Slide presentation
Law of Conservation of Mass
General Chemistry • Chemical Compounds
Topic 1 · Reaction mass
Law of Conservation of Mass
During a chemical reaction, matter is not created or destroyed. Atoms are rearranged, so the total mass of reactants equals the total mass of products in a closed system.
Learning target
Use mass before and after a reaction to explain conservation of matter and connect it to balanced chemical equations.
when no matter enters or leaves the system.
Why it matters
Mass conservation makes chemical reactions measurable.
Chemists use conservation of mass to check experiments, predict product amounts, balance equations, and explain why an apparent mass change often means matter escaped or entered the system.
Laboratory evidence
A sealed reaction container should have the same total mass before and after reaction.
Balanced equations
The same number of each atom must appear on both sides of a chemical equation.
Stoichiometry
Mass relationships in reactions depend on atoms being conserved.
Core concept
A chemical reaction rearranges atoms.
Reactants are the starting substances. Products are the substances formed. The atoms can bond in new ways, but the number of each type of atom remains constant.
Reactants
Atoms begin in one arrangement.
Reaction
Bonds break and form, changing how atoms are connected.
Products
The same atoms appear in a new arrangement.
Vocabulary
Mass conservation depends on system boundaries.
Before deciding whether mass is conserved, define what belongs to the system. A gas escaping an open flask can make the measured mass decrease even though matter still exists elsewhere.
| Term | Meaning | Common unit | Example |
|---|---|---|---|
| Reactants | Starting substances in a chemical reaction | g, mol | Mg and O2 before MgO forms |
| Products | New substances formed by the reaction | g, mol | MgO after magnesium burns |
| Closed system | No matter enters or leaves during the reaction | system description | Sealed flask |
| Open system | Matter can enter or leave during the reaction | system description | Open beaker releasing CO2 |
| Balanced equation | Chemical equation with equal atom counts on both sides | coefficients | 2H2 + O2 → 2H2O |
Main relationship
In a closed system, total mass before equals total mass after.
Mass conservation is a macroscopic result of atom conservation. If every atom remains inside the measured system, the total mass cannot change.
\(m_{\text{reactants}}\) is the total starting mass. \(m_{\text{products}}\) is the total final mass in the same closed system.
Balanced equation meaning
Coefficients are chosen so each element has the same atom count before and after reaction.
Mass calculation meaning
If 12.0 g of reactant A combines with 8.0 g of reactant B, the closed-system products must total 20.0 g.
Interactive simulation
Test mass conservation in a closed or open system.
Adjust the reactant masses. In a closed system, all matter stays inside. In an open system, escaping gas can make the measured product mass appear smaller.
Reaction mass tester
Measured mass result
20.0 g before = 20.0 g after
Closed system: mass is conserved in the container.Static fallback model
For 12.0 g reactant A and 8.0 g reactant B in a closed system, the products must total 20.0 g.
Reactants
20.0 g
Products
20.0 g
Dynamic relationship
Closed-system data fall on the line \(y=x\).
On a graph of mass before versus mass after, conserved mass appears on the diagonal line. Open-system measurements can fall below that line if gas escapes.
The animated point uses the reactant masses from the simulation. It shows how measurement depends on whether matter remains inside the system.
Worked example
Find the missing product mass.
A closed container has 15.0 g of calcium carbonate before heating. After reaction, the container contains 8.4 g of calcium oxide and a trapped gas. What mass of gas formed?
Identify total mass before
The closed container starts with \(15.0\ \text{g}\) of reactant.
Use conservation of mass
\(15.0\ \text{g} = 8.4\ \text{g} + m_{\text{gas}}\)
Solve
\(m_{\text{gas}} = 15.0\ \text{g} - 8.4\ \text{g} = 6.6\ \text{g}\)
Final answer: The trapped gas has a mass of \(6.6\ \text{g}\).
Common misconception
A lower measured mass does not mean matter was destroyed.
If a reaction produces a gas in an open container, some matter may leave the measured system. The law is not broken; the measurement boundary changed.
Incorrect reasoning
“The product mass is lower, so some matter disappeared.”
This ignores gas escaping from the system.
Correct reasoning
Matter is conserved overall. In an open system, escaped gas must be included if you want the total mass after the reaction.
Practice check
Use mass conservation to find a missing mass.
Question: In a sealed flask, 6.5 g of zinc reacts completely with 4.0 g of sulfur. What mass of zinc sulfide product should be present?
Show answer
Add reactant masses
\(6.5\ \text{g} + 4.0\ \text{g} = 10.5\ \text{g}\)
Apply closed-system conservation
No matter leaves the sealed flask, so the product mass equals the total reactant mass.
Final answer
The flask should contain \(10.5\ \text{g}\) of zinc sulfide.
Reasonableness check
The answer must be greater than either single reactant mass because the product contains both zinc and sulfur atoms.
Apply the topic
Use mass conservation whenever a reaction has missing information.
In future chemistry problems, first decide whether the system is closed. Then compare total mass before and after, and use the balanced equation to track atoms.
Open the calculator
Practice finding missing reactant or product masses in reaction data.
Try related questions
Check your ability to interpret closed systems, open systems, and balanced equations.
Define system
Closed or open?
Track atoms
Use the balanced equation.
Compare mass
Before equals after when all matter is included.
Final summary
Mass is conserved because atoms are conserved.
Matter is not destroyed.
Chemical reactions rearrange atoms into new substances.
Closed systems reveal conservation.
If no matter enters or leaves, total mass before equals total mass after.
Balanced equations show atom conservation.
The same number of each atom appears on both sides.
Open systems can mislead.
Escaped gas must be included to account for all mass.
Key idea: \(m_{\text{reactants}} = m_{\text{products}}\) when all matter in the reaction system is counted.