Pure and impure substances

The scientific definition of a pure substance is based on its composition, not its appearance. A pure substance contains only one type of particle — it is made entirely of one element or one compound.

The option 'a substance that contains only one type of particle throughout' matches the scientific definition exactly. It focuses on composition, which is what determines purity in chemistry.

Looking clean, being filtered, or dissolving easily do not determine purity. A filtered liquid can still be a mixture if dissolved substances remain in it. The scientific meaning of 'pure' is specific and different from everyday usage.

The correct answer is 'a substance that contains only one type of particle throughout' because this is the precise scientific definition of a pure substance.

The question asks for the best scientific description of a pure substance. We need the option that captures the key scientific meaning of 'pure'.

This is eliminated because 'pure' in science does not mean clean in appearance. Tap water can look perfectly clear but is still a mixture containing dissolved minerals.

This is eliminated because filtration only removes insoluble particles. After filtering, dissolved substances remain, so the result is still a mixture — not necessarily a pure substance.

This is eliminated because solubility has nothing to do with purity. Sugar and salt both dissolve easily, but neither describes purity itself.

'A substance that contains only one type of particle throughout' is correct. This is the precise scientific definition — one element or one compound, with no other substances mixed in.

A pure substance melts at a single, sharp, fixed temperature. A mixture (impure substance) melts over a range of temperatures, and its melting point is usually lower than expected for the pure material.

The powder melts sharply at exactly $135°C$ — this is a single fixed temperature with no range described. This is exactly what we expect from a pure substance.

The sharp, fixed melting point is evidence of purity. We say 'likely' because melting point alone cannot tell us whether it is an element or a compound — but it strongly indicates the substance is pure.

The correct answer is 'likely a pure substance because it has a sharp, fixed melting point.' The sharpness and consistency of the melting point are the key evidence.

The question tests whether students can interpret melting point data as evidence for purity or impurity. The key fact is a sharp melt at exactly $135°C$.

This is eliminated because the melting temperature itself (high or low) does not indicate purity. The sharpness of the melting point is what matters, not the temperature value.

This is eliminated because melting point data alone cannot tell us whether the substance is an element or a compound. It can only indicate purity.

This is eliminated because nothing in the question suggests visible settling particles. A white powder is a solid being tested, not a liquid mixture.

'Likely a pure substance because it has a sharp, fixed melting point' is correct. A single fixed melting temperature is the hallmark of a pure substance.