In science, a law is a general statement that summarizes a large number of observations. Before being accepted, a law must be verified many times under many conditions.  A scientific theory is en explanation that explains what under-girds laws and observations. Scientific laws and theories form the core of scientific knowledge and are the products of scientific inquiry.  While laws and theories are widely accepted based on the evidence available at a given time, laws and theories may be modified in light of new evidence.

One scientific law that provides the foundation for understanding in chemistry is the law of conservation of matter. It states that in any given system that is closed to the transfer of matter (in and out), the amount of matter in the system stays constant. A concise way of expressing this law is to say that the amount of matter in a system is conserved.  Atomic theory underlies this law: if matter is composed of small particles we cannot see that are not created or destroyed, then it stands to reason that macroscopic matter which we can see is conserved as well since it is made up of those particles.

What does this mean for chemistry? In any chemical change, one or more initial substances change into a different substance or substances. Both the initial and final substances are composed of atoms because all matter is composed of atoms. According to the law of conservation of matter, matter is neither created nor destroyed, so we must have the same number and type of atoms after the chemical change as were present before the chemical change.

Before looking at explicit examples of the law of conservation of matter, we need to examine the method chemists use to represent chemical changes.