Topic 4.4 Notes – Physical and Chemical Changes

What Makes a Change Chemical vs Physical

There are two main types of interactions:

• Intramolecular bonds
  These are bonds within a substance.
  • Covalent bonds
  • Ionic bonds
• Intermolecular forces (IMFs)
  These are attractions between particles.
  • Hydrogen bonding
  • Dipole-dipole forces
  • London dispersion forces
  • Ion-dipole forces

The classification rule:

• Chemical change → involves breaking and/or forming intramolecular bonds.
• Physical change → involves changes in intermolecular forces or particle arrangement only.

Connecting Macroscopic and Particulate Levels

On tests, you’re almost never done after saying “chemical” or “physical.” You need to explain why.

• Macroscopic level = what you observe (color change, solid forming, temperature change).
• Particulate level = what atoms, ions, or molecules are doing.

Strong AP-style reasoning sounds like:

A precipitate forms, which indicates new ionic bonds formed between ions, so the process is chemical.

The bond explanation is what earns the point.

Chemical Processes

A chemical process involves breaking and/or forming covalent or ionic bonds.

At the particle level:

• Atoms rearrange.
• Old bonds break.
• New bonds form.
• A new substance with new properties appears.

Common macroscopic evidence:

• Gas formation (not just boiling)
• Precipitate formation
• Unexpected color change
• Heat or light produced (like combustion)

Example:
When propane burns,

$$\ce{C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g)}$$

• C-H and O=O bonds break.
• C=O and O-H bonds form.
• Completely new substances form.
  That is clearly chemical.

Energy changes do not determine the classification. Physical processes can also absorb or release energy.

Physical Processes

A physical process changes the form, phase, or arrangement of matter without changing chemical identity.

At the particle level:

• Molecules stay the same.
• Only IMFs are broken or formed.
• Particle spacing or motion changes.

Classic example: melting ice. The $\ce{H2O}$ molecules are the same in all three phases. What changes is how closely they are packed and how strongly they interact.

In melting:

• Hydrogen bonds between water molecules are partially broken.
• The $\ce{H2O}$ molecules themselves do not change.
• Still water → physical change.

Other examples:

• Boiling ethanol
• Crushing a crystal
• Mixing sand and sugar

Dissolution of Ionic Compounds

This is where students hesitate.

When $\ce{KBr(s)}$ dissolves in water:

• Ionic bonds in the crystal lattice are broken.
• Ion-dipole interactions form between ions and water.

The diagram shows a crystal breaking apart and an individual ion surrounded by water molecules in a hydration shell.

Hydration shell around an ion in aqueous solution

You are breaking chemical bonds in the solid, which sounds chemical. But:

• No new substance forms.
• The ions were already $\ce{K+}$ and $\ce{Br-}$.
• They are just separated and hydrated.

AP-level reasoning: this process is typically classified as physical, but you must justify it by discussing both the breaking of ionic bonds and formation of ion-dipole forces. The explanation matters more than the label.

Intramolecular Bonds vs Intermolecular Forces

If a question mentions covalent bonds breaking, it’s chemical.
If it only mentions hydrogen bonding changing, it’s physical.

How to Justify Your Answer on a Test

When given a scenario:

1. Identify what changes macroscopically.
2. Ask what particles must be doing.
3. State which interactions are broken and/or formed.
4. Classify the process.

Example: A solid forms when two solutions mix.
That means ions combined to form a new ionic compound. New ionic bonds formed. That’s chemical.

Students often lose points by saying “a reaction happened” without mentioning bonds. Always bring it back to bond interactions.