Conservation of Mass and Formula Mass » Mass Changes When Gases Are Involved

When the balance seems to lie

Remember, mass is conserved in every reaction. No atoms are lost or made. So why do some reactions show a different reading on the balance at the end?

The answer is nearly always a gas. Gases are made of particles that are spread out and moving quickly in all directions. You cannot see most gases, so it is easy to forget they have mass. But every gas particle has mass, just like the particles in a solid.

Many experiments are done in a container that is open to the air, such as a crucible or an open flask. This is called a non-enclosed system. Gas particles can get in from the air, or get out into the air, without anyone noticing. The balance only weighs what is actually sitting on it.

Key terms:

• Non-enclosed system: a container that is open to the air, so gases can enter or leave it.
• Enclosed system: a sealed container, so nothing can enter or leave it.

There are two ways the mass can seem to change:

Mass going up: a metal reacting with oxygen

When copper powder is heated strongly in air, its surface turns from orange-brown to black. The black solid is copper oxide. If you weigh the solid before and after, the copper oxide is heavier than the copper you started with.

Here is the balanced symbol equation:

2Cu(s) + O₂(g) → 2CuO(s)

Look at the state symbols. The copper is a solid and so is the product. But the oxygen is a gas, and it comes from the air around the crucible. At the start, you only weighed the copper. The oxygen was in the air, not on the balance.

Explaining it with the particle model:

• Oxygen molecules from the air hit the hot copper and react with it.
• The oxygen atoms become bonded to the copper atoms in the solid copper oxide.
• The solid now contains the copper atoms plus the oxygen atoms, so it has more mass.
• No atoms have been made. The extra mass is exactly the mass of the oxygen that reacted.

This is true for any metal reacting with oxygen. The mass of the metal oxide produced is greater than the mass of the metal. You will meet more about metals and oxygen in the lesson Metal Oxides and the Reactivity Series.

Mass going down: heating a metal carbonate

Breaking down a substance by heating it is called thermal decomposition. Metal carbonates break down this way to make a metal oxide and carbon dioxide.

Copper carbonate is a green powder. When you heat it, it turns black. The black solid is copper oxide. If you weigh the solid before and after, it is lighter at the end.

CuCO₃(s) → CuO(s) + CO₂(g)

Again, the state symbols tell the story. One product is a solid, but the other is carbon dioxide gas. In an open test tube or crucible, the carbon dioxide escapes into the atmosphere. That leaves the metal oxide as the only solid product.

Explaining it with the particle model:

• Heating gives the particles in the copper carbonate enough energy to break some bonds.
• Some carbon and oxygen atoms form carbon dioxide molecules.
• Gas particles move quickly and spread out, so they leave the open container and mix with the air.
• The solid left behind has fewer atoms in it, so its mass is less. The atoms have not been destroyed. They are now in the air.

Gases from reactions in solution

The same idea works for reactions in a flask. If sodium carbonate is added to hydrochloric acid in an open conical flask on a balance, it fizzes. The reading on the balance goes down as the fizzing happens.

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

The bubbles are carbon dioxide. They rise out of the liquid and leave through the open neck of the flask. The reading stops falling when the reaction stops making gas.

Now imagine doing it in a sealed flask, an enclosed system. The carbon dioxide cannot get out, so the reading on the balance stays the same. This shows the mass was never really lost. It had simply moved somewhere the balance could not weigh it.

Exam-style question

A student heats some zinc carbonate in an open crucible. The equation for the reaction is:

ZnCO₃(s) → ZnO(s) + CO₂(g)

The mass of the crucible and its contents before heating is 32.50 g. After heating, it is 31.62 g.

(a) Explain why the mass decreased. Use the particle model in your answer. [3 marks]

(b) Calculate the mass of carbon dioxide produced. [1 mark]