The ''''law of conservation of mass/matter'''' aka Law of Mass Conservation, (The Lomonosov-Lavoisier law) states that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. An equivalent statement is that matter changes form, but cannot be created or destroyed. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
http://en.wikipedia.org/wiki/Conservation_of_mass
Rest Energy
The fact that feeding energy into a body increases its mass suggests that the mass m0 of a body at rest, multiplied by c2, can be considered as a quantity of energy. The truth of this is best seen in interactions between elementary particles. For example, there is a particle called a positron which is exactly like an electron except that it has positive charge. If a positron and an electron collide at low speed (so there is very little kinetic energy) they both disappear in a flash of electromagnetic radiation. This can be detected and its energy measured. It turns out to be 2m0c2 where m0 is the mass of the electron (and the positron).
Thus particles can "vaporize" into pure energy, that is, electromagnetic radiation. The energy m0c2 of a particle at rest is called its "rest energy". Note, however, that an electron can only be vaporized by meeting with a positron, and there are very few positrons around normally, for obvious reasons-they just don't get far. (Although occasionally it has been suggested that some galaxies may be antimatter!)
http://galileo.phys.virginia.edu/classe ... nergy.html