Chemical Change
Understanding how substances transform into entirely new materials with different properties
Chemical change occurs when substances are transformed into entirely new substances with different chemical properties. Bonds break and form, atoms rearrange, and new materials are created.
Key Insight: Antoine Lavoisier's work on combustion in the 1770s helped establish that chemical changes involve the rearrangement of atoms, not their creation or destruction.
1. Definition and Characteristics
Formation of New Substances
The chemical identity of reactants is lost, and new products with different properties are formed.
2H₂ + O₂ → 2H₂O
Irreversibility
Chemical changes are typically irreversible through simple physical means.
Cannot "un-burn" ash back into wood
Large Energy Changes
Chemical reactions involve significant energy changes - much larger than physical changes.
Energy Comparison: Chemical changes involve energy changes 10-100 times larger than physical changes because chemical bonds are being broken and formed.
2. Molecular Level: Breaking and Making Bonds
Chemical Bonds: The Key to Change
Intramolecular Bonds
Unlike physical changes, chemical changes break the strong bonds within molecules.
H-H bond breaks
Rearrangement of Atoms
Atoms are not created or destroyed - they are rearranged into new combinations.
Interactive Bond Breaker
Click to break the bonds and see atoms rearrange:
Energy required to break H-H bond: 436 kJ/mol
3. Indicators of Chemical Change
In a laboratory setting, these signs indicate a chemical change has occurred:
Color Change
Temperature Change
Gas Production
Precipitate Formation
Spot the Indicator
A clear liquid is mixed with another clear liquid, and a solid forms at the bottom. What indicator is this?
4. Examples of Chemical Changes
Synthesis Reaction
Two or more substances combine to form a more complex compound.
2H₂ + O₂ → 2H₂O
Hydrogen + Oxygen → Water
Everyday Chemical Changes
Rusting
Iron + Oxygen → Iron oxide
Baking
Baking soda + acid → CO₂ bubbles
Digestion
Food broken down by enzymes
Photosynthesis
CO₂ + H₂O → Glucose + O₂
5. Conservation Laws in Chemical Change
Conservation of Mass
Total mass of reactants = Total mass of products
Conservation of Atoms
Number of each atom type remains constant
CH₄ + 2O₂ → CO₂ + 2H₂O
Non-conservation of Molecules
Number of molecules can change
3 molecules reactants → 3 molecules products
CH₄ + 2O₂ (3) → CO₂ + 2H₂O (3)
Mass Conservation Calculator
If 12g of carbon reacts with 32g of oxygen, what mass of CO₂ is produced?
Test Your Understanding
1. Which of the following is a chemical change?
2. What is conserved in a chemical reaction?
3. Which indicator shows a gas is being produced?
Key Terms
Key Takeaways
- Chemical changes form new substances with different properties
- Chemical changes are typically irreversible
- Bonds break and form - intramolecular bonds are involved
- Indicators include color change, temperature change, gas production, and precipitate formation
- Examples: synthesis, decomposition, combustion, rusting, baking
- Mass and atoms are conserved; molecules may change in number
