Elements, compounds and mixtures Study Guide

Chromatography is a powerful analytical technique used to separate components of a mixture. It works on the principle that different substances have varying affinities for two phases: a stationary phase and a mobile phase. The stationary phase is a fixed material, such as paper or a solid packed in a column, that does not move. The mobile phase is a fluid (either a liquid or a gas) that moves through the stationary phase, carrying the components of the mixture with it. The separation occurs because substances distribute themselves differently between these two phases. Substances that are more soluble in the mobile phase and/or have less affinity for the stationary phase will travel further and faster. Conversely, substances that are less soluble in the mobile phase and/or have a stronger affinity for the stationary phase will move slower or barely move at all. This differential distribution based on solubility and adsorption allows for the effective separation of complex mixtures into their individual components. For example, in paper chromatography, the paper acts as the stationary phase and a solvent acts as the mobile phase, separating pigments in an ink.

The melting point of a substance is a specific temperature at which it changes from a solid to a liquid state. For a pure substance, this change occurs sharply at a single, distinct temperature. For example, pure water melts at exactly 0 °C under standard atmospheric pressure. This precise melting point is a characteristic physical property that can be used to identify pure substances.

However, when a substance is impure, meaning it contains one or more different substances (impurities), its melting behaviour changes significantly. Impurities disrupt the regular arrangement of particles in a solid, which affects the energy required to break the forces holding them together. As a result, an impure substance will melt over a range of temperatures, rather than at a single, sharp point. Furthermore, the presence of impurities generally causes the melting point to be lower than that of the pure substance. This phenomenon is known as melting point depression. For instance, if you add salt (an impurity) to water, the water will freeze (and therefore melt) at a temperature below 0 °C. The greater the amount of impurity, the larger the depression in the melting point and the wider the melting range. This principle allows chemists to use melting point determination as a quick and effective method to assess the purity of a sample. If a substance melts sharply at its known characteristic melting point, it is likely pure. If it melts over a broad range of temperatures and at a lower temperature than expected, it is impure.