Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level 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.
Questions students ask about Identity and Purity
How do you tell whether a measured melting point supports identity or purity?
Identity and purity are separate conclusions from the same measurement. A measured value that agrees with a known value supports identity. A value that is sharp, rather than spread over a range, supports purity, because a pure substance melts or boils at one fixed temperature under stated conditions while an impure sample changes state over a range. A question can ask for either conclusion, and a measurement need not support both.