Law of Conservation of Mass: Statement, Formula, Examples and Importance

The law of conservation of mass is one of the fundamental laws of science that explains what happens to matter during a physical or chemical change. It states that the total mass remains the same because mass can neither be created nor destroyed. This simple idea helps explain many everyday activities, such as burning wood, rusting iron, and forming water.

This article explains what is the law of conservation of mass, who gave the law of conservation of mass, its statement, formula, and simple real-life examples to help you learn the concept easily.

Table of Contents

• What is the Law of Conservation of Mass
• Law of Conservation of Mass Explained with an Example
• Who Gave the Law of Conservation of Mass First
• Mathematical Formula of the Law of Conservation of Mass
• Common Examples of the Law of Conservation of Mass
• Frequently Asked Questions on Law of Conservation of Mass

What is the Law of Conservation of Mass

The total mass before a reaction is always equal to the total mass after the reaction. The law of conservation of mass states that,

“During a chemical reaction, mass can neither be created nor destroyed in a closed system. It can only change from one form to another.”

The fact is that atoms do not disappear during a reaction. They only rearrange to form new substances. Because of this, the total mass always remains the same.

Law of Conservation of Mass Explained with an Example

Have you ever noticed what happens to a piece of wood after a bonfire? It may look as if the wood has completely disappeared, but that is not what actually happens.

When wood burns, it reacts with oxygen from the air. This reaction produces ash, carbon dioxide, water vapour, and heat. Most of the products, such as carbon dioxide and water vapour, escape into the air, while only the ash is left behind.

Now, the obvious question is, has the mass decreased?

The answer is no. If you could measure the mass of the wood and the oxygen before burning, and then measure the mass of the ash along with the gases produced after burning, you would find that the total mass remains exactly the same.

Mass of wood + Mass of oxygen = Mass of ash + Mass of carbon dioxide + Mass of water vapour

This example clearly explains the law of conservation of mass, which states that mass can neither be created nor destroyed during a chemical reaction. It only changes from one form to another. Even though the wood changes into different substances, the total mass before and after the reaction remains equal.

Who Gave the Law of Conservation of Mass First

The law of conservation of mass was given by Antoine Lavoisier in 1789 after performing several careful experiments.

He burned phosphorus inside a sealed container and carefully measured its mass before and after the reaction.

At that time, many scientists believed that a substance called phlogiston escaped from materials during burning, causing them to lose mass. The phosphorus actually gained mass because it combined with oxygen present in the air inside the sealed container.

Since the container was sealed, no matter could enter or leave. When Lavoisier measured the total mass of the container and its contents after the reaction, it remained exactly the same as before the experiment.

This experiment showed that mass is neither created nor destroyed during a chemical reaction. Instead, the reactants combine to form new substances, and the total mass remains constant. 

Mathematical Formula of the Law of Conservation of Mass 

The formula is very simple.

Mass of Reactants = Mass of Products

Or,

m₁ + m₂ + … = m₃ + m₄ + …

Where reactants are the substances present before the reaction and products are the substances formed after the reaction.

This equation shows that the total mass remains unchanged.

Common Examples of the Law of Conservation of Mass

The law of conservation of mass can be seen in many everyday activities and chemical reactions. Let's look at a few simple examples.

Burning of Wood

When wood burns, it reacts with oxygen from the air and forms ash, carbon dioxide, and water vapour. While the gases escape into the atmosphere, the ash remains behind. If the mass of the wood, oxygen, ash, and gases is measured together, the total mass before and after burning is the same.

Formation of Water

Hydrogen reacts with oxygen to produce water.

2H₂ + O₂ → 2H₂O

Here, the total mass of hydrogen and oxygen before the reaction is equal to the total mass of the water formed. The atoms only rearrange to form a new substance, so no mass is lost or gained.

Rusting of Iron

Iron reacts with oxygen present in the air to form rust.

Although the iron changes into a new substance, the total mass of the iron and oxygen together remains equal to the mass of the rust produced. This also follows the law of conservation of mass.

Dissolving Salt in Water

Adding salt to a glass of water. The salt seems to disappear, but it is still present in the solution.

If you measure the total mass of the salt and water before mixing and compare it with the mass of the salt solution after mixing, both will be the same. This shows that mass is conserved even during physical changes.

So far, we've seen that the law of conservation of mass says that mass is always conserved during a reaction. This law helps explain why the total mass remains the same, even when substances change into new forms. We also studied its formula and real-life examples in a simple way.