Elements, compounds and mixtures

O Level / IGCSE Chemistry · Atoms, elements and compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll O Level / IGCSE Chemistry topics

The idea

An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by chemical means. Examples are copper, Cu, and oxygen, O2.

A compound is made of two or more elements chemically bonded together in fixed proportions, such as water, H2O. Its properties are different from those of its elements, and it can only be split up by a chemical reaction.

A mixture contains two or more elements or compounds that are not chemically bonded. The proportions can vary, each substance keeps its own properties, and the parts can be separated by physical methods such as filtration or distillation.

Rules to remember

Common mistake

Do not treat every molecule as a compound. O2, N2 and Cl2 are molecules but they are elements, because each contains only one type of atom.

Worked example

A colourless liquid starts to boil at 78 °C and the temperature keeps rising to 100 °C as it boils. Fractional distillation separates it into two different liquids. Is the liquid an element, a compound or a mixture?

  1. A single pure substance (element or compound) boils at one fixed temperature. This liquid boils over a range, so it is not pure.
  2. Distillation is a physical method. It can separate the parts of a mixture but cannot split a compound into its elements.
  3. Two different liquids are obtained by a physical method, so they were not chemically bonded to each other.

Answer: The liquid is a mixture of two liquids.

Practice questions

Try each one, then open the answer.

1. Iron filings and sulfur powder are mixed. Part of the mixture is then heated strongly to form iron(II) sulfide. Which statement is correct?

  1. A
    a magnet removes iron from neither, because the iron is chemically bonded in both
  2. B
    a magnet removes iron from both, because the iron is still present in the iron(II) sulfide
  3. C
    the iron(II) sulfide has the same properties as the mixture, because both contain iron and sulfur
  4. D
    a magnet removes iron from the mixture but not from the iron(II) sulfide
Show answer

Answer: D. In the mixture the iron keeps its own properties, so it is still magnetic. In the compound the iron is chemically bonded to sulfur and the product has new properties.

2. Which substance is an element?

  1. A
    brass
  2. B
    water
  3. C
    sodium chloride
  4. D
    sulfur
Show answer

Answer: D. Sulfur contains only one type of atom. Water and sodium chloride are compounds; brass is a mixture (an alloy) of copper and zinc.

3. Hydrogen is a flammable gas and oxygen helps things burn, yet water is used to put out fires. What does this show?

  1. A
    compounds have different properties from their elements
  2. B
    water is just a mixture of hydrogen and oxygen gases held together loosely
  3. C
    hydrogen and oxygen atoms are destroyed when they combine to form water
  4. D
    water contains no hydrogen or oxygen, only a new element
Show answer

Answer: A. When elements combine chemically they form a new substance with its own properties. The atoms are still there, but bonded differently.

More questions on this topic

Read the full notes

Keep going

← DiffusionAtomic structure and the Periodic Table →All rules on one pageStuck? Ask a question