How many elements are involved in the law of reciprocal proportions?

Correct answer: Three

The direct answer is B: three elements are involved. The law of reciprocal proportions compares the masses of three elements. Suppose elements A and B separately combine with a fixed mass of element C. The masses of A and B that combine with the same mass of C are then compared with the ratio in which A and B combine with each other. This is why the law is called reciprocal: one relationship is checked against another related combination. Step by step, the key clue is not simply “a compound has two elements”; it is the comparison of three elements across different compounds or combinations. Option A, one element, is impossible because a combination law must compare substances. Option B, three elements, is correct because the law studies their mass relationships. Option C, two elements, describes the basic participants in some individual compounds but not the complete reciprocal-proportion relationship. Option D, four elements, is not the standard definition of this law. Do not confuse it with conservation of mass, which says total mass remains constant, or definite proportions, which concerns a fixed composition of one pure compound. Memory cue: reciprocal proportions = three elements and linked mass ratios.