Conservation of Mass

Law of Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. This means the total mass of the substances before the reaction (reactants) is exactly equal to the total mass of the substances after the reaction (products).

This happens because atoms are simply rearranged in a chemical reaction; no atoms are lost or gained. The number and type of atoms remain constant, only their connections change.

For example, when hydrogen reacts with oxygen to form water, the total mass of hydrogen and oxygen before the reaction is the same as the mass of water produced.

This principle is fundamental in chemistry and is used to understand and predict the outcomes of reactions.

For instance, if 10 g of hydrogen reacts with 80 g of oxygen, the total mass of water formed will be:

Mass of water = mass of hydrogen + mass of oxygen = 10 g + 80 g = 90 g

A balanced chemical equation for this reaction is: $2H_2 + O_2 \rightarrow 2H_2O$, showing that atoms are conserved and rearranged to form new substances.

For example, measuring the mass of substances before and after a reaction is done using a balance to ensure accuracy.

Mass Changes in Reactions

In a closed system, where no substances can enter or leave, the mass remains constant during a chemical reaction. This is because all reactants and products are contained, so no atoms escape.

In contrast, an open system allows gases or other substances to escape or enter. This can cause the measured mass to change during the reaction.

For example, if a metal reacts with oxygen in an open container, the mass may increase because oxygen from the air combines with the metal. However, if a gas is produced and escapes, the mass may appear to decrease.

To accurately demonstrate conservation of mass, reactions should be carried out in sealed containers such as closed flasks or using a gas syringe to capture escaping gases.

Example: When magnesium ribbon burns in air inside a sealed container, the mass before and after the reaction remains the same because the oxygen is trapped and reacts with magnesium to form magnesium oxide.

Practical Evidence

Experiments can demonstrate the conservation of mass by measuring the mass of reactants before a reaction and the mass of products after the reaction.

Common examples include:

• Burning magnesium in air inside a sealed container
• Reaction of hydrochloric acid with sodium carbonate in a closed flask
• Heating copper carbonate in a closed system

In these experiments, the total mass remains constant, confirming the law of conservation of mass.

For example, when copper carbonate is heated in a sealed container, it decomposes into copper oxide and carbon dioxide. The mass stays the same because the carbon dioxide gas cannot escape.

Example: If 20.0 g of copper carbonate is heated in a sealed container and decomposes, the total mass of copper oxide and carbon dioxide inside remains 20.0 g.

Example: If 5.0 g of hydrogen reacts with 40.0 g of oxygen, the total mass of water formed will be 45.0 g, demonstrating conservation of mass in a simple calculation.