Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 9. Atomic Foundations of Matter

CBSE Notes New Syllabus: Class 9

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Chapter 9. Atomic Foundations of Matter

This section explains the Law of Constant Proportions, one of the fundamental laws of chemistry. It states that a pure chemical compound always contains the same elements combined in a fixed proportion by mass, irrespective of its source or method of preparation. This law helped scientists understand the definite composition of chemical compounds. 

Law of Constant Proportions

The Law of Constant Proportions explains that the composition of a pure compound never changes.

Statement

A pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of its source or method of preparation. 

Scientist

Joseph Proust proposed the Law of Constant Proportions.

Main Features of the Law

• Fixed Composition – Every pure compound has a definite composition.
• Constant Mass Ratio – The ratio of the masses of the elements remains unchanged.
• Independent of Source – The composition remains the same even if the compound is obtained from different sources.
• Independent of Method – The method of preparation does not change the composition of the compound.

Examples of the Law

• Water (H₂O) – Water always contains hydrogen and oxygen in the same fixed ratio by mass.
• Sodium Chloride (NaCl) – Common salt always contains sodium and chlorine in a fixed proportion by mass.
• Carbon Dioxide (CO₂) – Carbon and oxygen combine in a definite mass ratio to form carbon dioxide.

Importance of the Law

• Explains the Composition of Compounds – Every pure compound has a fixed chemical composition.
• Supports Atomic Theory – It provides evidence that atoms combine in fixed proportions.
• Foundation of Modern Chemistry – It helped scientists develop theories about atoms and molecules.
• Useful in Chemical Analysis – It helps identify and verify pure compounds.

Difference Between the Two Fundamental Laws

Applications of the Law

• Identification of pure chemical compounds.
• Verification of chemical composition.
• Preparation of compounds in laboratories.
• Development of Dalton's Atomic Theory.
• Understanding chemical reactions and formulas.

ATP Education Concept Builder

Do not confuse the two fundamental laws. The Law of Conservation of Mass deals with the total mass during a chemical reaction, whereas the Law of Constant Proportions deals with the fixed composition of a chemical compound.

ATP Education Exam Booster

• Remember the statement of the Law of Constant Proportions.
• Learn the name of Joseph Proust.
• Understand why water and sodium chloride always have a fixed composition.
• Differentiate between the Law of Conservation of Mass and the Law of Constant Proportions.
• CBSE competency-based questions often ask students to identify which fundamental law is applicable in a given situation.