Assessing Purity and Separating Mixtures

Assessing Purity

Defining Pure Substances and Mixtures

A pure substance contains only a single substance, either an element or a compound, with definite and constant properties.
A mixture, on the other hand, is composed of two or more substances brought together that do not chemically combine; these substances maintain their identities and can be separated by physical techniques.

Considering Physical Properties

Physical properties like melting point and boiling point can be used to assess the purity of a substance. Pure substances have fixed melting and boiling points.
In a mixture, the substances present may each affect the boiling and melting points, causing a range rather than a fixed point.

Separating Mixtures

Understanding Filtration and Crystallisation

Filtration is a physical technique used to separate mixtures composed of a solid suspended in a liquid.
The method of crystallisation involves heating a solution until the solvent evaporates, leaving behind a crust of crystals that can then be collected.

Chromatography

Chromatography separates components of a mixture based on the relative affinity of each component towards a mobile phase and a stationary phase.
Fundamentally, the substances that are more attracted to the mobile phase travel faster and further, while those attracted to the stationary phase lag behind.
The strength of a substance’s affinity toward these phases can be described with a retention factor (Rf) value.
Same retention factors across different chromatograms are one indicator of substance identity, but these should not be used alone to confirm identity.

Distillation

Distillation separates liquids in a mixture, exploiting their differences in boiling points.
The liquid with the lowest boiling point evaporates first and is then cooled and collected as a pure liquid.
There are two commonly used types of distillation: simple distillation (for liquids with significantly different boiling points) and fractional distillation (for liquids with close boiling points).
On an industrial scale, fractional distillation is used in oil refineries to separate crude oil into useful fractions.

Separating Funnel

When two immiscible liquids are in a mixture, a separating funnel may be used.
The mixture is poured into the funnel, and after being allowed to stand and separate, the denser liquid can be drained off from the bottom.

Remember these are physical separation methods, relying on differences in physical properties such as solubility, boiling point and adhesive forces without changing the substances chemically.