2. Methods of Purification
Determining purity
Pure substance: Has a fixed and constant melting & boiling point.
Impure substance:
How to determine whether a substance is pure?
EXAM TIP: Only these 3 methods are allowed to determine purity.
Effect of impurities on melting and boiling point
Effect of pressure on boiling point (Optional)
- A pure substance is made up of only one substance and is not mixed with any other substance.
- A mixture is a substance that contains two or more substances that are not chemically combined.
Pure substance: Has a fixed and constant melting & boiling point.
Impure substance:
- Melting point is lower than that of the pure solid, melts over a range of temperature
- Boiling point is higher than that of the pure liquid, boils over a range of temperature
How to determine whether a substance is pure?
- By checking its melting and boiling point: Pure substances have fixed melting and boiling points.
- By using chromatography
EXAM TIP: Only these 3 methods are allowed to determine purity.
Effect of impurities on melting and boiling point
- Cause melting and boiling to take place over a range of temperatures.
- Decrease melting point: The greater the amount of impurities, the lower the melting point of the substance.
- Increase boiling point: The greater the amount of impurities, the higher the boiling point of the substance.
Effect of pressure on boiling point (Optional)
- As pressure acting on liquid increases, the boiling point increases.
- As pressure acting on liquid decreases, the boiling point decreases.
