Lesson 5, Topic 1
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Purity

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What is a pure substance ?

 A pure substance contains only one thing 

  • If you have more than one compound or element present than you have a mixture

Testing for purity

  • Every pure substance has a specific, sharp melting point and boiling point. For example, pure ice melts at 0°C, and pure water boils at 100°C

  • You can use this to test the purity of a sample of a substnace, by comparing the actual melting point of the sample to the expected value

  • If a substance is a mixture then it will melt gradually over a range of temperatures rather than having a sharp melting point , like a pure substance

  • Example: The melting points of four powdered solids A,B,C and D are shown below

  • Which of the four solids A, B, C or D is a mixture ?

  • Answer: B, must be a mixture because it melts over a range of temperatures