The law of definite proportion or not the law of constant combination) states that a given compound is composed of elements in a given proportion and with the same types of atom.
A pure water and pond water has the same composition which is H2O
where hydrogen and oxygen are combined in 2:1 ratio.

The Law of Definite Proportions is a fundamental principle in chemistry that states that a chemical compound will always contain the same elements in the same proportions by mass regardless of the sample size or source of the compound.

Key Points:
- Definition: The Law of Definite Proportions states that a compound will always have the same elements in fixed proportions by mass.
- Example: For example, water (H2O) will always consist of 2 hydrogen atoms for every 1 oxygen atom, no matter the source of the water.
- Mass Ratios: The law ensures that the mass ratios of the elements in a compound are constant.
- Consistency: This principle applies to all compounds, whether they are naturally occurring or synthesized in a laboratory.
- Implications: The Law of Definite Proportions is crucial for determining the composition of compounds and predicting their chemical behavior.
In summary, the Law of Definite Proportions is a foundational concept in chemistry that helps scientists understand the consistent and predictable nature of chemical compounds. It provides a basis for analyzing and interpreting the composition of substances, leading to advancements in various fields such as materials science, pharmaceuticals, and environmental studies.