Cambridge IGCSE Chemistry · Syllabus 0620 · Experimental Techniques and Chemical Analysis
Identity and Purity
What is Identity and Purity?
Two separate conclusions drawn from the same melting point or boiling point measurement. Identity comes from comparison: a measured value that agrees with a known value for a substance supports the sample being that substance. Purity comes from sharpness: a pure substance melts and boils at a fixed characteristic temperature under stated conditions, while an impure sample changes state over a range of temperatures. A measurement can support one conclusion without supporting the other.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge IGCSE Chemistry.
Common mistakes with Identity and Purity
- 10. Identity and purity Fix A melting point that matches a known value supports identity. A melting point that is sharp rather than spread over a range supports purity. They are separate deductions from the same measurement, and a question often asks for only one of them.