Revision notes for AQA GCSE Chemistry Mass changes when a reactant or product is a gas. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Mass changes when a reactant or product is a gas
What you'll learn
- Why mass can appear to change during a reaction involving gases.
- How to tell whether mass will increase, decrease, or stay the same.
- How to explain observations using balanced symbol equations and the particle model.
- Why open and closed apparatus give different balance readings.
The big idea: conservation of mass
In a chemical reaction, atoms are rearranged to make new substances. The atoms themselves do not disappear, get created, or change into different atoms.
Definition
Conservation of mass
Conservation of mass means that the total mass of the reactants is equal to the total mass of the products, as long as every reactant and product is included in the measurement.
So if a reaction happens in a sealed container, the balance reading should stay the same. The particles may be rearranged, but the total number of atoms of each element is unchanged.
For example, calcium carbonate can thermally decompose:
CaCO3(s) → CaO(s) + CO2(g)
The carbon dioxide is a gas, but if the container is sealed, the gas is still inside the container, so its mass is still being measured.
Example
Checking conservation in a sealed container
A sealed container contains 5.00 g of calcium carbonate. After heating, it contains 2.80 g of calcium oxide and 2.20 g of carbon dioxide.
- Because the container is sealed, both products are still inside the measured system: CaO(s) and CO2(g).
- Add the product masses: 2.80 g+2.20 g=5.00 g2.80\,\text{g} + 2.20\,\text{g} = 5.00\,\text{g}2.80g+2.20g=5.00g.
- Compare this with the starting mass: 5.00 g of reactant has become 5.00 g of products, so mass has been conserved.
Open and closed systems
A system is the part of the experiment you are considering, usually the reacting chemicals plus the apparatus on the balance.
A closed system is one where substances cannot enter or leave. A sealed flask is a good example.
An open system, also called a non-enclosed system, is one where substances can enter or leave. An open crucible or test tube is an open system.
This matters because gases can move in or out of open apparatus. A balance only measures what is actually sitting on it. If a gas escapes into the air, its mass is no longer included. If a gas from the air joins the chemicals, the measured mass may increase.
Key Idea
The balance measures what is included
Apparent mass changes usually happen because a gas has either entered the measured system or left it. Conservation of mass has not stopped working.
When mass increases: a gas is a reactant
Some metals react with oxygen from the air when heated. For example, magnesium reacts with oxygen to form magnesium oxide:
2Mg(s) + O2(g) → 2MgO(s)
The magnesium oxide has a greater mass than the magnesium you started with because oxygen atoms from the air have joined with the magnesium atoms.
In an open crucible, the oxygen gas was not part of the original solid sample on the balance. Once it reacts and becomes part of the solid oxide, it is included in the measured mass.
Using the particle model: oxygen particles from the air move into the crucible. Their atoms combine with metal atoms. No atoms are created — extra atoms have entered the measured system.
Example
Finding the mass of oxygen gained
A student heats 2.40 g of magnesium in a crucible. After heating, the magnesium oxide has a mass of 3.98 g. Find the mass of oxygen that reacted.
- The product, magnesium oxide, contains the original magnesium plus oxygen from the air.
- Use the difference between the product mass and the starting metal mass: moxygen=mmagnesium oxide−mmagnesiumm_\text{oxygen} = m_\text{magnesium oxide} - m_\text{magnesium}moxygen=mmagnesium oxide−mmagnesium.
- Substitute the values: moxygen=3.98 g−2.40 g=1.58 gm_\text{oxygen} = 3.98\,\text{g} - 2.40\,\text{g} = 1.58\,\text{g}moxygen=3.98g−2.40g=1.58g.
- The mass increased by 1.58 g because that mass of oxygen entered the system and became part of the solid product.
Common Mistake
Saying mass has been created
Do not say the reaction “made extra mass”. The extra measured mass came from oxygen gas in the air, which was not counted in the original mass of the metal.
When mass decreases: a gas is a product
A thermal decomposition reaction is a reaction where one compound breaks down into simpler substances when heated.
Many metal carbonates thermally decompose to form a metal oxide and carbon dioxide. For example:
CuCO3(s) → CuO(s) + CO2(g)
If this happens in an open container, the carbon dioxide gas escapes into the atmosphere. The solid left behind is the metal oxide, so the mass on the balance decreases.
Using the particle model: the atoms from the carbonate are rearranged. Some atoms form solid metal oxide, while carbon and oxygen atoms form carbon dioxide gas particles. In an open system, these gas particles spread out and leave the apparatus.
Example
Finding the mass of carbon dioxide lost
A student heats 6.20 g of copper carbonate in an open test tube. After heating, the solid copper oxide has a mass of 4.15 g. Find the mass of carbon dioxide that escaped.
- From the equation, CuCO3(s) → CuO(s) + CO2(g), carbon dioxide is a gaseous product.
- Because the test tube is open, the CO2(g) can escape and is no longer measured on the balance.
- Calculate the loss in mass: mcarbon dioxide=6.20 g−4.15 g=2.05 gm_\text{carbon dioxide} = 6.20\,\text{g} - 4.15\,\text{g} = 2.05\,\text{g}mcarbon dioxide=6.20g−4.15g=2.05g.
- The apparent decrease in mass is 2.05 g, which is the mass of carbon dioxide that left the system.
Using the balanced equation to predict the mass change
A state symbol tells you the physical state of a substance in an equation:
- (s) means solid
- (l) means liquid
- (g) means gas
- (aq) means aqueous, meaning dissolved in water
When you are given a balanced symbol equation, look for substances with the state symbol (g). Then ask whether the gas is a reactant or a product, and whether the apparatus is open or closed.
If a gas is a reactant and comes from outside the measured apparatus, the mass can increase.
If a gas is a product and escapes from the measured apparatus, the mass can decrease.
If the apparatus is sealed and all products remain inside, the total mass stays the same.
Example
Predicting the observed mass change
A student heats solid zinc carbonate in an open crucible:
ZnCO3(s) → ZnO(s) + CO2(g)
Predict what happens to the mass shown on the balance.
- Identify the gas in the equation: CO2(g) is carbon dioxide gas.
- Decide which side of the equation the gas is on: it is on the products side, so gas is made during the reaction.
- Use the apparatus information: the crucible is open, so CO2(g) can escape into the air.
- Therefore, the balance reading decreases because some product mass has left the measured system.
Tip
Quick decision rule
Gas reactant entering an open system usually means measured mass increases. Gas product escaping from an open system usually means measured mass decreases. Sealed system means total mass stays the same.
Practical observations
You may see this idea in experiments using balances, crucibles, conical flasks, test tubes, lids, bungs, balloons, or gas syringes.
For a metal reacting with oxygen, a crucible lid may be used to reduce loss of solid powder while still allowing oxygen from the air to enter. The final solid oxide can have a greater mass than the original metal.
For a carbonate decomposition, the apparatus is often open, so carbon dioxide escapes. If the gas were trapped in a sealed container or collected as part of the measured apparatus, the total mass would not decrease.
A cotton wool plug sometimes appears in experiments that produce gas. It can stop liquid spray or solid particles escaping, but it still lets gas pass through. So the mass may still decrease if a gaseous product leaves.
Common Mistake
Mass change depends on what is on the balance
If gas is collected in a gas syringe that is not on the balance, the balance reading for the reaction flask may still fall. Always be clear about which substances and apparatus are included in the measured system.
The particle model explanation to write
The particle model explains substances as tiny particles such as atoms, molecules, or ions.
For this topic, the key particle-model points are:
- Atoms are rearranged in a chemical reaction.
- No atoms are created or destroyed.
- Gas particles can move freely and spread out.
- In an open container, gas particles can leave or enter the measured system.
- The balance reading changes only because some particles are no longer included, or new particles have become included.
Exam technique
In the exam
- Check whether the reaction is in an open system or a closed system.
- Use the state symbols in the balanced equation to identify any gases.
- If a gas escapes, explain the decrease in mass; if a gas enters and reacts, explain the increase in mass.
- Always link your answer to particles: atoms are conserved, but gas particles may enter or leave the measured system.
Self review
Check yourself
- Why does magnesium gain mass when heated in air in an open crucible?
- Calcium carbonate is heated in an open test tube. Why does the mass of solid left behind decrease?
- What would happen to the total mass if a carbonate decomposition happened in a sealed container?
Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations
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