Lesson focus: Mass is conserved because reactions rearrange atoms; apparent changes arise when matter crosses the measured system boundaryThe imaginary line between the system you are studying and its surroundings..
Apply conservation of massThe rule that the total mass stays the same in a reaction, because atoms are not made or destroyed. to closed and open systemA set-up where substances, such as gases, can get in or out. and particle equations.
Build on earlier learning
Thermal decompositionEscaping carbon dioxide provides a clear test of system boundaries and apparent mass loss.
You will be successful when you can…
- State conservation of mass.
- Count atoms before and after.
- Explain open-system mass changes.
- Predict mass in a sealed reaction.
Words that carry the science
- conservation of mass
- Total mass remains constant in a closed system during a reaction.
- closed system
- A defined system that exchanges no matter with its surroundings.
- open system
- A system across whose boundary matter can move.
- system boundary
- The chosen division between system and surroundings.
- coefficient
- A number showing relative particle amount in an equation.
Atoms account for conserved mass
01Atoms account for conserved mass
Chemical reactions rearrange existing atoms. The number of atoms of each element is the same before and after, so total mass is conserved in a closed systemA container or set-up where no substances can get in or out..
Balanced symbol equations represent this count. coefficientA big number in front of a formula in an equation that tells you how many of that particle there are, such as the 2 in 2H₂O. can change; formula subscripts cannot.
02Measurements depend on the boundary
If gas escapes an open flask, its measured mass can fall. If oxygen enters and joins a solid, the measured solid can gain mass.
These observations do not break conservation. Matter crossed the boundary of what was placed on the balance.
03Sealed apparatus reveals the total
A sealed bag or stoppered flask retains gaseous products, so total mass before and after should agree within measurement uncertainty.
A small difference may reflect balance resolution, leaks, evaporation before sealing or material left outside the measured setup.
Conservation of mass
Marble chips (calcium carbonate) react with hydrochloric acid to make calcium chloride, water and carbon dioxide gas. Does the mass change?
Controls
Masses are illustrative (0.5 g of marble makes 0.22 g of carbon dioxide). Atoms are not made or destroyed: the total mass of products equals the total mass of reactants.
Predict product mass
12.0 g carbon reacts completely with 32.0 g oxygen in a sealed vessel.
- Define the sealed vessel and contents as the system.
- All reactant matter remains within the boundary.
- Total reactant mass = 12.0 + 32.0 = 44.0 g.
- Therefore total product mass = 44.0 g.
How to make the reasoning visible: The closed-system product mass is 44.0 g.
Track matter across boundaries
Explain each balance reading using matter movement.
| Setup | Before / g | After / g | Boundary event |
|---|---|---|---|
| sealed flask | 125.4 | 125.4 | none detected |
| open carbonate tube | 42.8 | 41.7 | gas leaves |
| iron wool open to air | 8.0 | 8.9 | oxygen enters solid |
| leaking gas bag | 63.2 | 62.6 | gas leaves |
Which row best tests conservation directly?
Explain the iron-wool gain.
Check your understanding
Answer from memory, then check against the suggested answer.
Why is mass conserved?
What is a closed system?
Why can open mass fall?
5.0 g reactants form gas in a sealed bag. Predict final total mass.
Why is “mass lost” weaker than “mass left the measured system”?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A reaction that makes a gas is done twice: in an open flask the mass goes down, but in a sealed flask it stays the same.
Explain why the mass goes down in the open flask.
(3)Explain why the mass stays the same in the sealed flask.
(2)Total for this question: 5 marks
Command word: Use the system boundary explicitly.