Laws of chemical combination

Correct answer: A. Law of multiple proportions

The governing concept is the law of multiple proportions. H₂O and H₂O₂ are two different compounds formed from the same elements, hydrogen and oxygen. If the mass of hydrogen is kept constant, the masses of oxygen that combine with it are in a simple whole-number ratio: in water the oxygen-to-hydrogen mass ratio is 8:1, while in hydrogen peroxide it is 16:1. The oxygen masses are therefore in the ratio 8:16, or 1:2. This directly satisfies the law of multiple proportions, so option A is correct. The law of definite proportions applies to one pure compound at a time, conservation of mass concerns total reactant and product mass, and the pressure law concerns gases rather than composition of these compounds.

Correct answer: A. It compares mass relationships among three elements

The law of reciprocal proportions involves three elements. Suppose elements A and B combine separately with a fixed mass of element C. The masses of A and B that combine with C are related to the ratio in which A and B combine directly, or to a simple multiple of that ratio. Hence, the key identifying feature is a comparison of mass relationships among three elements, making option A correct. The colour of a compound is unrelated to a quantitative combination law. Temperature comparison belongs to gas behaviour or thermodynamics, not reciprocal proportions. A mixture recipe does not describe fixed chemical combination between elements.

Correct answer: A. An experiment involving conservation of mass

The open or closed nature of a system is especially important in an experiment testing conservation of mass. In a closed system, no material can enter or leave, so the measured total mass before and after a reaction can be compared fairly. In an open system, gases may escape or enter from the surroundings, causing the measured mass to change even though mass is conserved overall. Therefore, option A is correct. Percentage composition and formula comparison primarily require composition or mass-ratio calculations, while Gay-Lussac’s law focuses on gas volumes at the same temperature and pressure. Those topics may involve experimental precautions, but the open-versus-closed distinction is central to a mass-conservation measurement.