The Law of Conservation of Mass states that mass cannot be created or destroyed during a chemical reaction.
This means:
- Atoms do not disappear
- New atoms do not appear
- Atoms only rearrange during reactions
Because of this, the total mass stays the same.
Examples
You can see this law in daily life.
Burning Wood
When wood burns, it changes into:
- Ash
- Smoke
- Gases
The total mass still remains the same.
Melting Ice
Ice changes into water, but the mass does not change.
Rusting Iron
Iron reacts with oxygen to form rust. The atoms rearrange, but matter remains conserved.
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Why Must Chemical Equations Balance?
Chemical equations must balance because atoms cannot be created or destroyed.
Example:
Atom Count
| Element | Reactants | Products |
|---|---|---|
| Hydrogen | 4 | 4 |
| Oxygen | 2 | 2 |
The number of atoms stays equal. This follows the Law of Conservation of Mass.
How Does This Law Link to Balanced Equations?
Balanced equations follow the Law of Conservation of Mass.
When balancing equations:
- The number of atoms must stay equal
- Matter must remain conserved
This is why coefficients are added in equations.
Why Does Mass Stay the Same During a Reaction?
Mass stays the same because:
- Atoms remain present
- No atoms are destroyed
- No new atoms are made
Only the arrangement changes.
Why Can Mass Appear to Change?
This is another very common student question.
Mass can appear to decrease if:
- Gas escapes from the container
Mass can appear to increase if:
- A gas from the air enters the reaction
The actual total mass still remains conserved.
Common GCSE Exam Question
Explain Why Mass Stays Constant
The atoms rearrange during the reaction, but they are not created or destroyed.
Explain Why Mass Appears to Change
The mass decreased because gas escaped from the container.
Explain Why Chemical Equations Must Balance.
Chemical equations must balance because atoms are not created or destroyed during reactions. The number of atoms must stay equal on both sides of the equation.
A Student Observed a Decrease in Mass During a Reaction. Explain Why.
The measured mass decreased because gas escaped from the container during the reaction. The gas still has mass, but it left the system.
Balance the Following Equation: Mg+O2→MgO
Explanation
- Oxygen has 2 atoms on the left side.
- Therefore, 2MgO is needed on the right side.
- Then magnesium must also become 2Mg.
Why should the flask remain sealed During an Experiment??
The flask should remain sealed so gases cannot escape. This helps keep the total mass constant during the reaction.
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Common A-Level Exam Question
Explain How a Balanced Equation Demonstrates the Law of Conservation of Mass.
A balanced equation shows that the number of atoms is equal on both sides of the reaction. This proves atoms are not created or destroyed, so mass remains constant.
Explain Why Mass May Appear to Decrease During a Chemical Reaction.
Mass may appear to decrease if gas escapes from the reaction container. The gas still contains mass, but it is no longer measured in the system.
Explain Why Iron Gains Mass When It Rusts.
Iron gains mass during rusting because oxygen atoms from the air combine with the iron to form iron oxide.
Explain Why Stoichiometry Calculations Depend on Conservation of Mass.
Stoichiometry calculations use balanced equations because atoms and mass are conserved during chemical reactions.
This allows scientists to calculate reacting masses and product amounts accurately.
Common Student Misconceptions
Mass decreases when gas forms
Incorrect.
"The gas still has mass. It simply escaped."
Atoms disappear during reactions
Incorrect.
"Atoms rearrange into new substances."
Mass and volume are the same
Incorrect.
"Volume can change while mass stays constant."
Read More How to Balance Equations in Chemistry?
FAQs
Does mass ever disappear?
No. Matter cannot disappear during chemical reactions.
Why does rust weigh more than iron?
Because oxygen atoms join the iron.
Is conservation of mass always true?
Yes, for ordinary chemical reactions.