Law of Definite Proportions
Definition and meaning of Law of Definite Proportions in chemistry.
The Law of Definite Proportions states that a chemical compound always contains the same elements in the same proportions by mass, regardless of the amount of compound or its source of preparation.
In more detail
This law means that pure water always contains hydrogen and oxygen in the same ratio by mass, whether isolated from a glacier, ocean, or laboratory synthesis. The proportions reflect the fixed ratios of atoms present in each molecule of the compound. This law is fundamental to chemistry because it demonstrates that compounds have definite, constant compositions and distinguishes them from mixtures, which have variable compositions. The law provides evidence that atoms combine in fixed numerical ratios to form compounds.
Key facts
| Also known as | Law of Constant Proportions or Law of Constant Composition |
|---|---|
| Field | General Chemistry |
| Classic example | H2O is always 11.2% H and 88.8% O by mass |
| Key distinction | Applies to pure compounds, not physical mixtures |
Water always contains 11.2% hydrogen and 88.8% oxygen by mass. A 100 g sample of pure water contains 11.2 g of hydrogen and 88.8 g of oxygen; a 200 g sample contains 22.4 g hydrogen and 177.6 g oxygen. The proportion remains constant regardless of sample size.
Frequently asked questions
How does this differ from the Law of Multiple Proportions?
The Law of Definite Proportions states that a specific compound has constant elemental ratios, while the Law of Multiple Proportions applies when two elements form multiple compounds, each with its own definite proportions.
Why is this law important to chemistry?
It proves that compounds have fixed compositions and helped establish atomic theory by showing that atoms combine in specific, fixed ratios to form compounds.