How does mass decrease in a chemical reaction?

Mass does not decrease in a closed system during a typical chemical reaction. This is because of the Law of Conservation of Mass.

The Law of Conservation of Mass

The Law of Conservation of Mass, formulated by Antoine Lavoisier, states that mass is neither created nor destroyed in ordinary chemical reactions. In a closed system, the total mass of the reactants (starting materials) must equal the total mass of the products (substances formed).

Why it Might Seem Like Mass Decreases

While the total mass remains constant, it may appear as though mass decreases in some reactions if you are not accounting for all reactants and products. Here are some scenarios:

• Gases are produced and escape: If a reaction produces a gas that escapes into the atmosphere, the apparent mass of the remaining solid or liquid products will be less than the original reactants. For example, burning wood results in ash (a solid) but also releases carbon dioxide and water vapor (gases). If you only weigh the ash, it will weigh less than the original wood.
• Reactions in open containers: If a reaction is performed in an open container, it is not a closed system. Reactants can enter from the atmosphere, and products can escape. This can lead to apparent mass increases or decreases.
• Nuclear Reactions: The Law of Conservation of Mass, as described above, applies to chemical reactions. In nuclear reactions, mass can be converted into energy according to Einstein's famous equation, E=mc². However, these are not chemical reactions.

Examples

Let's illustrate with an example:

Reactants: Methane (CH₄) + Oxygen (O₂)

Products: Carbon Dioxide (CO₂) + Water (H₂O)

If this reaction is performed in a closed container, the total mass of the CO₂ and H₂O produced will equal the total mass of the CH₄ and O₂ that reacted.

However, if the reaction is performed in an open container, the gases (CO₂ and H₂O) can escape, making it seem like mass has been lost.

Summary

The perception of mass decreasing in a chemical reaction typically stems from not accounting for all reactants and products, particularly gases escaping the system. In a closed system, the total mass always remains constant, adhering to the Law of Conservation of Mass (in chemical reactions).