In combustion, the law of combining masses indicates what?

The law of combining masses, often associated with the law of definite proportions, states that when compounds form, they do so in fixed, specific proportions by mass. This principle reflects that only elements can react in particular ratios to create a new substance. Each chemical compound has a unique composition, defined by the ratio of the different types of elements present, which does not vary regardless of the sample size or source.

For example, water (H2O) is always made up of two hydrogen atoms for every oxygen atom in a 1:8 mass ratio. This law reinforces the idea that elements react in specific proportions, which is essential for predicting how they will combine in a chemical reaction. In industrial and laboratory settings, understanding this law helps in correctly measuring reactants to ensure complete reactions and to yield predictable results.

This principle fundamentally underpins the concept of stoichiometry, which governs the relationships between reactants and products in chemical reactions, emphasizing the significance of accurate ratios based on mass.