Conservation Laws
Understanding what stays constant during physical and chemical changes
Conservation Laws are fundamental principles that govern all physical and chemical changes. They explain why we must balance equations and help us understand the behavior of matter and energy.
1. Law of Conservation of Mass
Definition
The Law of Conservation of Mass states that mass cannot be created or destroyed in a chemical reaction.
Application: The total mass of the reactants must exactly equal the total mass of the products. This is essential for accurate chemical equations.
Mass Conservation Calculator
Enter reactant masses to calculate product mass:
Total Mass of Products = 15 g
Reactants: 10g + 5g = 15g
Products must also be 15g
Worked Example
If 10g of Reactant A reacts with 5g of Reactant B, what is the total mass of the products?
2. Law of Conservation of Atoms
Definition
The Law of Conservation of Atoms states that the number of atoms of each element remains constant throughout a chemical reaction.
Key Concept: Atoms are simply rearranged to form new compounds during reactions. This is why we use coefficients to balance chemical equations.
Interactive Atom Counter
Verify atom conservation in: CH₄ + 2O₂ → CO₂ + 2H₂O
Try It Yourself
Count atoms in: 2H₂ + O₂ → 2H₂O
3. Law of Constant Composition
Definition
The Law of Constant Composition states that a specific chemical compound always contains the same elements in fixed proportions by mass.
Example: Water (H₂O)
- ■ Oxygen: 88.8% by mass
- ■ Hydrogen: 11.2% by mass
Example: Carbon Dioxide (CO₂)
- ■ Carbon: 27.3% by mass
- ■ Oxygen: 72.7% by mass
Key Point: Pure water from any source—rain, river, or laboratory—always has the same composition: 88.8% oxygen and 11.2% hydrogen by mass.
Quick Check
A sample of pure water is found to contain 8g of oxygen. How many grams of hydrogen should it contain?
4. Conservation of Energy
Definition
The Conservation of Energy principle asserts that energy cannot be created or destroyed; it can only be transformed from one form to another.
Exothermic Reactions
Chemical potential energy → Thermal energy (heat released)
CH₄ + 2O₂ → CO₂ + 2H₂O + Energy
Endothermic Reactions
Thermal energy → Chemical potential energy (heat absorbed)
CaCO₃ + Heat → CaO + CO₂
Energy Transformation Simulator
Click to see energy transformation:
5. What is NOT Conserved?
While mass, atoms, and energy are conserved, other properties can change during chemical reactions.
Number of Molecules
Can change during reactions
NOT conserved
Volume
Especially in gases, volume can change
NOT conserved
Physical State
Solid, liquid, gas can change
NOT conserved
Color
Can change during reactions
NOT conserved
Summary: What's Conserved vs What's Not
| Property | Conserved? | Explanation |
|---|---|---|
| Mass | ✓ Yes | Law of Conservation of Mass |
| Atoms | ✓ Yes | Same number of each element |
| Energy | ✓ Yes | Only transformed, not destroyed |
| Molecules | ✗ No | Number can change |
| Volume | ✗ No | Especially for gases |
Test Your Understanding
1. According to the Law of Conservation of Mass, if 12g of carbon reacts with 32g of oxygen, what mass of CO₂ is produced?
2. What is conserved in a chemical reaction?
3. The Law of Constant Composition means that water always contains:
4. Which of the following is NOT conserved in a chemical reaction?
Key Terms
Key Takeaways
- Mass is conserved: total mass of reactants = total mass of products
- Atoms are conserved: same number of each element on both sides of equation
- Constant composition: compounds always have fixed proportions by mass
- Energy is conserved: only transformed from one form to another
- Number of molecules is NOT conserved - can change during reactions
- Volume is NOT conserved - especially for gases
- These laws explain why we must balance chemical equations
