Chemical formulae represent the reactants and products. The equation shows how many atoms are rearranged and joined together differently, and gives the relative amounts of reactants and products.
Chemical bond
Force that holds atoms together in molecules
Chemical reaction
A change in which a new substance is formed. In a chemical reaction, atoms are rearranged and joined together differently.
Combustion
A chemical reaction in which a substance reacts quickly with oxygen and gives out light and heat. Also called burning.
Conservation of mass
In a chemical reaction, the total mass of reactants is equal to the total mass of products. This is conservation of mass. Mass is conserved in chemical reactions and in physical changes.
Conserved
When the quantity of something does not change after a process takes place.
Decomposition
A chemical reaction in which a compound breaks down to form more than one product.
Endothermic reaction
An endothermic reaction takes in energy, usually as heat. In other words, it transfers energy from the surroundings.
Energy level diagram
Diagram showing the relative energies of the reactants and products. It shows whether a reaction is endothermic or exothermic.
Exothermic reaction
An exothermic reaction gives out energy, usually as heat or light. In other words, it transfers energy to the surroundings.
Physical change
One that changes the physical properties of a substance, but no new substance is formed. A physical change is reversible.
Products
Substances that are formed in a chemical reaction, shown on the right of the arrow in a chemical equation.
Reactants
Substances that react together, shown on the left of the arrow in a chemical equation.
