1.9- Understand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures

Pure substances, such as elements and compounds, melt and boil at fixed temperatures. For example, the melting point of water is 0⁰C and the boiling point 100⁰C.

However, mixtures usually melt or boil over a range of temperatures. The main reason for this is the presence of impurities, which can lower the melting point and raise the boiling point. For instance, adding sodium chloride to water can affect its boiling point. Therefore, this can sometimes be useful to determine whether a substance is pure. If the melting point should be one number, but is much below, the sample is impure.