Short Answer

The Law of Constant Proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, no matter where it is found or how it is prepared. This means the composition of a compound does not change.

For example, water (H₂O) always contains hydrogen and oxygen in the fixed ratio of 1:8 by mass. Whether the water comes from a river, rain, or laboratory, this ratio remains the same. This law shows that compounds have a definite and constant composition.

Detailed Explanation :

Law of Constant Proportion

The Law of Constant Proportion is one of the fundamental laws of chemical combination. It was proposed by the French scientist Joseph Proust in 1799. According to this law, a chemical compound always contains the same elements in the same proportion by mass, regardless of how the compound was prepared or where it was obtained. This means that the composition of a compound is fixed and definite.

The law helps us understand that chemical compounds are not random mixtures but have a specific and constant composition. It forms the basis for understanding formulas of compounds, chemical reactions, and stoichiometry. This law is essential for predicting how elements combine to form compounds and how much of each element is required.

1. Meaning of constant proportion

To understand the law, we must know that elements combine in specific mass ratios. For example:

• Water always contains hydrogen and oxygen in a 1:8 mass ratio.
• Carbon dioxide always contains carbon and oxygen in a 3:8 mass ratio.
• Ammonia always contains nitrogen and hydrogen in a 14:3 mass ratio.

These ratios do not change for a given compound. Even if the sources or preparation methods are different, the proportions remain constant.

2. Reason behind the law

The reason a compound has constant proportions is because:

• Atoms combine in simple whole-number ratios.
• The number and type of atoms in each molecule are fixed.
• Since the atomic masses of elements are constant, the mass ratios also remain constant.

For example, one molecule of water (H₂O) always has 2 hydrogen atoms and 1 oxygen atom. Since the atomic mass of hydrogen is 1 and oxygen is 16, the ratio is always:
2 × 1 : 16 = 2 : 16 = 1 : 8

This fixed ratio ensures that water everywhere has the same composition.

3. Examples showing constant proportion
4. Water (H₂O)

Water from any source—river, rain, ocean, or distilled in a lab—always contains hydrogen and oxygen in the mass ratio 1:8.

1. Carbon dioxide (CO₂)

Carbon dioxide always has carbon and oxygen in the ratio 3:8 by mass. No matter how it is formed—burning fuel or exhalation—the ratio remains constant.

1. Sodium chloride (table salt)

Sodium chloride always contains sodium and chlorine in the ratio 23:35.5. Salt from seawater and salt from a laboratory are identical in composition.

1. Ammonia (NH₃)

No matter how ammonia is produced, it always contains nitrogen and hydrogen in a fixed ratio by mass.

4. Importance of the law

The Law of Constant Proportion is important because:

• It proves that compounds have definite chemical composition.
• It helps in writing chemical formulas correctly.
• It supports the concept of atoms combining in fixed ratios.
• It led to the development of atomic theory by John Dalton.
• It helps chemists calculate the mass of elements involved in reactions.

Without this law, chemistry would not have a clear understanding of how substances mix and combine.

5. Limitations of the law

Although the law applies to most compounds, there are a few cases where it does not strictly apply:

• Some compounds show non-stoichiometric composition, such as metal oxides.
• Isotopes may slightly change mass ratios due to different atomic masses.
• Some natural compounds may have small variations due to impurities.

However, these cases are exceptions, and the law is valid for the majority of chemical compounds.

6. Relation with modern atomic theory

Dalton’s atomic theory supports the law by stating that:

• Atoms of an element are identical in mass.
• Atoms combine in simple whole-number ratios.

Because of this, the mass ratio of elements in a compound remains constant. This explains why compounds always have fixed formulas such as H₂O, CO₂, or NaCl.

Conclusion

The Law of Constant Proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, regardless of how or where it is obtained. This law is essential in chemistry because it shows that compounds have definite composition and predictable formulas. It forms the foundation of chemical reactions, stoichiometry, and atomic theory, helping us understand how elements combine to create different compounds.