Atoms are the basic units of elements and compounds. In normal chemical reactions, atoms retain their identity. In this section, we will present a quick review of some of the basic terms and concepts such as elements, compounds, and mixtures.
An element is defined as matter that is made of only one type of atom. Elements are the basic building blocks of more complex matter. Some examples of elements include hydrogen (H), helium (He), potassium (K), carbon (C), and mercury (Hg).
A compound is matter formed by the combination of two or more elements in fixed ratios. Let's consider an example. Hydrogen peroxide (H2O2) is a compound composed of two elements, hydrogen and oxygen, in a fixed ratio.
A combination of different elements, or a combination of elements and compounds, or a combination of different compounds is called a mixture. For example, an aqueous solution of potassium hydroxide (KOH + H2O). In this example, the two components are potassium hydroxide and water.
Though these definitions illustrate basic ideas, you need to understand them fully; otherwise it will be almost impossible to decipher higher concepts that are based on these simple ideas. The MCAT tests your understanding of basic concepts by incorporating simple ideas into passages. So in order to succeed on the test, you need to thoroughly understand the basics.