Gases

One of the important properties of gases is that they diffuse into all available space. This is because their molecules have a lot of kinetic energy, so they have a lot of translational freedom.

Non-enclosed system: A system where the gas is free to escape

Example: An open beaker or flask

Reaction that produces gas

There may be an apparent decrease in mass between reactants and products if this reaction is carried out in a non-enclosed system.

This is because some of the reactants have been turned into gas and have escaped the reaction vessel.

If we could weigh the gas produced, we would find that no mass is lost.

Reactions that involve heating of solids

There may be an apparent increase in mass between reactants and products if this reaction is carried out in a non-enclosed system.

This apparent gain is because some of the solid reactants have reacted with gases to form solid products.

Calculations vs observations