A substance is said to be impure if

Analyze the question: The question asks which observation tells us that a substance is impure. This is about the physical criteria of purity.

Concept: A pure substance has fixed, sharp physical constants:

• It melts at one definite temperature, or over a very narrow range of about 1°C.
• It boils at a definite temperature at a given pressure.

Impurities disturb the arrangement of particles in the substance. This has two effects:

1. The melting point is lowered.
2. The melting happens over a wide range of temperature instead of at a sharp point.

This is why chemists test purity by measuring melting point. A sharp melting point means the substance is pure, and a broad range means it is impure.

Evaluate each option:

• A. Its melting point range is wide. Correct. A wide range is the standard sign of impurity.
• B. It dissolves in water with difficulty. Incorrect. Solubility depends on the nature of the substance. Many pure substances, such as pure sulphur or calcium carbonate, are insoluble or sparingly soluble in water.
• C. It has a low melting point. Incorrect as a general test. Many pure substances have naturally low melting points, such as ice at 0°C or naphthalene at about 80°C. Impurities lower a substance's melting point relative to its pure value, but a low value alone does not show impurity.
• D. It is coloured. Incorrect. Many pure substances are coloured, such as copper(II) sulphate (blue) and potassium permanganate (purple).

Answer: A.

Exam tip: Remember "Pure = Sharp, Impure = Wide." If a question asks about purity, think of a sharp melting point and a fixed boiling point as the signs of a pure substance. A melting or boiling range that is wide or varying signals impurity.