Introduction for Reactions
Separate observations from evidence that a new chemical species has formed.
AP Chemistry Topic 4.1 · CED printed p. 78 / PDF p. 85. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
Imagine watching ice melt. The sample looks very different, but each water molecule is still H2O. A physical change changes arrangement, phase, or mixture composition without creating a different substance. Filtering a mixture separates existing components; evaporating water leaves the dissolved solute behind. Ask what species were present before and after, rather than whether the change was dramatic or reversible.
A chemical change produces different chemical species by rearranging atoms and electrons. A new solid, gas, color, heat, or light can be a useful clue, but a clue is not a verdict. Boiling produces bubbles of the same substance; mixing two colored solutions can change color without reaction; dissolving can warm or cool a sample. A convincing claim connects an observation with an independent identity check, such as showing that a gas has properties different from water vapor. In an open vessel, escaped gas can lower the measured mass even though atoms are conserved across vessel plus surroundings.
Let's work through it
A clear liquid forms bubbles on warming. Has a reaction occurred? Let’s slow down before naming it.
- List two models: vaporization of the liquid, and formation of a new gaseous substance. Both predict bubbles.
- Collect identity evidence from supplied observations: the bubbles condense back to the original liquid on cooling and its composition is unchanged. This supports a physical phase change.
- A different result—gas with a different measured composition—would support chemical change. Bubble formation alone does not distinguish the models.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
A salt solution becomes more concentrated when water evaporates. What species changed identity?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
A salt solution becomes more concentrated when water evaporates. What species changed identity?
Worked answer and reason
None need change identity. Water molecules leave the liquid phase and dissolved solute remains; concentration changes because solvent volume changes.
Species identity and phase must be tracked separately.
Retrieval 2
Why is a temperature rise only possible evidence of reaction?
Worked answer and reason
Both chemical transformations and physical mixing/dissolution can transfer energy. Show that a new species formed before concluding a reaction.
A single observation can fit competing explanations.
Catch the tempting mistake
- Any bubbles prove a chemical reaction.
- Boiling also forms bubbles. Identify the gas or compare competing particle models.
- A decrease in vessel mass means mass was destroyed.
- Include any escaped gas in the system boundary; its atoms carry the missing mass.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 1 · ap application · macroscopic
- Unit 4 problem 2 · ap application · multiple
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For the teacher
- Ask for an observation and an inference in separate sentences.
- If students cite reversibility, use ice melting and a recoverable precipitate to challenge it.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.1.A
- Identify evidence of chemical and physical changes in matter.
- 4.1.A.1
- A physical change occurs when a substance undergoes a change in properties but not a change in composition. Changes in the phase of a substance (solid, liquid, gas) or formation/separation of mixtures of substances are common physical changes.
- 4.1.A.2
- A chemical change occurs when substances are transformed into new substances, typically with different compositions. Production of heat or light, formation of a gas, formation of a precipitate, and/or color change provide possible evidence that a chemical change has occurred.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Writing and Balancing Chemical Equations — Scientific check: conserved atoms/charge and complete/net ionic conventions.
- OpenStax Chemistry 2e: Classifying Chemical Reactions — Scientific check: precipitation, oxidation-number conventions, proton transfer; AP exclusions applied separately.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
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Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
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Filtration
Filtration separates suspended particles from a fluid using a barrier that retains appropriate particle sizes. It generally does not remove individually dissolved ions or molecules. For example, a filter can separate an undissolved precipitate from the surrounding solution.