Chemistry is the study of matter and the changes that it undergoes, and scientists who study chemistry are called chemists. When chemists study matter they want to find out what materials are made of, how they behave, and why they have those characteristics.

Mass vs. Weight

        Matter is defined as anything that takes up space and has mass. Mass is the "stuff" something has even if it is in outer space where there is no gravity. Weight, on the other hand, changes depending on gravity. Since gravity determines what something weighs, we use a spring scale to measure weight. We use balance scales to measure mass, because the balance will show the same mass no matter where we are in the universe.

Alchemy

        Chemistry has not always been an exact science. The first "chemists" were called alchemists. They were as much wizards as they were scientists. They hoped to turn cheap metals into gold by doing experiments using potions. Although they never did succeed in finding the magic potion to make gold, called the Philosopher’s Stone, they did discover many things about the materials found on our planet.

Atomic Models

        As wizardry gave way to real science, chemists used many models for what they thought matter was made up of. The few basic building blocks that all matter is made of were called elements. These elements were thought to be made of small particles called atoms. Rutherford thought of the atom as being like a miniature solar system with a nucleus of protons and electrons orbiting around it. Later chemists changed the model to include neutrons in the nucleus and used mathematics to describe more complicated orbits. Scientists have discovered the nucleus of an atom holds many types of particles, not just protons and neutrons. The latest models of the atom use the idea of an electron cloud swarming around the nucleus.

Elements and Compounds

        Elements react with each other to form compounds. Compounds are made of molecules of different atoms. Molecules are made of two or more atoms joined together chemically. A compound is not the same thing as a mixture or a solution. A mixture can easily be separated into the original materials by a physical change, such as evaporation.

Chemical Changes

        When elements combine to make a compound something new is formed. It cannot be changed back into its original parts without tremendous effort. Sometimes there is no way of undoing the combination. You can suspect that a chemical change has taken place if a gas is given off, if there is a temperature change, or if the new material has completely different characteristics from the old ones.

A World of Chemistry

        Our universe is full of elements that we take for granted. Grandmother’s skillet may have been made of the element iron. Aluminum foil and Tin roofing are also made of elements. We breathe air that contains the elements Nitrogen and Oxygen in pairs of like atoms. Our sun is made mostly of the gaseous elements Helium and Hydrogen. Our pencil "lead" isn’t really lead. It’s graphite, a form of the element Carbon. Diamonds are also a form of this same element Carbon.

        We use thousands, maybe millions, of compounds every day. We breathe out the compound carbon dioxide, made from the elements Carbon and Oxygen. The water we drink is made of Hydrogen and Oxygen atoms combined in a molecule. Table salt is made of the elements Sodium and Chlorine combined in a molecule. Each of these compounds is made of molecules. The molecules have completely different characteristics from the atoms they are made of.