Which statement correctly uses definite proportions?

Correct answer: Element ratio in a pure compound remains fixed

The direct answer is B: the element ratio in a pure compound remains fixed. The law of definite proportions states that a particular pure compound always contains the same elements in the same fixed proportion by mass, regardless of where the sample comes from or how it was prepared. For example, pure water always contains hydrogen and oxygen in a fixed mass ratio. The important words are “pure compound” and “fixed composition.” Option A is wrong because a mixture does not have one fixed chemical composition; its components can be present in different amounts. Also, the total amount of a mixture can change. Option B is correct because it states the exact principle of definite proportions. Option C is wrong because gas volumes are not always equal; gas volume depends on conditions such as temperature, pressure and amount. Option D is wrong because chemical reactions do not make mass double in every case; mass is conserved in a closed system. Do not confuse a compound with a mixture: salt water can have changing amounts of salt and water, but pure sodium chloride has a fixed composition. The law concerns ratios of elements, not the size of the sample. Memory cue: definite proportions means “same compound, same element ratio,” not “same amount of mixture.”