medium difficulty6 min

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.

In one line: Same atoms, same total mass.

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A reaction uses 5 g of A and 8 g of B.
2If 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 you can reuse

1Add masses of all reactants.
2Add known product masses.
3Equate total reactant mass to total product mass.
4Find the missing mass by subtraction.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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.

Common wrong answer

Writing 10 g as the total product mass in the worked example.

Why it loses marks: It ignores the second product.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

1
Enough for 1 mark
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
Add for 2 marks
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.
3
Add for 3 marks
In a chemical reaction, atoms are rearranged to form new substances, but atoms are neither created nor destroyed.

MCQ Quiz

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