Elements, compounds and mixtures Study Guide

Chromatography is a powerful analytical technique used to separate and identify components in a mixture. It relies on the principle that different substances have different solubilities in a 'mobile phase' and different affinities for a 'stationary phase'. Paper chromatography and thin-layer chromatography (TLC) are common types. In paper chromatography, the stationary phase is the paper itself (cellulose), and the mobile phase is a solvent that moves up the paper. When a sample, such as an ink spot or food dye, is placed on the baseline of the paper, the solvent starts to move upwards by capillary action. As the solvent front moves, it dissolves the components of the sample. Components that are more soluble in the mobile phase and have less affinity for the stationary phase will travel further up the paper. Less soluble components, or those with stronger affinity for the stationary phase, will travel shorter distances. This differential movement causes the components to separate into distinct spots or bands.

To distinguish between a pure substance and an impure substance using chromatography, we look at the resulting chromatogram. A pure substance, when subjected to chromatography, should ideally produce only one spot on the chromatogram at a specific height, assuming it is not a mixture of isomers or polymorphs that might separate. This single spot indicates that all the material present is uniform and moves together. In contrast, an impure substance, which is a mixture of two or more different compounds, will typically separate into multiple distinct spots or bands on the chromatogram. Each spot represents a different component present in the mixture. Even if some impurities are present in very small amounts, they might still show up as faint separate spots. Therefore, the presence of more than one spot on a chromatogram is a clear indication that the substance under investigation is impure, while a single spot suggests a pure substance.

Chemical bonding is how atoms join together to form compounds and materials. The type of bonding significantly influences the physical and chemical properties of a substance. There are several key types of bonding and resulting structures. Ionic bonding occurs between metals and non-metals, involving the transfer of electrons to form charged ions which are strongly attracted in a giant lattice structure. This leads to high melting and boiling points and good electrical conductivity when molten or dissolved. Simple molecules, formed by covalent bonding between non-metal atoms, involve shared electrons. These molecules typically have low melting and boiling points because the intermolecular forces between individual molecules are weak, even though the covalent bonds within the molecules are strong. Giant covalent structures, also formed by covalent bonding, create a vast network of atoms linked by strong covalent bonds, leading to very high melting points and hardness (e.g., diamond, silicon dioxide). Polymers are large molecules composed of repeating smaller units (monomers) joined by strong covalent bonds. The properties of polymers vary widely depending on their structure and intermolecular forces. Finally, metallic bonding occurs in metals, where positive metal ions are arranged in a lattice and surrounded by a 'sea' of delocalised electrons. This structure accounts for metals' excellent electrical and thermal conductivity, malleability, and ductility. Understanding these differences allows us to predict and explain the behaviour of various materials.