Gases Have Mass

What students typically write

“The gas escapes into the air so it has no mass.” Or: “When the reaction produces a gas, the mass disappears because gases are weightless.”

Students often believe that because gases are invisible and seem to float, they have no mass. This error surfaces most often in conservation of mass questions where a gas is produced in an open container and the balance reading drops.

Why it is wrong

Gases are made of particles, and particles have mass. Oxygen molecules (Mr = 32), carbon dioxide molecules (Mr = 44), and even the lightest gas, hydrogen (Mr = 2), all have measurable mass. A litre of air at room temperature and pressure has a mass of roughly 1.2 g.

When a reaction in an open container produces a gas, the balance reading decreases not because mass has been destroyed but because the gas escapes into the surrounding atmosphere. The total mass of all products (including the escaped gas) still equals the total mass of all reactants. This is the law of conservation of mass: matter cannot be created or destroyed in a chemical reaction.

If the same reaction is carried out in a sealed container so the gas cannot escape, the balance reading stays constant. That contrast is exactly how examiners test whether students understand the principle or are just memorising “mass stays the same”.

What the mark scheme actually wants

A typical question: “When calcium carbonate is heated in an open crucible, the mass decreases. Explain why, in terms of conservation of mass.” [2]

Marking points:

1. Carbon dioxide gas is produced and escapes from the crucible (1 mark)
2. The total mass of all products (calcium oxide + carbon dioxide) equals the mass of the calcium carbonate / mass is conserved (1 mark)

An answer that says “the gas has no weight so mass is lost” contradicts the conservation of mass and scores 0.

Worked example: wrong vs right

Question: A student adds dilute hydrochloric acid to magnesium ribbon on a balance. The reading decreases during the reaction. Explain this observation. [2]

Wrong answer: “Hydrogen gas is produced and it has no mass, so the reading goes down.” This scores 0/2. It correctly identifies hydrogen but then claims gas has no mass.

Right answer: “Hydrogen gas is produced (1) and escapes from the open beaker into the atmosphere, so the mass recorded on the balance decreases, though the total mass of all products equals the total mass of reactants (1).” This scores 2/2. It explains the decrease without violating conservation of mass.

How to avoid this mistake

Whenever a question involves a change in mass during a reaction, ask two things:

1. Is the container open or sealed?
2. Is a gas produced?

If open and a gas is produced, the balance reading decreases because gas escapes, not because mass is destroyed. If sealed, the reading stays constant. If no gas is produced (for example, a precipitation reaction in solution), the reading stays constant regardless of container type.

Remember the particle model: gases are particles with mass, moving quickly and far apart. “Far apart” explains low density and why gases disperse into the air. It does not mean zero mass.

For the full treatment of conservation of mass and how it connects to mole calculations, see the mole and Avogadro constant. The kinetic particle theory page covers why gas particles behave as they do.