Elements, compounds and mixtures Study Guide

Many of the useful materials we encounter daily are not pure substances but are, in fact, formulations. A formulation is a mixture that has been designed to give it specific properties for a particular purpose. These properties are often improved or tailored by combining different substances in precise ratios. For example, paints are formulations consisting of a pigment for colour, a binder to hold the pigment to the surface, a solvent to allow for application, and various additives to control drying time, gloss, or durability. Medicines are another critical example, where the active drug ingredient is combined with excipients (inactive substances) to control dosage, stability, taste, and absorption rate. Similarly, cleaning products, fuels, and fertilisers are all formulations. The process of creating a formulation involves careful selection of ingredients and precise measurement of their quantities to achieve the desired performance characteristics. This allows manufacturers to produce materials that are optimised for their intended use, often exhibiting properties that none of the individual components could achieve on their own. The exact composition of a formulation is often a closely guarded trade secret because the specific combination and proportions of ingredients are what make the product effective and unique.

The term 'pure' has different meanings in everyday language and in scientific contexts. In everyday use, a substance is often considered pure if it is untainted or unadulterated; for example, 'pure orange juice' suggests no added sugar or water, but it's still a mixture of many chemicals like water, sugars, acids, and vitamins. Similarly, 'pure gold' in jewellery might be 24-carat gold, meaning it contains only gold atoms, but in common speech, it could also refer to a high-carat alloy that is mostly gold but contains other metals to improve durability. This everyday understanding focuses on the absence of unwanted contaminants or diluting substances, rather than strict chemical composition.

Scientifically, a pure substance is defined as a single chemical element or compound, not mixed with any other substance. A pure element consists of only one type of atom (e.g., pure iron contains only iron atoms), while a pure compound consists of only one type of molecule or formula unit (e.g., pure water contains only H₂O molecules). Each pure substance has a unique and fixed set of physical properties, such as a sharp, specific melting point and boiling point, density, and refractive index. The presence of impurities (other substances) will alter these physical properties, typically lowering the melting point and increasing the boiling point over a range of temperatures, rather than at a single sharp temperature. For example, pure water boils at exactly 100 °C at standard pressure, whereas salty water will boil at a higher temperature and over a range. This difference in physical properties is a key way scientists determine the purity of a substance.