An excellent example of the law of constant proportion is the formation of water (H₂O), where hydrogen and oxygen atoms always combine in a fixed ratio, leading to a consistent composition by mass.
Understanding the Law of Constant Proportion
The Law of Constant Proportion, also known as the Law of Definite Proportions, is a fundamental principle in chemistry. It states that a pure chemical compound always contains its component elements in a fixed, characteristic ratio by mass, regardless of its source or method of preparation. This means that if you analyze any sample of a specific compound, the proportion of each element by mass will always be the same.
Illustrative Examples of the Law
The consistent composition of compounds like water and salt perfectly demonstrates this law.
Water (H₂O)
Water is one of the most common and clear examples. Regardless of whether water comes from a tap, a river, or is created in a laboratory, its chemical composition remains unchanged:
- Atomic Proportion: Every molecule of water is consistently composed of two hydrogen atoms combined with one oxygen atom.
- Mass Proportion: Because each atom has a specific atomic mass (hydrogen ≈ 1 atomic mass unit, oxygen ≈ 16 atomic mass units), the mass ratio of hydrogen to oxygen in water is always fixed. For two hydrogen atoms (2 × 1 = 2 amu) and one oxygen atom (1 × 16 = 16 amu), the mass ratio of hydrogen to oxygen is 2:16, which simplifies to 1:8. This ratio holds true for every pure water sample.
Sodium Chloride (NaCl)
Common table salt, or sodium chloride, also adheres to the law of constant proportion:
- Atomic Proportion: For sodium chloride to form, one sodium (Na) atom must combine with one chlorine (Cl) atom. This creates a 1:1 atomic ratio between the elements in the compound.
- Mass Proportion: Due to this fixed atomic ratio, the mass proportion of sodium to chlorine in any sample of pure sodium chloride is always constant (approximately 23 parts sodium to 35.5 parts chlorine by mass).
Key Takeaways
- The Law of Constant Proportion highlights that the chemical identity of a compound is defined by the precise ratios in which its constituent elements combine.
- This law is crucial for understanding chemical formulas and stoichiometry, as it ensures that compounds have predictable and consistent compositions.
- Any deviation from these fixed proportions would result in a different chemical substance or a mixture, not the pure compound itself.
