Which observation best illustrates the law of conservation of mass?

• A. When 15 g of sugar dissolves in 100 g of water, the resulting solution has a mass of 110 g.
• B. When 20 g of calcium carbonate is heated and produces 10 g of calcium oxide and 5 g of carbon dioxide, the total mass decreases.
• C. When 5 g of sodium is heated in air and produces 7 g of sodium oxide, the mass increases.
• D. When 10 g of hydrogen reacts with 80 g of oxygen to form 90 g of water, the total mass remains unchanged.

Answer: (D)

Verified step by step guidance

1. Understand the law of conservation of mass: it states that mass cannot be created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
2. Analyze each observation to see if the total mass of reactants equals the total mass of products.
3. For the first observation, check if the mass of sugar plus water equals the mass of the solution: 15 g + 100 g should equal 110 g.
4. For the second and third observations, note any discrepancies where the total mass of products does not equal the total mass of reactants, which would violate the law.
5. For the fourth observation, verify that the sum of the masses of hydrogen and oxygen (10 g + 80 g) equals the mass of water formed (90 g), illustrating the law of conservation of mass.