# Physical and chemical changes

> AP Chemistry · AP Chemistry CED Unit 4: Chemical Reactions
> Source: https://www.owlsprep.com/study/ap-chemistry-u4-physical-and-chemical-changes/

This subtopic covers distinguishing physical vs chemical changes, comparing intermolecular/intramolecular bond changes, applying conservation of mass, interpreting particulate diagrams, and using physical changes for mixture separation for AP Chemistry.

**Prerequisites:** Basic definition of chemical bonds; Pure substances vs mixtures classification; Particulate representation of matter

## Learning objectives

- Distinguish between physical and chemical changes based on intramolecular vs intermolecular changes
- Apply the law of conservation of mass to physical and chemical changes in open/closed systems
- Classify changes from AP-style particulate diagrams
- Avoid common exam pitfalls when classifying changes

## Core Definitions and Exam Context

This foundational topic makes up 1-3% of the total AP Chemistry exam score, per the official CED. It appears in both multiple-choice (MCQ) and conceptual warm-up sections of free-response questions (FRQ), most often paired with particulate diagram reasoning.

**Physical Change** — A change to the physical form or properties of a substance that does not alter its underlying chemical identity

*Example:* Melting of solid sucrose, dissolving sugar in water

**Chemical Change** — A process that produces new chemical substances with distinct compositions and properties via rearrangement of atoms

*Notation:* Also called a chemical reaction

*Example:* Decomposition of sucrose into carbon and water, combustion of fuel

> **info**
>
> AP Chemistry exam questions emphasize conceptual reasoning over memorization of examples. Always rely on the intramolecular bond/new substance rule rather than oversimplified heuristics.

## Key Difference: Intermolecular vs Intramolecular Changes

The most reliable method to classify any change is to assess which interactions are altered during the process. Intermolecular forces (IMFs) are weak attractive forces between separate molecules or formula units that hold bulk matter together. Intramolecular bonds (covalent, ionic, metallic) are strong bonds that hold atoms together within a single molecule or formula unit, giving a substance its unique chemical identity.

In a physical change, only intermolecular forces are broken or formed; intramolecular bonds remain completely intact, so chemical identity does not change. In a chemical change, intramolecular bonds are broken, atoms rearrange, and new intramolecular bonds form to create new chemical substances.

**Worked example:** A student heats a sample of solid lead iodide (PbI₂) until it melts to form liquid lead iodide. No gas or solid residue is observed, and the liquid can be cooled to re-form solid PbI₂. Classify this change as physical or chemical, and justify your answer.

1. Identify the change: melting of a pure ionic solid.
2. Assess which interactions are altered: In solid PbI₂, Pb²⁺ and I⁻ ions are held in a crystal lattice by ionic (intramolecular) bonds. When melted, the ions separate, but the ions themselves remain chemically unchanged; no new chemical species are formed.
3. Confirm: The only change is to the arrangement of ions; intermolecular forces between ions in the lattice are broken, but the ionic bonds between Pb²⁺ and I⁻ remain intact, forming the same substance in liquid form.
4. Conclusion: This is a physical change.

> **tip**
>
> Never use reversibility to classify a change. Cutting a tree into lumber is irreversible but physical, while many acid-base reactions are reversible but chemical. Always use the new substance/intramolecular bond rule.

## Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed in any physical or chemical change, as long as the system being measured is closed (no mass can enter or leave the system from the surroundings). This rule holds equally for both types of change: all atoms present before the change remain present after, just rearranged.

AP exam questions frequently test understanding of mass changes in open systems, where mass can be exchanged with the surroundings. A measured mass change in an open system does not violate conservation of mass—it just means matter entered or left the system.

**Worked example:** A student weighs 25.0 g of sand and 100.0 g of water into an open beaker with a mass of 85.0 g. They stir the mixture to suspend the sand, then filter the sand out of the water and dry it completely. The dried sand is weighed in the same beaker. What is the expected mass of the beaker and dried sand?

1. Identify the process: filtering is a physical separation of sand and water, no mass is created or destroyed.
2. All sand is recovered after drying, so the mass of dried sand equals the original mass of sand: 25.0 g.
3. Calculate total mass:
4. $$85.0\ \text{g} + 25.0\ \text{g} = 110.0\ \text{g}$$
5. Water removed from the sand is in the filtrate, so it does not contribute to the mass of the beaker and dried sand. The expected mass is 110.0 g.

> **exam_tip**
>
> When asked to explain an unexpected mass change, always first check if the system is open, then account for any escaped gas (from chemical reactions) or evaporated solvent (physical change) rather than claiming conservation of mass does not hold.

## Interpreting Particulate Diagrams

A common AP Chemistry question type asks you to classify a change as physical or chemical based on a particulate diagram showing particles before and after the change. The key to solving these problems is to check whether the same chemical species exist before and after the change.

If the particles are just separated, spread out, or rearranged but the same molecules or formula units are present, the change is physical. If new molecules or formula units are present that did not exist before (atoms rearranged into new combinations), the change is chemical.

**Worked example:** A particulate diagram shows 6 diatomic AB molecules before a change. After the change, the container has 3 A₂ molecules and 3 B₂ molecules, with no intact AB molecules remaining. Classify the change.

1. Identify species before the change: only intact AB molecules, held together by A-B intramolecular bonds.
2. Identify species after the change: A₂ and B₂ molecules are new chemical species that did not exist before.
3. Original A-B bonds were broken, and new A-A and B-B bonds formed to make new molecules.
4. Conclusion: this is a chemical change, because new substances were formed by breaking and re-forming intramolecular bonds.

> **tip**
>
> When interpreting a diagram of dissolving solid NaCl, remember separating Na⁺ and Cl⁻ ions from the crystal lattice is still a physical change—no new chemical substances are formed, only hydrated separated ions.

**Check your understanding**

Test your understanding with this AP-style multiple choice question:

1. Which of the following processes is correctly classified as a chemical change?

   - A) Sublimation of solid carbon dioxide to carbon dioxide gas
   - B) Separation of a mixture of ethanol and water by distillation
   - C) Decomposition of solid potassium chlorate into potassium chloride and oxygen gas
   - D) Dissolution of solid potassium nitrate in water to form an aqueous solution

   *Why:* A: Sublimation only breaks intermolecular forces between CO₂ molecules, so it is physical. B: Distillation uses physical changes to separate mixtures, no new substances form. D: Dissolving KNO₃ only rearranges ions, no new substances are formed. C is correct: new substances are formed via bond rearrangement.

## Common pitfalls

- **Wrong:** Classifying the dissolving of NaCl in water as a chemical change because ions separate and are hydrated.
  - Why it fails: Students confuse breaking ionic bonds between formula units in a crystal lattice with breaking intramolecular bonds to form new substances; hydration only forms new intermolecular interactions, not new chemical species.
  - Correct: Always check if new chemical compounds are formed; if only ions are separated and hydrated, it is a physical change.
- **Wrong:** Claiming a change is physical because it is reversible, and chemical because it is irreversible.
  - Why it fails: Students memorize an oversimplified rule from introductory chemistry that is not universally true.
  - Correct: Always use the 'new substance / intramolecular bond change' rule to classify, never rely on reversibility.
- **Wrong:** Claiming conservation of mass is violated in an open beaker when a reaction produces gas that escapes.
  - Why it fails: Students forget the law of conservation of mass applies to closed systems that do not exchange matter with the surroundings.
  - Correct: If the system is open, explain the mass change by accounting for any matter that entered or left the system, rather than saying mass was created or destroyed.
- **Wrong:** Classifying the boiling of liquid hydrogen peroxide (H₂O₂) as a physical change, assuming all phase changes are physical.
  - Why it fails: Students assume all phase changes are physical without considering if decomposition occurs at boiling temperature.
  - Correct: Always check if the substance undergoes a chemical reaction at the conditions of the change before classifying.
- **Wrong:** Interpreting melting of solid molecular iodine (I₂) as a chemical change because I₂ molecules are separated.
  - Why it fails: Students confuse separation of molecules (breaking intermolecular forces) with separation of atoms within a molecule (breaking intramolecular bonds).
  - Correct: Check if the same molecules exist before and after; if I₂ molecules are still intact after melting, the change is physical.

## Cheatsheet

| Category | Rule | Notes |
| --- | --- | --- |
| Physical Change | No new chemical substances formed | Only intermolecular forces are broken/formed; intramolecular bonds stay intact |
| Chemical Change | New chemical substances formed | Intramolecular bonds are broken/formed to rearrange atoms into new compounds |
| Conservation of Mass | $m_{\text{total, initial}} = m_{\text{total, final}}$ | Only applies to closed systems (no mass exchange with surroundings) |
| Particulate Diagram: Physical | Same chemical species before/after | Molecules/ions just rearranged, not split into new species |
| Particulate Diagram: Chemical | New chemical species after change | Atoms rearranged into new bonded combinations |
| Dissolving Ionic Compounds | Classified as physical change | Ions separate but no new chemical compounds are formed |
| Phase Change | Almost always physical | Check for decomposition at the new temperature before confirming |
| Mixture Separation | All techniques use physical changes | No chemical change occurs during distillation, filtration, or chromatography |

## What's next

This topic is the foundational classification for all of Unit 4: Chemical Reactions, and it connects closely to core concepts in Unit 3: Intermolecular Forces and Properties. Mastering the distinction between physical and chemical changes is critical for interpreting particulate representations of reactions, a high-frequency question type that appears on almost every AP Chemistry exam. Next, you will build on this foundation to learn how to write balanced chemical equations for chemical changes, where you will rely on your ability to identify that reactants are converted to new products. This topic also underpins all separation techniques used in experimental chemistry, which are commonly tested in the FRQ section of the exam.

- [Unit 4: Chemical Reactions Overview](https://www.owlsprep.com/study/ap-chemistry-u4-overview/)
- [Intermolecular Forces and Properties](https://www.owlsprep.com/study/ap-chemistry-u3-intermolecular-forces/)
- [AP Chemistry Stoichiometry](https://www.owlsprep.com/study/ap-chemistry-u4-stoichiometry/)

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