Specification Point 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 Substance vs Mixture
- Contains only one substance, with no other substances mixed together
- Has fixed melting and boiling point as they consist of only one substance so all molecules will have the same melting and boiling points
- E.g. Water has a boiling point of 100°C and a melting point of 0°C
- A combination of two or more substances (elements and/or compounds) that are not chemically joined together
- Has a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
- The physical property of boiling/melting point can therefore be used to test if a substance is pure or not.
- For example, if a sample of water melts at exactly 0°C and boils at exactly 100°C then the water is pure.
- If the melting and boiling points of the water aren’t these exact values then the water must be impure and contain other substances i.e. it must be a mixture.
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Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.
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