Conservation of mass

Antoine Lavoisier stated that "The law of conservation of mass means that the atoms of an object cannot be created or destroyed, but can be moved around and be changed into different particles."

Mechanics

Laws Conservation of mass Conservation of momentum Conservation of energy Entropy inequality Solid mechanics Solids Stress · Deformation Compatibility Finite strain · Infinitesimal strain Elasticity (linear) · Plasticity Bending · Hooke's law Failure theory Fracture mechanics Frictionless/Frictional Contact mechanics Fluid mechanics Fluids Fluid statics · Fluid dynamics Surface tension Navier–Stokes equations Viscosity: Newtonian, Non-Newtonian Rheology Viscoelasticity Smart fluids: Magnetorheological Electrorheological Ferrofluids Rheometry · Rheometer Scientists Bernoulli · Cauchy · Hooke Navier · Newton · Stokes

This law says that when a chemical reaction rearranges atoms into a new product, the mass of the reactants (chemicals before the chemical reaction) is the same as the mass of the products (the new chemicals made). More simply, whatever you do, you will still have the same amount of stuff.

However, certain nuclear reactions (fusion/fission), can convert a small part of the mass into energy. But if this energy is turned back into mass, you will have the same amount of mass.