The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.
The Law of Definite Proportions holds true for compounds because in a compound, the elements always combine in a fixed ratio by mass. Irrespective of the source or how it is formed, a compound always contains the same elements in definite proportions. For example, water from any source always contains hydrogen and oxygen in a mass ratio of 1:8.
On the other hand, mixtures do not follow this law because their constituents can be mixed in varying proportions. Since there is no fixed ratio of combining elements in a mixture, the Law of Definite Proportions does not hold true for them.
Explanation
The textbook context highlights Proust's Law, stating that chemical compounds always contain elements in fixed ratios by mass (like water's 1:8 ratio of hydrogen to oxygen). Because compounds have a definite, unvarying composition, the law applies to them. Mixtures, conversely, are not chemically bound in fixed ratios; their components can be mixed in any proportion, making the law inapplicable to them.
Solution Steps
Step 1: State that compounds consist of elements combined in a fixed ratio by mass, which satisfies the Law of Definite Proportions.
Step 2: Explain that mixtures do not have a fixed ratio of constituents by mass, which is why the law does not apply to them.