Chapter 5 · Molecules and Compounds
5.2Compounds Display Constant Composition
8 min · two checks
Predict
Can you make water that is 50% hydrogen by mass if you mix the gases carefully?
The idea
- State the law of constant composition.
- Tell a compound from a mixture using composition.
A compound is not a recipe you can adjust. The law of constant composition says that any pure sample of a given compound has the same mass percent of each element. Every sample of water, from a glacier or a combustion experiment, is about 1 g of hydrogen for every 8 g of oxygen. If a sample of “water” analyzes differently, it is impure or it is not water.
Mixtures refuse this rule. Salt water can be 1% salt or 10% salt. Brass can be richer or poorer in zinc. Constant composition is how you know the substance in the bottle is a compound rather than a blend, and it is why a chemical formula has fixed subscripts. When those subscripts change, you have named a different compound: CO is not CO₂, and their mass percents of oxygen differ.
Keep these
- Pure compounds have fixed mass ratios of elements.
- Mixtures have variable composition.
- Changing a subscript changes the compound.