Class 9 Science Exploration Solutions English Medium

NCERT Solutions Class 9 Science Exploration English Medium

Atomic Foundations of Matter

Topic: Lows of Chemical Combination

Chapter 9. Atomic Foundations of Matter

This section introduces the fundamental laws of chemical combination that explain how substances react during chemical reactions. These laws show that matter is conserved and that elements combine in fixed proportions to form compounds. They form the basis of modern chemistry. 

Physical and Chemical Changes

Changes occurring around us are broadly classified as physical changes and chemical changes.

Physical Change

• Definition – A change in which no new substance is formed.
• Nature – Only the physical properties such as shape, size or state change.
• Reversible – Most physical changes are reversible.
• Examples – Melting of ice, boiling of water and cutting paper.

Chemical Change

• Definition – A change in which one or more new substances are formed.
• Nature – Chemical properties of the substances change.
• Irreversible – Most chemical changes cannot be reversed easily.
• Examples – Burning of paper, rusting of iron and digestion of food.

Laws of Chemical Combination:

The laws of chemical combination were established by Lavoisier and Joseph L. Proust. 

The laws of chemical combination are two;

1. Law of conservation of mass: Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

2. Law of constant proportions: In a chemical substance the elements are always present in definite proportions by mass.

1. Law of Conservation of Mass

The Law of Conservation of Mass is one of the fundamental laws of chemistry. It explains that matter is neither created nor destroyed during a chemical reaction.

Statement

During a chemical reaction, the total mass of the reactants is always equal to the total mass of the products, provided the reaction takes place in a closed system. 

Scientist

Antoine Lavoisier proposed the Law of Conservation of Mass.

Importance of the Law

• Mass Remains Constant – Matter is neither created nor destroyed during a chemical reaction.
• Foundation of Chemistry – It forms the basis for balancing chemical equations.
• Supports Atomic Theory – It confirms that atoms are only rearranged during chemical reactions.

Activity to Verify the Law

• Take two solutions in separate containers.
• Measure the total mass before mixing them.
• Mix the solutions in a closed container.
• Measure the mass again after the reaction.
• The total mass remains unchanged, verifying the law.

Examples of the Law

• Burning of Magnesium – The total mass of magnesium and oxygen equals the mass of magnesium oxide formed.
• Reaction in a Closed Flask – The combined mass before and after the reaction remains the same.

Applications of the Law

• Balancing chemical equations.
• Studying chemical reactions.
• Industrial chemical processes.
• Environmental and laboratory investigations.

ATP Education Concept Builder

The Law of Conservation of Mass does not mean that substances remain unchanged. During a chemical reaction, atoms are only rearranged to form new substances, but the total mass remains constant.

ATP Education Exam Booster

• Remember the statement of the Law of Conservation of Mass.
• Know the name of Antoine Lavoisier.
• Understand why the law is valid only for a closed system.
• Differentiate between physical and chemical changes.
• Practice questions based on conservation of mass in simple chemical reactions.

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