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Maharashtra State BoardSSC (English Medium) 9th Standard
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Syllabus

Laws of Chemical Combination

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Estimated time: 15 minutes
  • Introduction
  • Experiment

Introduction

When a chemical reaction occurs, the composition of substances changes as they form new compounds. Scientists in the 18th and 19th centuries conducted detailed experiments to understand these changes. By carefully measuring the amounts of substances involved in chemical reactions, they discovered certain patterns and rules, now known as the laws of chemical combination.

These laws helped scientists understand how elements combine to form compounds and provided the foundation for writing molecular formulas. The discoveries were guided by Dalton's atomic theory, which explained the behaviour of atoms during chemical reactions. These laws are fundamental to the study of chemistry and explain how matter interacts and transforms.

Experiment

1. Aim: To verify the Law of Conservation of Mass using chemical reactions.

2. Requirements

  • Apparatus: conical flask, test tube, balance, airtight rubber cork.
  • Chemicals: Calcium oxide (CaO), water (H₂O), calcium chloride (CaCl₂), sodium sulphate (Na₂SO₄).

3. Procedure

I: Reaction Between Calcium Oxide and Water

  • Take 56 g of calcium oxide in a conical flask.
  • Add 18 g of water to the flask.
  • Observe the reaction and measure the mass of the substance formed.

Observation: A vigorous reaction occurs, producing a substance. The total mass of the substance formed equals the combined mass of calcium oxide and water.

Conclusion: The experiment confirms that the mass remains constant during the reaction, validating the Law of Conservation of Mass.

II: Reaction Between Calcium Chloride and Sodium Sulphate

  • Add a solution of calcium chloride to a conical flask.
  • Place a solution of sodium sulphate in a test tube and insert the test tube into the conical flask without mixing.
  • Seal the flask with an airtight cork and weigh it using a balance.
  • Tilt the flask to mix the solutions and allow the reaction to occur. Weigh the flask again after the reaction.

Observation: The reaction forms a white precipitate of calcium sulphate. The total mass of the flask and its contents remains unchanged before and after the reaction.

Conclusion: The mass remains constant during the reaction, confirming the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.

Verification of law of the chemical combination

Maharashtra State Board: Class 12

Laws: Laws of Chemical Combination

Five fundamental laws govern how elements and compounds combine chemically:

Law 1 — Law of Conservation of Mass (Antoine Lavoisier)

Mass is neither created nor destroyed during any chemical reaction. The total mass of reactants always equals the total mass of products.

Law 2 — Law of Definite Proportion (Joseph Proust)

A specific chemical compound always contains its elements combined in a fixed ratio by weight, regardless of where the compound comes from or how it was made.

Exception: This law does not hold for compounds made from different isotopes of an element.

Law 3 — Law of Multiple Proportion (John Dalton)

When two elements combine to form more than one compound, the different masses of one element that combine with a fixed mass of the other are always in a simple whole-number ratio. Example: CO and CO₂.

Law 4 — Gay Lussac's Law of Gaseous Volumes

When gases react or are produced in a chemical reaction, their volumes bear a simple whole-number ratio to each other — provided temperature and pressure remain the same.

Law 5 — Avogadro's Law

At the same temperature and pressure, equal volumes of all gases contain the same number of molecules, regardless of the type of gas.

V ∝ n (at constant T and P)
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