Question Video: Identifying Which Properties of a Sample Can Be Used to Help in Testing Its Purity Chemistry

Video Transcript

Which of the following can help in testing the purity of a sample? (A) The sample’s boiling point only. (B) The sample’s melting point only. (C) The sample’s boiling point and melting point. (D) The sample’s color. (E) The sample’s physical state.

Purity is a measure of the extent to which a substance is free from impurities or undesired substances. The question asks which of the following can be used to help in testing the purity of a sample. Let’s consider two beakers, one containing pure water and one containing salt water, which could be said to contain salt impurities. The two substances have the same physical appearance and physical state. So the sample’s color and the sample’s physical state cannot help us in testing the purity of the sample.

Let’s consider the boiling point of these two samples. Boiling point is the temperature at which the vapor pressure of a liquid equals the surrounding pressure. For pure water at one atmosphere, the boiling point is 100 degrees Celsius. But when salt, an impurity, is added to water, the boiling point is no longer 100 degrees Celsius. This is because the impurities interfere with the ability of the water molecules to escape the liquid phase, which tends to decrease the vapor pressure. When the vapor pressure decreases due to adding a solute, the solution will need to be heated to a higher temperature in order for the liquid to boil. In general, the more solute that is dissolved or the more impure the substance, the higher the boiling point.

So we now know that boiling point can be used to help in testing the purity of a sample. But what about melting point? To help us understand how impurities might affect the melting point of a sample, let’s examine pure sucrose and impure sucrose. The molecules of sucrose in a pure sample, represented by the squares in this figure, are arranged in an orderly crystal lattice that maximizes the intermolecular forces between the molecules. In order for the sample to melt, energy must be supplied to overcome the intermolecular forces between the molecules. A pure sample of sucrose will melt at a temperature of 186 degrees Celsius.

In an impure sucrose sample, the impurities disrupt the orderly crystal lattice, weakening the intermolecular forces in the solid. Because the intermolecular forces are weaker and the impurities are irregularly dispersed throughout the solid, an impure sample will have a slightly lower melting point that occurs over a range of temperatures instead of sharply at just one value. So, the melting point of a sample can also be used to help test the purity.

Therefore, to help in testing the purity of a sample, we can use both the sample’s boiling point and melting point, answer choice (C).