Hello, I am an oxygen atom. I cannot dissapear or suddenly appear, just like humans!
The Law of Conservation of mass is a law that states this: Atoms can't be created or destroyed.
Look there are my two hdrogen friends walking down the streets!
Right now we are just one oxygen atom and two hydrogen atoms.
Now that we have gone through a chemical change we are H2o, which is water.
Before the chemical change we were three individual atoms, which you can call the reactants. We then became water, which is the product. In a chemical equation the reactants have to be equal to the product. We were all still there, but we became bonded as a water molecule.