourless gas that could explode if lit, and the gases could be separated by physical means. In water, each molecule has two hydrogen atoms chemically bonded to one oxygen atom, so the ratio is always 2 : 1. Water has new properties: it is a liquid at room temperature and puts out fires rather than burning. It can only be broken down into hydrogen and oxygen by a chemical reaction, such as passing electricity through it (electrolysis).

Success criteria:

  • Mixture: particles not chemically joined; compound: atoms chemically bonded
  • Mixture ratio can vary; compound ratio fixed at 2 : 1
  • Compound has different properties from its elements
  • Mixture separated physically; compound separated only by a chemical reaction
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