Physical and Chemical Changes
i. Chemical processes
ii. Physical processes
Quick Notes
- Chemical processes involve the breaking and/or formation of chemical bonds between atoms.
- Physical processes involve changes in intermolecular forces only — no change in chemical composition.
- Common examples:
- Chemical: combustion, oxidation, acid-base reactions.
- Physical: melting, boiling, dissolving molecular substances.
- Some processes, like dissolution of ionic compounds, can involve both intermolecular and chemical bond changes — they may be considered borderline cases.
- Macroscopic evidence can be used for chemical change: color change, gas production, temperature change, precipitate formation.
Full Notes
To distinguish between chemical and physical changes, it's important to understand what is happening at the molecular level — specifically, whether chemical bonds between atoms are being altered or if only intermolecular forces are changing.
Chemical Processes
Chemical processes involve:
- Breaking of intramolecular bonds (e.g., covalent or ionic bonds between atoms).
- Formation of new bonds to create different substances.
These changes result in:
- New substances with different chemical properties.
- Often, observable macroscopic changes like heat/light release, color change, precipitate, or gas formation.
Example:Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O
C–H and O=O bonds broken, C=O and O-H bonds formed
Physical Processes
Physical changes involve:
- No breaking of intramolecular (chemical) bonds.
- Only changes in intermolecular forces (e.g., hydrogen bonding, dispersion forces).
These changes do not produce new substances, just different forms or arrangements of the same substance.
Example:Melting and boiling water (molecular substance)
Hydrogen bonds between water molecules break, molecules themselves remain the same.
Borderline Cases
Some processes blur the line between chemical and physical such as the dissolution of ionic salts in water.
- Ionic bonds in the crystal lattice are broken.
- Formation of ion-dipole interactions between ions and water molecules.
However, no new substance is formed so it can be argued as being both a physical and chemical process, depending on context.
When deciding if a process is chemical or physical, ask:
- “Are the chemical bonds between atoms changing?”
- If yes then chemical process.
- If no, and only phase or structure changes then physical process.
- Watch for clues: production of gas, new color, or a new solid often means chemical.
Summary
- Chemical changes involve breaking or forming chemical bonds and result in new substances with different compositions.
- Physical changes involve only changes in intermolecular forces and do not alter the chemical identity of the substance.
- Understanding the type of bond interactions occurring allows us to predict whether a process is physical or chemical and relate this to what we observe at the macroscopic level.