Do change that cannot be easily reversed, such ad burning, observe the law of conservation of mass? Explain

Yes

Explanation:

All chemical processes must obey the Law of Conservation of Mass.

The burning of paper appears to disobey this law. The paper disappears in flame, leaving only a small amount of ash.

Paper (s) + O₂ (g) ⟶ CO₂ (g) + H₂O (g) + ash (s)

However, that's because the main products, carbon dioxide and water vapour, are invisible gases.

Nevertheless,

mass of (paper + oxygen) = mass of (CO₂ + O₂ + ash)