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Law of Conservation of Mass

The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products.

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Student-friendly explanation

In a chemical reaction, atoms are rearranged to form new substances, but atoms are neither created nor destroyed. Therefore, if the reaction happens in a closed system, the total mass before and after the reaction remains the same.

How to write this in exams

  1. 1

    Start with the exact idea

    The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products.

  2. 2

    Then show how to use it

    Add masses of all reactants. Add known product masses. Equate total reactant mass to total product mass. Find the missing mass by subtraction.

  3. 3

    Add one concrete example

    If 12 g of carbon combines completely with 32 g of oxygen, 44 g of carbon dioxide is formed. Total reactant mass is 44 g and product mass is also 44 g.

  4. 4

    Avoid this incomplete answer

    Writing 10 g as the total product mass in the worked example is wrong because it ignores the second product.

Definition

The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products.

Example

If 12 g of carbon combines completely with 32 g of oxygen, 44 g of carbon dioxide is formed. Total reactant mass is 44 g and product mass is also 44 g.

Rule to remember

Total mass of reactants = total mass of products, provided no matter enters or leaves the system.

Memory hook

Same atoms, same total mass.

Examples and method

Worked example

A reaction uses 5 g of A and 8 g of B. If 10 g of product C is formed, the remaining product D must have mass 3 g because 5 + 8 = 13 g and 13 - 10 = 3 g.

Method to apply

Add masses of all reactants. Add known product masses. Equate total reactant mass to total product mass. Find the missing mass by subtraction.

Diagram support

A simple closed-flask diagram can show reactants before reaction and products after reaction with the same total mass reading on a balance.

How CBSE asks it

Usually asked as a numerical mass-balance question, a reason-based question on closed systems, or an explanation of why apparent mass changes when gas escapes.

Avoid common mistakes

Common confusion

Students often think mass decreases because a gas escapes. The law is checked correctly only when all products, including gases, are included.

Common wrong answer

Writing 10 g as the total product mass in the worked example is wrong because it ignores the second product.

Exam tip

When a question gives masses before and after a reaction, add all reactant masses and all product masses before comparing.

Quick check

Why should a gas produced in a reaction be included while checking conservation of mass?

The gas must be included because it is also a product made from the same atoms. If it escapes and is not measured, the mass may look smaller, but total mass is still conserved in a closed system.

Answer writing and exam use

1-mark answer

The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products.

2-mark answer

The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products. Total mass of reactants = total mass of products, provided no matter enters or leaves the system. If 12 g of carbon combines completely with 32 g of oxygen, 44 g of carbon dioxide is formed. Total reactant mass is 44 g and product mass is also 44 g.

3-mark answer

In a chemical reaction, atoms are rearranged to form new substances, but atoms are neither created nor destroyed. Therefore, if the reaction happens in a closed system, the total mass before and after the reaction remains the same. Total mass of reactants = total mass of products, provided no matter enters or leaves the system. A reaction uses 5 g of A and 8 g of B. If 10 g of product C is formed, the remaining product D must have mass 3 g because 5 + 8 = 13 g and 13 - 10 = 3 g. Usually asked as a numerical mass-balance question, a reason-based question on closed systems, or an explanation of why apparent mass changes when gas escapes. Writing 10 g as the total product mass in the worked example is wrong because it ignores the second product.
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