A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain. [Pg. No. 166]

No, the Law of Conservation of Mass is not violated. When ethanol burns in air, it reacts with oxygen to form carbon dioxide ( $CO_2$ ) and water vapour ( $H_2O$ ). In an open beaker, these gaseous products escape into the surroundings, so no residue is left behind in the beaker.
If we consider the total mass of the reactants (ethanol + oxygen) and the total mass of the products (carbon dioxide + water vapour), the mass remains the same.
Therefore, mass is conserved; it only appears to decrease because the gaseous products have left the beaker.