Pure and Impure Substances

In science, a pure substance is defined as one that contains only one type of particle — either a single element or a single compound.

The everyday meaning of 'pure' (natural, clean, safe) is NOT the scientific meaning. Science requires that every particle in the sample is identical.

The option stating 'only one type of particle, either an element or a compound' matches the scientific definition exactly.

This is the everyday/colloquial meaning of pure, not the scientific definition. A natural substance can still be a mixture.

Safety has nothing to do with chemical purity. Pure substances can be toxic (e.g. pure arsenic).

Filtration removes insoluble solids but leaves dissolved substances — the result is still a mixture, not necessarily a pure substance.

Only the option referencing 'one type of particle, element or compound' correctly captures the scientific definition.

A compound is a pure substance made of two or more elements chemically bonded in a fixed ratio. Water ($H_{2}O$) is such a compound.

The key test for purity is whether every particle in the sample is identical. In pure water, every particle is an $H_{2}O$ molecule — there is only one type of particle present.

Because all particles are identical $H_{2}O$ molecules, water is a pure substance (specifically a compound), regardless of being made from two elements.

The physical states of hydrogen and oxygen are irrelevant to whether water is a pure substance.

'Natural' is an everyday concept, not a scientific criterion for purity. Seawater is natural but is a mixture.

While pure substances do have fixed properties, colour and odour alone do not define purity. Some mixtures also have consistent appearance.

The correct explanation is that every particle is an identical $H_{2}O$ molecule, making it a single compound and therefore a pure substance.