Some liquids can be distilled, but only at temperatures that are so high that it is impractical or so high the compound decomposes. Explain why distilling such compounds at reduced temperatures (some degree of vacuum) would solve this problem.

Explanation:

Distillation is a method of purification of miscible liquids, or liquids with some solid contaminants, based on the differences on the boiling points.

By heating the mixture, the component with the lower boiling point will evaporate first. Then, by collecting the stream of vapor and later cooling it, an almost pure substance can be obtained.

Nevertheless, some times, the boling point of the substance at atmospheric  pressure either is too high or exceeds the temperature at which the substance that you want obtain decomposes.

In the case of a high boiling point, the process may be antieconomical, mainly by energetic and economical reasons: elevated amounts of heat energy means a high consumption of fuel, which is scarce, expensive.  and a source of pollution. Which is why the distillation is impractical.

In the case of decomposition of the desired substance, it does not make any sense, even if the economical analysis were favorable.

The boiling temperature may be lowered by lowering the gas pressure above a liquid, which is reached by working at reduced pressures (some degree of vacuum), making the distillation a viable process for those liquids with the mentioned condition.