AQA GCSE FOUNDATION Chemistry: The Law of Conservation of Mass

The Law of Conservation of Mass

The Law of Conservation of Mass states that in a closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. This means that matter cannot be created or destroyed, only transformed.

Closed and Non-Closed Systems

Closed system: A system where no matter can enter or leave. This is important because it ensures that the mass of the reactants and products remains constant.

Non-closed system: A system where matter can enter or leave. This means the total mass of the reactants and products may not be equal.

Understanding the Law

Here's an example to illustrate the concept:

Imagine burning a piece of wood in a closed container.

• Reactants: Wood and oxygen
• Products: Ash, carbon dioxide, and water vapor

In a closed container:

• The total mass of the wood and oxygen before burning will equal the total mass of the ash, carbon dioxide, and water vapor after burning.

In an open container:

• Some of the products, like the water vapor, can escape into the atmosphere. This would make the mass of the products seem less than the mass of the reactants.

Calculations

To understand the Law of Conservation of Mass in calculations, consider the following:

• Reactants: The substances that are present before a chemical reaction.
• Products: The substances that are formed after a chemical reaction.

For a closed system:

• Total mass of reactants = Total mass of products

For a non-closed system:

• Total mass of reactants ? Total mass of products

This is because some matter may have entered or left the system, resulting in a difference in mass.

Example Calculation

Consider the following chemical reaction:

Sodium + Chlorine -> Sodium Chloride 

Experimentally, we observe:

• 2.3 g of Sodium reacts with 3.6 g of Chlorine.
• The reaction produces 5.9 g of Sodium Chloride.

Applying the Law of Conservation of Mass:

• Total mass of reactants (2.3 g + 3.6 g) = 5.9 g
• Total mass of product = 5.9 g

Therefore, the Law of Conservation of Mass is validated in this example.

Conclusion

The Law of Conservation of Mass is a fundamental principle in chemistry. It emphasizes that matter is not created or destroyed, only transformed, and helps us understand and predict the outcomes of chemical reactions. Understanding this principle is crucial for calculating the masses of reactants and products in chemical reactions.