Test: Elements, compounds and mixtures - Year 7 MCQ

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Oxygen is an element because it consists of only one type of atom. Water (H₂O) and salt (NaCl) are compounds, while air is a mixture of different gases.

A mixture is a combination of two or more substances where each substance retains its own chemical identity. The components of a mixture can be separated by physical means such as filtration, evaporation, or decantation. Mixtures do not have a fixed composition and can be either homogeneous (uniform composition) or heterogeneous (non-uniform composition).

A compound is a substance formed when two or more elements are chemically bonded together in a specific ratio. Compounds have different properties from the elements that make them up. For example, water is a compound made of hydrogen and oxygen in a 2:1 ratio (H₂O).

A homogeneous mixture has a uniform composition throughout. When salt is dissolved in water, it forms a homogeneous mixture because the salt is evenly distributed throughout the water. Sand and iron filings, oil and water, and salad are all examples of heterogeneous mixtures where the components are not uniformly distributed.