Which example is not suitable for the law of definite proportions?

Correct answer: Sugar water

The direct answer is A, sugar water. The law of definite proportions says that a pure chemical compound always contains the same elements in the same fixed mass ratio, regardless of where or how much of it is taken. Sugar water is not one new compound; it is a mixture or solution. We can put a little sugar or a lot of sugar into water, so its composition changes. Therefore it cannot obey one fixed ratio. Option A is correct because sugar water has variable composition. Option B, pure HCl, is a compound made of hydrogen and chlorine in a fixed composition, so it is suitable. Option C, pure MgO, contains magnesium and oxygen in a fixed chemical ratio, so it is suitable. Option D, pure CaO, contains calcium and oxygen in a fixed ratio, so it is also suitable. A common mistake is to call every clear liquid a compound. In this question, the word pure and the fixed formula are important clues. Remember: mixture can vary, but a pure compound has a definite composition.