Substance (chemistry)

A chemical substance is a unique form of matter. It always has a constant composition, which means its parts are always the same. This substance will also show characteristic properties. These are special traits that help us identify it.

Substances can exist in different physical states. These states are called phases. A substance might be a solid, a liquid, or a gas. It can even be a plasma. A substance changes between these phases when the temperature or pressure changes. For example, water can be liquid or steam. Even though it looks different, the chemical parts stay the same.

There are two main ways substances are built. One way is through elements. An element is a substance made of only one kind of atom. You cannot break an element down using a chemical reaction.

The other way is through chemical compounds. A compound is formed when two or more elements join together. They do this through a chemical reaction. This creates a new substance with its own set of rules. All compounds are substances, but not all substances are compounds.

Some compounds are organic, meaning they use carbon and hydrogen. Others are inorganic. Some are even called organometallic if they use a bond between carbon and a metal.

Scientists have worked for a long time to understand these rules. In the late eighteenth century, a chemist named Joseph Proust did important work. He studied pure chemical compounds like basic copper carbonate. He found that all samples of a compound have the same proportions of elements. This idea is called the law of constant composition.

Later, researchers faced a hard job with something called isomerism. Isomers are substances that have the same parts but a different arrangement. Friedrich August Kekulé helped explain the structure of benzene. This helped people understand how atoms are put together in different ways.

We can find many different substances in our world. There are 118 known elements in total. About 80 of these are stable, so they do not change on their own.

Many elements are metals like gold, iron, or copper. Metals often shine and carry heat or electricity well. Other elements, like oxygen or nitrogen, are non-metals. Some substances are very common, like table salt or refined sugar. Pure water is a great example of a substance. It always has a ratio of two hydrogen atoms to one oxygen atom. This is written as H2O.

It is easy to confuse a substance with a mixture. A mixture contains two or more substances that are just mixed together. They do not have a fixed composition.

You can often separate a mixture using mechanical ways. For instance, you can use a magnet to pull iron away from sulfur. A mixture like butter or soil is very different from a pure substance. Even things that look smooth can be mixtures. Cranberry glass looks like one color, but it contains tiny gold particles. Understanding the difference helps us see how the world is built.