Pure naphthalene melts sharply at 80 °C. A student measured the melting point of a sample of naphthalene to test whether it was pure. Fig. 10.1 shows the th...

This question uses a melting point to test purity. The central idea: a pure substance melts sharply at one temperature, while an impure substance melts over a range and at a lower temperature.

(a) The thermometer in Fig. 10.1 reads 80 °C [1] (the mercury reaches the line marked 80).

(b) The completed table:

Sample P = pure [1]; sample Q = impure [1].

(c) A pure substance melts at a single, sharp temperature [1]. Sample Q starts melting at 72 °C and does not finish until 77 °C, so it melts over a range (and below the pure value of 80 °C) rather than sharply, which shows it is impure [1].

(d) An impurity raises the boiling point, so the liquid boils above its normal boiling point [1]. (Impurities lower the melting point but raise the boiling point.)

(e) One precaution to make the reading more reliable (any one) [1]: heat slowly and stir so the whole sample is at the thermometer's temperature; use a small amount of sample; keep the bulb right next to the sample.

(f) A digital thermometer is more accurate because it removes the error of reading a scale by eye (no parallax error) and gives a clear numerical value [1].

Exam tip: "sharp melting point at the expected value" = pure; "melts over a range / below the expected value" = impure. State the actual range from the data to earn the second mark.