This deep dive covers Melting and Boiling Points as Purity Tests within Purity & Formulations for GCSE Chemistry. Revise Purity & Formulations in Chemical Analysis for GCSE Chemistry with 22 exam-style questions and 12 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 3 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
🌡️ Melting and Boiling Points as Purity Tests
Pure substances have sharp, fixed melting and boiling points. All the particles are identical, so they all need the same energy to change state. The temperature holds perfectly steady during the change.
- Pure ice melts at exactly 0 °C
- Pure water boils at exactly 100 °C (at 1 atm)
Impure substances melt and boil over a range of temperatures because the different particles have slightly different intermolecular forces, disrupting the pattern.
Practice questions for Purity & Formulations
In chemistry, what does it mean for a substance to be described as 'pure'?
Explain why the presence of impurities in a substance lowers its melting point and causes it to melt over a range of temperatures.