Chemical and Physical Changes
Chemical Reactions produce
chemical changes.
Reactants - These are the substances that exist before the reaction. They are the substances that react.
Products - These are the substances that form as a result of the chemical reaction.
The reaction is shown with a chemical equation, with reactants on the left, products on the right and an arrow separating them.
Chemical Formula - Shorthand used in place of the names of chemicals
Subscripts tell how many of each atom there are in a chemical compound.
This  shows there are 3 atoms of hydrogen in sodium acetate.
Conservation of Mass - In a chemical reaction the mass of the reactants must equal the mass of the products. The number of atoms on each side is also the same.
Balancing Equations
Balance equations using coefficients, numbers in front of the formulas. All atoms in the formula are multiplied by the coefficient.
The 2 is a coefficient.
Energy in Chemical Reactions
Sometimes, energy is released from a chemical reaction. This is called an exothermic reaction.
In exothermic reactions, energy is released when bonds are broken in the reactants. It is released as heat or light.
When energy is absorbed in a chemical reaction, it is called an endothermic reaction.
An example of an exothermic reaction is combustion. That is when a substance burns and produces a flame.
During endothermic reactions, more energy is needed to make the bonds of the products than is produced by breaking bonds in the reactants.
A cold pack that you put on an injured muscle is an example of an endothermic reaction. It absorbs heat from your body and makes your skin feel cold.
Chemical Equation shows the reactants, the products, and how much of each there are in a reaction.
Examples of Physical Changes:
Ice Melting
Water Boiling
Chopping Wood
Crinkling Paper