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Cambridge IGCSE chemistry
Lesson 5, Topic 1
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