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 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). This means that mass can neither be created nor destroyed, only rearranged.

Simple Example

Quick Example

Imagine you have a plate of 2 samosas and 1 cup of chai. If you eat them, your body processes them. Even though they change form inside you, the total 'stuff' (mass) that went into your body is the same as the total 'stuff' that comes out (in different forms) plus the energy used. No mass simply vanishes or appears from nowhere.

Worked Example

Step-by-Step

Let's say we burn 12 grams of carbon in 32 grams of oxygen to form carbon dioxide.

Step 1: Identify the reactants and their masses. Reactant 1: Carbon = 12g. Reactant 2: Oxygen = 32g.
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Step 2: Calculate the total mass of reactants. Total reactant mass = Mass of Carbon + Mass of Oxygen = 12g + 32g = 44g.
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Step 3: According to the Law of Conservation of Mass, the total mass of the product (carbon dioxide) should be equal to the total mass of reactants.
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Step 4: Therefore, the mass of carbon dioxide formed will be 44g.

Answer: 44 grams of carbon dioxide will be formed.

Why It Matters

Understanding mass conservation is crucial for engineers designing new materials, chemists developing medicines, and even for scientists at ISRO calculating fuel requirements for rockets. It helps them predict how much product they will get or how much raw material they need, saving time and resources.

Common Mistakes

MISTAKE: Thinking that mass can be lost when a gas is produced or consumed. For example, if wood burns and turns into ash and smoke, students might think mass is lost. | CORRECTION: The smoke (gases) also has mass. If you collected all the ash and all the smoke, their combined mass would equal the original wood's mass plus the oxygen used from the air.

MISTAKE: Believing that mass changes during a physical change, like ice melting into water. | CORRECTION: The Law of Conservation of Mass applies to both physical and chemical changes. 100g of ice will melt into 100g of water; the mass remains the same, only the state changes.

MISTAKE: Not accounting for all reactants or products, especially gases. For instance, when iron rusts, students might only weigh the iron and the rust. | CORRECTION: Rusting involves iron reacting with oxygen from the air. The mass of the rust is the sum of the mass of the iron and the mass of the oxygen that reacted with it.

Practice Questions

Try It Yourself

QUESTION: If 20g of magnesium reacts completely with 16g of oxygen, what is the mass of magnesium oxide formed? | ANSWER: 36g

QUESTION: A student mixes 50g of Solution A with 70g of Solution B. After the reaction, a solid precipitate forms, and 10g of gas escapes. What is the total mass of the precipitate and any remaining liquid? | ANSWER: 110g

QUESTION: When 100g of calcium carbonate (limestone) is heated, it breaks down into calcium oxide and carbon dioxide gas. If 56g of calcium oxide is formed, how much carbon dioxide gas was produced? | ANSWER: 44g

MCQ

Quick Quiz

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

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

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

The total mass of reactants equals the total mass of products in a chemical reaction.

Mass is destroyed when energy is released in a reaction.

The Correct Answer Is:

Option C is correct because the law states that mass is neither created nor destroyed during a chemical reaction, meaning the total mass before and after the reaction remains constant. Options A, B, and D contradict this fundamental principle.

Real World Connection

In the Real World

In a kitchen, when your mother makes a cake, she uses specific amounts of flour, sugar, eggs, etc. (reactants). The total weight of these ingredients will be roughly equal to the weight of the baked cake (product). This principle is also used in industries to ensure efficient production and minimise waste, for example, in making medicines or fertilizers.

Key Vocabulary

Key Terms

REACTANTS: The starting materials in a chemical reaction. | PRODUCTS: The new substances formed after a chemical reaction. | CHEMICAL REACTION: A process that involves rearrangement of the molecular or ionic structure of a substance. | MASS: A measure of the amount of 'stuff' or matter in an object.

What's Next

What to Learn Next

Now that you understand mass conservation, you can move on to 'Balancing Chemical Equations'. This concept directly uses the Law of Conservation of Mass to ensure that the number of atoms of each element is the same on both sides of a reaction, just like mass is conserved!