The table gives the melting points and boiling points of four substances, P, Q, R and S, together with the way each substance melts. Substance Melting point...

This question tests how melting and boiling data distinguish a pure substance from a mixture, and how to read physical state from the data.

(a) A pure substance is a single substance with nothing else mixed in (made of only one type of particle, with a fixed composition) [1].

(b) The impure substance is Q [1]. A pure substance melts sharply at one fixed temperature, but Q melts over a range (-7 to 3 °C), which shows it is impure, a mixture [1].

(c) Compare each fixed point with 25 °C:

• R melts at 801 °C, which is above 25 °C, so at 25 °C R is a solid [1];
• S melts at -39 °C (below 25 °C) and boils at 357 °C (above 25 °C), so at 25 °C S is a liquid [1].

(d) Both elements and pure compounds melt sharply at a single fixed temperature [1]. A sharp melting point therefore only tells you the substance is pure; it does not reveal whether the substance is one element or two or more elements joined as a compound [1].

(e) Dissolving salt in ice lowers the melting point below 0 °C [1], and the mixture then melts over a range of temperatures instead of sharply [1]. This is exactly why salt is spread on icy roads.

(f) Measure the melting point: a pure solid melts sharply at its known melting point, so if the sample melts over a range or at a different temperature it is impure [1].

(g) The highest boiling point belongs to R (1413 °C) [1]. At 1000 °C, R is above its melting point (801 °C) but below its boiling point (1413 °C), so R is a liquid [1].