Try Reaction Lab free in your browser, with nothing to install and no account. Balance a chemical reaction and watch it happen, atom by atom, in a sealed flask on a scale. Add or take away whole molecules until every kind of atom matches, then press React: the molecules bump, their bonds let go, and each atom moves to its place in a new molecule. The scale never moves, because the same atoms are still there. Run an equation that is not balanced and see what it claims: atoms that vanish, or atoms from nowhere, and the scale jumps. Water, burning methane, burning magnesium, table salt, ammonia and more.