26. Conservation of Mass

Definition: In a chemical reaction, mass is always conserved. This means that no atoms are created or destroyed; only the chemical bonds between them change.
Balanced Equations: Because atoms are conserved, you must have the same number of atoms of each element on both sides of a chemical equation.
Mass Totals: The total relative formula mass (Mr) of the reactants will always be equal to the total relative formula mass (Mr) of the products.
Example: If 2.3g of sodium reacts completely with 3.5g of chlorine gas, it will produce exactly 5.8g of sodium chloride (2.3g + 3.5g = 5.8g).

In some experiments, it may look like mass has been gained or lost, but this is usually because a gas was involved that wasn't weighed on the scales.

Apparent Increase in Mass: * This usually happens when one of the reactants is a gas (like oxygen) that comes from the surrounding air.
- Example: Heating magnesium in the air to form magnesium oxide. The product weighs more than the original magnesium because it has reacted with oxygen from the air which wasn't on the scales at the start.
Apparent Decrease in Mass: * This usually happens when one of the products is a gas that escapes into the air.
- Example: The thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide (CO2). As the CO2 gas floats away, the remaining solid weighs less than the original calcium carbonate.

The Solution: To prevent mass from appearing to change, reactions can be carried out in a sealed container.
Result: Because all gases are trapped inside the container, the scales will accurately measure all reactants and products, showing that the mass stays exactly the same throughout the reaction.