For chemists, the most elemental building block of matter is the atom. While it is certainly true that the atom can be divided into even more elemental building blocks, it is at the level of the atom that the first distinctive "chemical" properties start appearing. There are many different types of atoms, as you can see in the periodic table of the elements, each with their own distinctive chemical properties. From these atoms molecules can be assembled. Molecules are groups of atoms held together by forces called chemical bonds.
Chemists classify matter into three different states:
A substance containing only one type of atom or one type of molecule is a pure substance. Most of the matter around us, however, consists of mixtures of pure substances. Air, wood, rocks and dirt are examples of such mixtures. Mixtures can be classified as Homogeneous and Heterogeneous.
Air is a homogeneous mixture (gaseous solution) of N2, O2, H2O, and CO2 gases. In contrast, a container of each gas by itself would be a pure substance. Only when they are mixed on an molecular level are they a homogeneous mixture (or gaseous solution).
Brass is a homogeneous mixture (solid solution) of copper and zinc. Again each metal by itself is a pure substance. Only when they are mixed on an atomic level are they a homogeneous mixture (or solid solution).
Beer is a homogeneous mixture (liquid solution) of H2O, C2H5OH, and a few other substances. (Sorry, there is no beer molecule!)
Heterogeneous mixtures are not uniformly mixed on an atomic or molecular level. For example,
Salt and pepper, chocolate chip cookies, or a Twix™ candy bar, ...
are all examples of heterogeneous mixtures, where substances are not mixed on a molecular level.
All mixtures, heterogeneous and homogeneous, can be separated into pure substances using physical methods.
Any change of matter that does not change the type of atoms and molecules within the matter is called a physical change. Water boiling is an example of a physical change. When water boils it is changing from a liquid state to a gaseous state. Chemists would represent this process as follows:
H2O(l) → H2O(g)
Here (l) stands for liquid and the (g) stands for gas. As this is a physical change, the H2O molecule does not change.
To help us understand the concept of chemical change let's examine why light bulbs are made the way they are. A lightbulb works by passing electrical current through a tungsten wire inside the bulb. The tungsten wire is sealed inside a glass bulb since if you did this in air the light bulb would burn out very quickly. This is because the tungsten wire undergoes a chemical reaction with the O2 gas in the air to form tungsten oxide.
2W + 3O2 → 2WO3
To prevent this reaction from happening, all oxygen is removed from the air sealed inside the bulb.
Chemisty, The Central Science, 10th Ed.