Chemistry is the study of matter and its changes and interactions. Matter is anything that has mass and takes up space. There are two broad categories of matter—Pure Substances and Mixtures.
Elements and compounds are both pure substances. A pure substance is when each particle is identical. Elements have each atom the same and compounds have each molecule the same. Compounds are atoms of more than one element chemically bonded together.
Mixtures are more than one type of pure substance physically mixed together. Mixtures can be categorized into homogeneous and heterogeneous. Homogeneous mixtures (also called solutions) look the same throughout; while heterogeneous have visible different types of matter.
Energy is the ability to produce heat or do work. There are two types of energy: Potential energy (or stored energy) and kinetic energy (energy due to motion).
Chemical changes produce a new substance while physical changes do not. Changes in state (melting, freezing, boiling, condensing, etc.) are physical changes. Dissolving is also a physical change, although it is often confused for a chemical change. Reacting with another type of matter, burning or rusting are examples of chemical changes. Often confused changes are melting (changing a solid to a liquid by adding heat), burning (chemically reacting with oxygen) and dissolving (combining two types of matter physically to produce a mixture). Mixtures can be separated by physical changes, compounds must be separated by chemical changes and elements cannot be separated by either.
There are many paths to follow when undertaking “science”—there is no one scientific method. Science involves observing, posing questions, forming possible explanations (hypothesis), experimenting, processing/analyzing data, looking for trends, more formation of possible explanations or question posing. Scientific processes form theories (which attempt to explain observed behavior) and laws (which describe or predict behavior, and are usually mathematical). A theory cannot become a law—one explains why and one describes what. A hypothesis is a proposed explanation for why something will occur (that may become a theory with enough evidence), while a prediction is simply a guess at what will happen—it does not attempt to say “why” it will happen.