Classification of matter

Chemistry has been defined as the study of matter and the changes it undergoes. Matter is everything that occupies space and has mass. Matter includes what we can see and touch, like water, earth, and trees, and what we cannot see or touch, like air. So, everything in the universe has a 'chemical' connection.

Chemists distinguish various subtypes of matter based on their composition and properties. The classification of matter includes substances, mixtures, elements, and compounds, in addition to atoms and molecules.

Substances and Mixtures

A substance is a form of matter that has a defined (constant) composition and distinctive properties. Examples of substances include water, ammonia, table sugar (sucrose), gold, and oxygen. Substances differ from each other in their composition and can be identified by their appearance, color, taste, and other properties.

A mixture is a combination of two or more substances in which they retain their properties. Some familiar examples of mixtures are air, carbonated beverages, milk, and cement. Mixtures do not have a constant composition. Therefore, samples of air obtained in different cities are likely to differ in composition due to differences in altitude, atmospheric pollution, etc.

Mixtures can be homogeneous or heterogeneous. When you dissolve a spoonful of sugar in water, you get a homogeneous mixture in which the composition of the mixture is uniform, and you cannot distinguish its substances by simple sight. However, when you mix sand with iron shavings, both remain separate. In such a case, it is called a heterogeneous mixture because its composition is not uniform, and you can observe the substances that make it up by simple sight.

Any mixture, whether homogeneous or heterogeneous, can be formed and then separated by physical means into its pure components without changing the identity of these components. Thus, sugar can be recovered from an aqueous solution by heating it and completely evaporating it. Condensing the vapor allows the recovery of water. As for the separation of the iron-sand mixture, it is possible to use a magnet to separate the iron shavings since the magnet does not attract the sand itself. After separation, the components of the mixture will have the same composition and properties as they did initially.

Elements and Compounds

Substances can be elements or compounds. An element is a substance that cannot be separated into simpler substances by chemical means. To date, 118 elements have been identified, with most of them naturally occurring on Earth. The others have been obtained through scientific means via nuclear processes.

For convenience, chemists use one or two-letter symbols to represent elements. The first letter of the symbol is always capitalized, but not necessarily the second letter. For example, Co is the symbol for the element cobalt, whereas CO is the formula for the molecule carbon monoxide.

In the table below, you can see the names and symbols of some common elements. The symbols of some elements are derived from their Latin names, for example, Au from "aurum" (gold), Fe from "ferrum" (iron), and Na from "natrium" (sodium). In many other cases, the symbols correspond to their names in English.

Atoms of many elements can interact with each other to form compounds. For example, the combustion of gaseous hydrogen with gaseous oxygen forms water, which has properties that clearly differ from those of the elements that make it up. Water consists of two parts of hydrogen to one part of oxygen. This composition does not change, whether the water comes from a tap in the United States, a lake in Mongolia, or the ice caps of Mars. Therefore, water is a compound, meaning it is a substance formed by atoms of two or more elements chemically bonded in fixed proportions. Unlike mixtures, compounds can only be separated into their pure components by chemical means.

The relationships between elements, compounds, and other categories of matter are summarized in the following concept map: