Purity of a Substance
Purity of a Substance
Definition
- Purity refers to a substance that consists of only one type of particle.
- A substance can be said to be pure if it has a fixed boiling and melting point, which corresponds to the standard value.
Impurities and Effect on Boiling Point
- Impurities in a substance lead to changes in the boiling and melting points.
- An impurity in a substance causes an increase in the boiling point and a decrease in the melting point of that substance.
- The presence of impurities in a substance can lead to a range of boiling and melting points, rather than a fixed point.
Impurities and Effect on Melting Point
- The melting point of an impure substance is lower than that of a pure one.
- The range over which the substance melts is also typically broader for impure substances compared to pure substances.
Testing for Purity
- Melting point and boiling point tests are the common methods for testing the purity of a substance.
- Chromatography is another technique used to check the purity of a substance. This technique separates out each component in a mixture and allows identification of each.
Using Chromatography to Determine Purity
- Chromatography builds on the premise that different substances move at different rates on a coated surface.
- The distance moved by each component is specific to its identity, size, and shape, enabling it to be identified.
- The retention factor (Rf) is a number that allows identification of a substance. It is calculated by dividing the distance moved by the substance by the total distance moved by the solvent.
Remember, a pure substance will always have the same properties and compositions, irrespective of its source or method of preparation. However, use rigorous testing methods to achieve conclusive results.