Introduction of chemistry

Chemistry, study of the composition, structure, properties, and interactions of matter. Chemistry arose from attempts by people to transform metals into gold beginning about ad 100, an effort that became known as alchemy (see Chemistry, History of). Modern chemistry was established in the late 18th century, as scientists began identifying and verifying through scientific experimentation the elemental processes and interactions that create the gases, liquids, and solids that compose our physical world. As the field of chemistry developed in the 19th and 20th centuries, chemists learned how to create new substances that have many important applications in our lives.

Chemists, scientists who study chemistry, are more interested in the materials of which an object is made than in its size, shape, or motion. Chemists ask questions such as what happens when iron rusts, why iron rusts but tin does not, what happens when food is digested, why a solution of salt conducts electricity but a solution of sugar does not, and why some chemical changes proceed rapidly while others are slow. Chemists have learned to duplicate and produce large quantities of many useful substances that occur in nature, and they have created substances whose properties are unique.

Much of chemistry can be described as taking substances apart and putting the parts together again in different ways. Using this approach, the chemical industry produces materials that are vital to the industrialized world. Resources such as coal, petroleum, ores, plants, the sea, and the air yield raw materials that are turned into metal alloys; detergents and dyes; paints, plastics, and polymers; medicines and artificial implants; perfumes and flavors; fertilizers, herbicides, and insecticides. Today, more synthetic detergent is used than soap; cotton and wool have been displaced from many uses by artificial fibers; and wood, metal, and glass are often replaced by plastics.

Chemistry is often called the central science, because its interests lie between those of physics (which focuses on single substances) and biology (which focuses on complicated life processes). A living organism is a complex chemical factory in which precisely regulated reactions occur between thousands of substances. Increased understanding of the chemical behavior of these substances has led to new ways to treat disease and has even made it possible to change the genetic makeup of an organism. For example, chemists have produced strains of food plants that are hardier than the parent strain.

Because the field of chemistry covers such a broad range of topics, chemists usually specialize. Thus, chemistry is divided into a number of branches, some of which are discussed at the end of this article. Nevertheless, the process of learning the properties of a substance and of taking it apart is fundamental to nearly all of chemistry.

The first step in investigating a complex material is to try to break it down into simpler substances. Sometimes this is easy. A mixture of brass and iron tacks, for instance, could be sorted with a magnet or even by hand. Getting the salt out of brine or seawater is a little harder, but the water can be evaporated, leaving the salt. Changes of this sort, which do not alter the fundamental nature of the components of the mixture but do modify their physical condition, are called physical changes. Grinding a rock, hammering a metal, or compressing a gas causes physical changes. Another example of physical change is the melting of ice, in which water changes from the solid to the liquid state.

Salt and water may not only be separated when in solution, but each may be broken down into other substances. This, however, involves a different kind of change—one that usually requires more energy than a physical change and that alters the fundamental nature of the material. This type of change is called a chemical change. By applying electrical energy, water can be broken down into two gases, hydrogen and oxygen. Hydrogen is a light gas that burns; oxygen is a gas that is necessary to sustain animal life. Salt can be broken down by melting it, then passing an electric current through it. This produces a pungent yellow-green gas called chlorine and a soft, silvery metal called sodium, which burns readily in air.

Some materials can be broken down simply by heating them. Other materials yield to attack by another substance; for example, iron oxide ore heated with coke yields metallic iron.