A pure substance (or simply substance) is the matter that has properties and composition characteristics that do not change from one sample to another. Water and salt (sodium chloride), the components of seawater, are examples of pure substances. Many forms of matter we know, for example the air we breathe (a gas), the water we drink (a liquid) and the bread we eat (a solid) are not chemically pure. We can separate these forms of matter in different pure substances.
All substances are either elements or compounds. The elements can not be decomposed into simpler substances. At the molecular level, each element is composed of a single type of atom. The compounds are substances formed by two or more elements, that is, containing two or more different kinds of atoms. Water, for example, is a compound formed by two elements: hydrogen and oxygen. Finally, mixtures are combinations of two or more substances wherein each chemical substance retains its own identity.
Currently are known 117 elements with a very varied abundance. Thus, 90% of the earth’s crust (including oceans and atmosphere) consists of only five elements: oxygen, silicon, aluminum, iron and calcium. Similarly, only three elements –oxygen, carbon and hydrogen– constitute more than 90% of the mass of the human body.
In the above table are shown some of the most common elements with their symbols or “abbreviations” used to represent them. The symbol for each element consists of one or two letters (the first always capitalized). The symbols are derived from their names either in English or Latin. It is very useful to know the symbols of chemical elements.
Many elements can interact with other elements to form compounds. When hydrogen burns in the presence of oxygen, the elements hydrogen and oxygen combine to form water compound. By contrast, water is decomposed into its components by passing through it an electric current. Pure water, regardless of their origin, contains 11% hydrogen and 89% oxygen by weight. This macroscopic composition corresponds to the molecular composition consisting of two hydrogen atoms combined with one oxygen.
As shown in the following table, the properties of water have no resemblance to those of its components. Hydrogen, oxygen and water are, each, unique substances due to the singularity of their respective molecules.
The observation that the elemental composition of a pure compound is always the same is known as the law of constant composition (or law of definite proportions). The French chemist Joseph Louis Proust (1754-1826) established this law by 1800. A pure compound has the same composition and properties regardless of their origin. Chemists and Nature use the same elements and work under the same natural laws. When two materials differ in composition and properties, or they are formed of different compounds either have different purities.
Most of the matter that we find are mixtures of different substances. In a mixture, each substance retains its chemical identity and properties. In contrast to a pure substance, having a fixed composition, the composition of a mixture can vary. For example a cup of sweetened coffee can have a little or a lot of sugar. The substances forming a mixture (such as sugar and water) are known as components of the mixture. Some blends do not have the same composition, properties and appearance as a whole. Rocks and wood, for example, vary in texture and appearance in any typical sample. Such mixtures are called heterogeneous. Those mixtures that are uniform as a whole, are called homogeneous. Air is a homogeneous mixture of gaseous nitrogen, oxygen and small amounts of other substances. Nitrogen from the air has the same properties as pure nitrogen because both the pure substance and the mixture contains the same nitrogen molecules. Salt, sugar and many other substances are dissolved in water to form homogeneous mixtures. Homogeneous mixtures are also called solutions. Although the term solution normally reminds us a liquid in a beaker or flask, the solutions can be solids, liquids or gases.
Since each component of a mixture retains its properties, we can separate a mixture into its components taking advantage of differences in their properties. For example, a heterogeneous mixture of gold and iron filings can be separated by the color difference of the two components. A simpler way would be using a magnet to attract the iron filings separating the gold. We could also use a chemical property; many acids dissolve iron but not the gold. Thus, if we put the mixture in a suitable acid, the acid will dissolve the iron leaving the gold unchanged. A filtration will separate the gold from the dissolved iron. Through a chemical reaction, as we will discuss in the future, the dissolved iron can be transformed in iron metal.