What is the Law of Conservation of Mass? | Simple Explanation for Class 10

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What is the Law of Conservation of Mass in Chemical Reactions?

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

The Law of Conservation of Mass states that during any chemical reaction, the total mass of the reactants (the starting materials) is always equal to the total mass of the products (the substances formed). It means mass can neither be created nor destroyed in a chemical reaction, only rearranged.

Simple Example

Quick Example

Imagine you have 1 kg of atta (flour) and 500 grams of water to make dough for rotis. When you mix them, you will get exactly 1.5 kg of dough. No atta or water disappears, and no extra dough appears. The total mass before mixing is equal to the total mass after mixing.

Worked Example

Step-by-Step

Let's say you burn a candle.

STEP 1: Measure the mass of a candle before burning. Let's say it is 50 grams.
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STEP 2: Burn the candle completely inside a closed container (like a sealed glass jar) to capture all gases and smoke produced.
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STEP 3: After burning, measure the mass of the remaining wax (if any), plus the mass of the soot, carbon dioxide gas, and water vapour produced (all captured inside the sealed jar).
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STEP 4: You will find that the total mass of all these products (remaining wax + soot + carbon dioxide + water vapour) will add up to approximately 50 grams.
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STEP 5: This shows that the initial mass of the candle (reactants) equals the total mass of the substances formed (products).

ANSWER: The total mass remains constant.

Why It Matters

This law is fundamental to all of chemistry and engineering, helping scientists understand how much material they need for reactions. Chemists use it to design new medicines, engineers use it to build safer rockets and vehicles, and even environmental scientists use it to track pollutants.

Common Mistakes

MISTAKE: Thinking that if something 'disappears' (like wood burning into ash and smoke), its mass is destroyed. | CORRECTION: The mass isn't destroyed; it just changes form. The smoke (gases) and ash still have mass, and if you collect all of them, their total mass will equal the original wood's mass plus the oxygen that reacted with it.

MISTAKE: Forgetting to include the mass of gases involved in the reaction. | CORRECTION: Gases like oxygen (from the air) are often reactants, and gases like carbon dioxide are often products. Their mass must be accounted for to balance the equation.

MISTAKE: Confusing conservation of mass with conservation of volume. | CORRECTION: Mass is conserved, but volume can change significantly during a chemical reaction, especially when gases are involved or when temperature changes.

Practice Questions

Try It Yourself

QUESTION: If 10 grams of calcium carbonate (CaCO3) breaks down to form 5.6 grams of calcium oxide (CaO) and some carbon dioxide (CO2), what is the mass of the carbon dioxide produced? | ANSWER: 4.4 grams

QUESTION: When 23 grams of sodium (Na) reacts completely with an unknown mass of chlorine (Cl2) to produce 58.5 grams of sodium chloride (NaCl), what mass of chlorine was used? | ANSWER: 35.5 grams

QUESTION: A student mixes 20 mL of a solution weighing 25 grams with 30 mL of another solution weighing 35 grams. They observe a chemical reaction. What is the total mass of the new solution formed? (Assume no gases escape or are absorbed from the air). | ANSWER: 60 grams

MCQ

Quick Quiz

Which of the following statements correctly describes the Law of Conservation of Mass?

Mass can be created but not destroyed in a chemical reaction.

The total mass of reactants is always less than the total mass of products.

Mass is neither created nor destroyed during a chemical reaction.

The total mass of products is always greater than the total mass of reactants.

The Correct Answer Is:

Option C correctly states that mass remains constant during a chemical reaction. Options A, B, and D contradict this fundamental principle, suggesting mass changes.

Real World Connection

In the Real World

When ISRO launches rockets, engineers precisely calculate the amount of fuel and oxidizer needed. They use the Law of Conservation of Mass to ensure the total mass of propellants equals the total mass of exhaust gases produced, plus the remaining rocket mass, ensuring efficient and successful missions.

Key Vocabulary

Key Terms

REACTANTS: The starting substances in a chemical reaction | PRODUCTS: The new substances formed after a chemical reaction | CHEMICAL REACTION: A process that transforms one set of chemical substances into another | CONSERVATION: The principle that a quantity remains constant over time

What's Next

What to Learn Next

Now that you understand mass conservation, you can explore 'Balancing Chemical Equations'. This next concept shows you how to represent chemical reactions symbolically while ensuring that atoms (and thus mass) are conserved on both sides of the equation. It's super important for all future chemistry studies!