Do changes that cannot be easily reversed, such as burning, observe the law of conservation of mass? Explain.

Yes! Conservation of mass applies for all just like conservation of energy. The explanation; if you weigh the burnt stuff its weight might be greater or less than its original weight, and this never violates the cons. of mass law as if you actually counted the masses of all the products like CO2 and reactants like O2, that is if you keep your system a closed system, you will actually get the same masses before and after the burning process.