GCSE Chemistry - Conservation of Mass (2026/27 exams)
TL;DR
The conservation of mass principle in chemistry states that mass is conserved in chemical reactions because atoms are rearranged, not created or destroyed; apparent mass changes in open systems are due to gases entering or leaving, which can be avoided by using sealed containers.
Key insights
The fundamental principle of conservation of mass states that in any chemical reaction, the total mass of reactants must equal the total mass of products.
- This is because atoms are neither created nor destroyed during a reaction; only the bonds between them change.
- This principle is directly applied when balancing chemical equations.
- The total relative formula mass (Mr) of reactants will always equal the total Mr of products.
Apparent changes in mass during a reaction can occur in non-closed systems, particularly when gases are involved.
- If a reactant is a gas that is absorbed from the surroundings (e.g., oxygen reacting with a metal), the product's mass may appear to increase.
- If a product is a gas that escapes into the surroundings (e.g., CO2 from decomposition), the product's mass may appear to decrease.
Conducting experiments in a sealed container is crucial for accurately demonstrating the conservation of mass.
- A sealed system ensures that all reactants and products, including gases, are contained and measured.
- This prevents the apparent gain or loss of mass due to gases entering or leaving the reaction vessel.
- Accurate measurements in a sealed system confirm that the total mass remains constant throughout the reaction.
Key moments
- 00:24Introduction to the conservation of mass principle: no atoms are created or destroyed in a chemical reaction, meaning the total mass of reactants equals the total mass of products.
- 00:44Demonstration of conservation of mass by comparing the total relative formula mass (Mr) of reactants and products, showing they are equal.
- 01:11Explanation of how actual masses are conserved, using an example of sodium reacting with chlorine, where the sum of reactant masses equals the product mass.
- 01:58Explaining why mass may appear to increase in non-closed systems, using the example of magnesium reacting with oxygen from the air.
- 02:57Explaining why mass may appear to decrease in non-closed systems, using the example of calcium carbonate decomposing into calcium oxide and carbon dioxide gas.
- 03:23Summary of key points: mass is always conserved, apparent changes are due to gases in open systems, and sealed containers ensure accurate measurement.