Pure and Impure Substances

Real Year 9 Science questions. No account needed.

A pure substance has a sharp, single melting point. An impure substance melts over a range of temperatures. Which of the following best explains why an impure substance melts over a range of temperatures?

• The impurities are different substances mixed together, so they disrupt the regular arrangement of particles and cause melting to start at a lower temperature and finish at a higher one.
• The impurities make the substance denser, so it needs more heat energy to melt all at once.
• The impurities turn the substance into a gas before it can fully melt.
• The impurities chemically react with the substance, creating a new compound that has no melting point.