Worked example
A pure sample contains only helium atoms. Which category fits?
Answer & why
Answer: an element
Helium contains only one atomic element.
Grade 8 · Science
Distinguish elements, compounds and mixtures
Use particle descriptions to classify elements, compounds and mixtures.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
A sample contains nitrogen gas and oxygen gas together. Which category fits?
- an element
- a compound
- a mixture
- insufficient composition evidence
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: a mixture
Two substances are present together without forming one pure compound.
Question 2
A pure sample contains only O₂ molecules. Which category fits?
- a compound
- insufficient composition evidence
- a mixture
- an element
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: an element
Both atoms in each molecule are oxygen; only one element is present.
Question 3
Sodium chloride is a pure substance with chemically combined sodium and chlorine in a fixed ratio. Which category fits?
- insufficient composition evidence
- an element
- a mixture
- a compound
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: a compound
Different elements are chemically combined in a fixed composition.
Question 4
A sample is described only as a clear liquid. Which conclusion about these composition categories follows?
- a compound
- insufficient composition evidence
- an element
- a mixture
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: insufficient composition evidence
Appearance alone cannot distinguish an element, compound or mixture.
Question 5
A pure sample contains only helium atoms. Which category fits?
- a mixture
- insufficient composition evidence
- a compound
- an element
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: an element
Helium contains only one atomic element.
Question 6
A pure sample contains only H₂O molecules. Which category fits?
- an element
- a mixture
- insufficient composition evidence
- a compound
A hint
An element contains one type of atom, even when its atoms occur in molecules.
Answer & why
Answer: a compound
Each water molecule contains hydrogen and oxygen bonded in a fixed ratio.