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Answers · Elements, compounds and mixtures

Elements, compounds and mixtures — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. ER1 · Return

Model answer. Row A. In a compound the elements are chemically combined, so it is a new substance with new properties, unlike those of its elements. Its elements are always in the same proportions, a fixed ratio. And because they are joined, only another chemical reaction can separate them. A mixture of the same two elements is the opposite in each column: it keeps the properties of its elements, they can be in any proportions, and a physical method can separate them.

A full answer includes

  • I chose row A.
  • I checked that a compound has new properties, unlike those of its elements.
  • I checked that a compound always has its elements in the same proportions.
  • I checked that only a chemical reaction can separate a compound into its elements.
  • I did not choose D: a physical method can separate a mixture, but not a compound.

Also accept

  • Row A

Watch for

  • “Row B.” — A compound has a fixed ratio. If more of one element is used, the extra is left over beside the compound; it does not go into it.
  • “Row C.” — Keeping the properties of its elements is what a mixture does. When elements combine, the new substance has new properties: iron sulfide isn't pulled by a magnet, although iron is.
  • “Row D.” — A physical method, such as a magnet or dissolving, only separates substances that are not joined. In a compound the atoms of its elements are joined, so only a chemical reaction can split it.

2. ER2 · Return

Model answer. The gas has two kinds of particle, helium atoms and neon atoms, so it is a mixture. It is not an element, because it has two kinds of atom. It is not a compound, because no helium atom is joined to a neon atom: the two elements are only side by side, and they could be mixed in any proportions.

A full answer includes

  • I explained that the gas has two kinds of particle, helium atoms and neon atoms.
  • I explained that it is not an element, because it has two kinds of atom.
  • I explained that it is not a compound, because no helium atom is joined to a neon atom.
  • I did not call it a compound just because it contains two elements.

Also accept

  • Two kinds of particle (helium atoms and neon atoms) not joined to each other: a mixture. Not an element (two kinds of atom); not a compound (no atoms joined).

Watch for

  • “All the atoms are single, so the gas is an element.” — Single atoms can belong to different elements. Here there are two kinds of atom, helium and neon, so the gas is not one element.
  • “It contains two elements, so it is a compound.” — A compound needs atoms of different elements joined together. The helium and neon atoms are not joined, so they are only mixed.