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Chemistry · Grades 5–8

Elements, compounds and mixtures

Separate chemical composition from physical arrangement.

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Element

An element is identified by its number of protons; its atoms can occur alone or in molecules.

Compound

A compound contains chemically combined elements in a characteristic composition.

Mixture

A mixture contains substances together without a single fixed chemical ratio.

Separation

A physical separation uses a property difference without requiring new substances.

Worked examples

Which description fits a compound?

A compound has chemical bonding and a characteristic composition.

Different elements chemically combined in a fixed composition.

O₂ contains only oxygen atoms. Is it an element or a compound?

More than one atom does not imply more than one element.

An elemental substance.

Check your understanding

  1. Which is a mixture?

    1. Salt dissolved in water.
    2. Pure oxygen gas.
    3. Pure sodium chloride.

  2. Which property could help separate iron filings from sand in a conceptual example?

    1. Magnetic attraction.
    2. The number of letters in their names.
    3. Both having the same color.

  3. Why is the formula for water not changed when extra salt dissolves in it?

Answers and marking notes

Answers and marking notes
  1. Salt dissolved in water.

    A salt solution contains water and dissolved ions in variable amounts.

  2. Magnetic attraction.

    The materials respond differently to a magnet; this is a property-based separation example.

  3. The mixture composition changes, but water molecules still have the formula H₂O.

    Distinguish mixture composition from the identity of the solvent.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.