States of matter and mixtures Study Guide

The physical state of a substance (solid, liquid, or gas) depends on its temperature and pressure. For most practical purposes in chemistry, we focus on temperature's impact when atmospheric pressure is assumed. A substance exists as a solid below its melting point, as a liquid between its melting and boiling points, and as a gas above its boiling point. The melting point is the temperature at which a solid changes to a liquid, while the boiling point is the temperature at which a liquid changes to a gas. These characteristic temperatures are specific to each substance and are crucial for predicting its physical state under different conditions. By comparing the given temperature to the substance's known melting and boiling points, we can determine its physical state and provide a justified prediction.

The term 'pure' carries different meanings in everyday language compared to scientific contexts. In daily life, 'pure' often implies something natural, unadulterated, or free from harmful additives. For instance, 'pure orange juice' suggests no added sugar or artificial ingredients. However, in chemistry, a pure substance is defined stringently as a single chemical element or compound not mixed with any other substance. This means it has a fixed, uniform composition and distinct, constant physical properties like a precise melting point and boiling point. Conversely, a mixture consists of two or more pure substances that are physically combined but not chemically bonded. Mixtures retain the properties of their individual components, and their physical properties, such as melting and boiling points, are not fixed but vary depending on the proportions of the constituent substances. For example, 'pure' orange juice, chemically speaking, is a complex mixture of water, sugars, acids, and other compounds. A sample of pure water, H₂O, would only contain H₂O molecules, with a sharp boiling point of 100 °C at standard pressure.