When Mass Seems to Change

Sometimes, it SEEMS like mass changes during a reaction, but there's always a reason:

1. If mass INCREASES, it could be due to a gas from the air becoming part of the product in an unsealed container.

e.g. when Sodium reacts with Oxygen:

The mass of the Sodium appears to increase. This is because the Oxygen in the air COMBINES with the Sodium to form Sodium Oxide.

The Oxygen’s mass can not be measured as it is a gas, but after it combines, it is a part of the solid so it CAN be weighed.

2. If mass DECREASES, a gas may have formed and ESCAPED from the reaction mixture in a non-enclosed system.

e.g. when Calcium Carbonate is heated in a THERMAL DECOMPOSITION reaction:

It releases Carbon Dioxide GAS which escapes, meaning it can’t be weighed. So when you weigh the products, the mass appears to decrease as the mass of the Carbon Dioxide is not measured once it escapes.