2.3 Chemical Properties and Reactions

Chemical Properties and Reactions

Structure of Matter

Compounds and Mixtures

Another type of substance is called a compound. Compounds form when two or more elements chemically combine in a different proportion. For example, water is a compound consisting of the elements hydrogen and oxygen. The proportion of hydrogen to oxygen in water is always 2:1. One molecule, or unit, of the compound water can be represented by the chemical formula H20. This means that one molecule of water consists of 2 hydrogen atoms and 1 oxygen atom.

The properties of a compound differ from the properties of elements of which it is made. For example, water, a liquid compound (at room temperature), is made of the elements of hydrogen and oxygen, which are gases at room temperature.

A third type of substance is called a mixture. Mixtures are combinations of two or more substances that keep their original properties. The substances that form mixtures do not combine chemically. Instead, they mix physically. Soil, salad dressing, and perfumed air are all mixtures. The substances in a mixture can combine in any proportion.

A special type of mixture is called a solution. In a solution, very tiny particles of two or more substances mix evenly. An example of a liquid solution is salt water. In salt water, the compound salt is the solute, the substance that dissolves. Not all solutions are liquid. Solutions can be solid or gaseous, too. For example, steel is a solid solution of carbon in iron. Air is a gaseous solution of nitrogen, oxygen, and other trace elements.

The ingredients of a solution, like those of any mixture, can be separated out. For example, if you heat salt water, the water will evaporate, leaving the salt behind. This is the principle behind extracting salt from seawater. Seawater sits in shallow ponds and evaporates in the heat of the sun. Blocks of sea salt are left behind.