Physical and chemical changes — AP Chemistry Study Guide

For: AP Chemistry candidates sitting AP Chemistry.

Covers: Distinguishing physical vs chemical changes, comparing intermolecular vs intramolecular bond changes, applying conservation of mass to both change types, identifying changes in particulate diagrams, and using physical changes for mixture separation.

You should already know: Basic definition of chemical bonds, pure substances vs mixtures, particulate representation of matter.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Chemistry style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Physical and chemical changes?

This foundational topic for Unit 4: Chemical Reactions accounts for approximately 1-3% of the total AP Chemistry exam weight per the official Course and Exam Description (CED). It appears in both multiple-choice (MCQ) and the conceptual warm-up sections of free-response questions (FRQ), often paired with particulate diagram reasoning. A physical change is a change to the physical form or properties of a substance that does not alter its underlying chemical identity. A chemical change (also called a chemical reaction) produces new chemical substances with distinct compositions and properties. The core AP-expected distinction between the two types of change centers on whether intramolecular (chemical) bonds are broken and re-formed to make new substances, versus only intermolecular forces between molecules being rearranged. This topic emphasizes conceptual reasoning over memorization, and it underpins all subsequent work with chemical reactions in the course.

2. Core Defining 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; they do not involve the sharing or transfer of electrons that create stable chemical bonds. Intramolecular bonds (covalent, ionic, metallic) are the 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 the chemical identity of the substance does not change. For example, when solid sucrose melts, the intermolecular forces between separate sucrose molecules are broken, but the covalent bonds holding the 12 carbon, 22 hydrogen, and 11 oxygen atoms together in each sucrose molecule remain whole. The product is still sucrose, just liquid instead of solid. In a chemical change, intramolecular bonds are broken, atoms are rearranged, and new intramolecular bonds are formed to create new chemical substances that did not exist before. For example, when sucrose is heated strongly enough to decompose into carbon and water, the covalent bonds within sucrose are broken and new bonds form to make carbon and water, two distinct new substances.

Worked Example

Problem: 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 to form the same substance, just in liquid form).
4. Conclusion: This is a physical change.

Exam tip: Never use reversibility to classify a change. Many students memorize the incorrect middle school rule that "physical changes are reversible and chemical changes are not"—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.

3. Conservation of Mass in Physical and Chemical Changes

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: in both cases, all atoms present before the change are still present after the change, just rearranged.

AP exam questions frequently test understanding of mass changes in open systems, where mass can be exchanged with the surroundings. For example, if you dissolve sugar in water in an open beaker, the total mass of the beaker and contents equals the sum of the mass of the sugar, water, and beaker, even though the sugar "disappears". If you leave the open beaker for a week, the water evaporates and the total mass decreases, but this is not a violation of conservation of mass—water vapor just escaped to the surroundings, not destroyed.

Worked Example

Problem: 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: mass of beaker + mass of dried sand = $85.0\text{g}+25.0\text{g}=110.0\text{g}$.
4. The water that was removed from the sand is now 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 gas that escaped (from a chemical reaction) or solvent that evaporated (physical change) rather than claiming conservation of mass does not hold.

4. Interpreting Particulate Diagrams of Changes

A common AP Chemistry question type asks you to classify a change as physical or chemical based on a particulate diagram showing the arrangement of atoms/molecules 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. For example, a diagram showing liquid water changing to water vapor has the same H2O molecules in both images, just spaced further apart, so this is physical. If new molecules or formula units are present that did not exist before the change (atoms rearranged into new combinations), the change is chemical. For example, a diagram showing H2 and O2 molecules reacting to form H2O molecules has new H2O molecules that did not exist before, so this is chemical.

Worked Example

Problem: 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 present, which are new chemical species that did not exist before.
3. The A-B bonds in the original AB molecules were broken, and new A-A and B-B bonds were formed to make the new molecules.
4. Conclusion: this is a chemical change, because new substances were formed by breaking and re-forming intramolecular bonds.

Exam tip: When interpreting a diagram of dissolving solid NaCl, remember that separating Na+ and Cl- ions from the crystal lattice is still a physical change—no new chemical substances are formed, the ions are just hydrated by water instead of packed in a solid.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Classifying the dissolving of NaCl in water as a chemical change because ions separate and are hydrated. Why: 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 move: Always check if new chemical compounds are formed; if only ions are separated and hydrated, it is a physical change.
• Wrong move: Claiming a change is physical because it is reversible, and chemical because it is irreversible. Why: Students memorize an oversimplified rule from introductory chemistry that is not universally true. Correct move: Always use the "new substance / intramolecular bond change" rule to classify, never rely on reversibility.
• Wrong move: Claiming conservation of mass is violated in an open beaker when a reaction produces gas that escapes. Why: Students forget the law of conservation of mass applies to closed systems that do not exchange matter with the surroundings. Correct move: 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 move: Classifying the boiling of liquid hydrogen peroxide (H2O2) as a physical change, assuming all phase changes are physical. Why: Students assume all phase changes are physical without considering if decomposition occurs at boiling temperature. Correct move: Always check if the substance undergoes a chemical reaction at the conditions of the change before classifying.
• Wrong move: Interpreting melting of solid molecular iodine (I2) as a chemical change because I2 molecules are separated. Why: Students confuse separation of molecules (breaking intermolecular forces) with separation of atoms within a molecule (breaking intramolecular bonds). Correct move: Check if the same molecules exist before and after; if I2 molecules are still intact after melting, the change is physical.

6. Practice Questions (AP Chemistry Style)

Question 1 (Multiple Choice)

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

Worked Solution: We apply the rule that a chemical change requires formation of new chemical substances via breaking and forming intramolecular bonds. Option A: sublimation is a phase change that only breaks intermolecular forces between CO2 molecules, so it is physical. Option B: distillation separates two liquids based on boiling point, no new substances are formed, so it is physical. Option C: potassium chlorate decomposes to form two new substances (potassium chloride and oxygen gas) that did not exist before, so this is a chemical change. Option D: dissolving KNO3 is a physical change that only rearranges ions, no new substances form. The correct answer is C.

Question 2 (Free Response)

A student performs two experiments in an open lab beaker placed on a digital balance, as described below. Experiment 1: 15.0 g of solid potassium bromide is added to 75.0 g of room temperature water. The potassium bromide dissolves completely, and the student records the total mass of the beaker and contents immediately after dissolution. Experiment 2: 15.0 g of solid magnesium carbonate is added to 75.0 g of 1.5 M nitric acid. The reaction produces aqueous magnesium nitrate, liquid water, and carbon dioxide gas. The student waits until bubbling stops, then records the total mass of the beaker and contents.

(a) Classify the change that occurs in Experiment 1 as physical or chemical, and justify your answer. (b) The mass of the empty beaker is 120.0 g for both experiments. Calculate the expected mass recorded by the student for Experiment 1 immediately after dissolution. Show your work. (c) Explain why the total mass recorded in Experiment 2 after the reaction is less than the total mass of the starting materials and the beaker.

Worked Solution: (a) The change in Experiment 1 is a physical change. When potassium bromide dissolves in water, only intermolecular forces between solid KBr formula units are broken, and new intermolecular forces form between K+ ions, Br- ions, and water molecules. No new chemical substances are formed, so the change is physical. (b) The measurement is taken immediately after dissolution, so no water has evaporated to the surroundings, meaning the system behaves as a closed system for this measurement. Total mass is the sum of all components: $$\text{Total Mass}=120.0\text{g}+15.0\text{g}+75.0\text{g}=210.0\text{g}$$ The expected mass is $\boxed{210.0\text{g}}$. (c) The reaction produces carbon dioxide gas, which escapes to the surroundings in the open beaker. Conservation of mass holds for the entire system (beaker + surroundings), but the open beaker only contains the liquid and solid products, so the measured mass is lower by the mass of the escaped CO2 gas.

Question 3 (Application / Real-World Style)

A baker bakes bread in a closed oven. The total mass of the raw dough and baking pan is 1.15 kg before baking. After baking, the cooked bread and pan has a mass of 1.08 kg. Baking causes yeast to ferment residual sugars into ethanol and carbon dioxide, which leavens the bread. Most of the carbon dioxide escapes the bread into the oven air during baking. Assuming all mass lost is from escaped CO2, calculate the mass of CO2 produced and escaped during baking. Is fermentation a physical or chemical change? Justify your answer.

Worked Solution: The change in total mass equals the mass of CO2 that escaped the bread/pan system into the oven surroundings. Calculate the mass difference: $${\text{Mass of CO}}_2=\text{Initial mass}-\text{Final mass}=1.15\text{kg}-1.08\text{kg}=0.07\text{kg}=70\text{g}$$ Fermentation is a chemical change: yeast converts sugar (sucrose) into new chemical substances (ethanol and carbon dioxide) by breaking intramolecular bonds in sucrose and forming new bonds in the products. In context, 70 grams of carbon dioxide escaped during baking, which is enough to create the air pockets that give bread its light texture.

7. Quick Reference Cheatsheet

8. 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. Next, you will 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, a core distinction you learned here. Without mastering the difference between physical and chemical changes, you will struggle to interpret particulate representations of reactions, a high-frequency question type on the AP exam. This topic also underpins all separation techniques used in chemistry, which are tested in lab-based FRQ questions. Balancing chemical equations Introduction to chemical reactions Net ionic equations Intermolecular forces