AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-1)
Unit 4 · Topic 4.1
4.1 Introduction for Reactions
A physical change alters a substance's form or phase but not what it's made of; a chemical change makes new substances. Gas forming, a precipitate appearing, a color change, or heat or light being given off are possible signs of a chemical change, but none of them proves it on its own.
Key terms
- physical change
- chemical change
- precipitate
- evidence of a reaction
Physical changes
In a physical change, a substance's properties change, but its composition doesn't. The particles at the end are the same particles you started with.
Changes of phase are the most common physical changes: melting ice, boiling water, freezing wax, dry ice (solid CO₂) turning into gas. Making or separating mixtures is also physical: stirring sugar into tea, filtering sand out of water, or distilling ethanol from a mixture. Cutting, grinding and bending are physical too.
Chemical changes
In a chemical change, atoms rearrange into new combinations, making substances with different compositions and properties. Iron rusting, wood burning, baking soda fizzing in vinegar and food digesting are all chemical changes.
You can't see atoms rearranging, so in the lab you look for evidence.
- A gas forms that wasn't there before, such as bubbles when a metal is added to acid.
- A precipitate forms: an insoluble solid appears when two clear solutions are mixed.
- The color changes, such as a solution turning from purple to colorless.
- Heat or light is given off (or heat is absorbed), such as a flame or a solution warming up.
Evidence isn't proof
Each of those clues can also come from a physical change. Bubbles form when water boils, but no new substance is made. A solution's color fades when you dilute it with water. Dissolving some salts makes a solution noticeably colder or warmer. So a single observation suggests a chemical change; it doesn't prove one.
The real test is whether a new substance with a different composition has formed. Strong evidence combines observations, or shows a property the starting materials didn't have: a gas that pops when lit, or a solid that wasn't present in either solution.
Don't rely on 'can it be reversed?' either. Many physical changes are easy to reverse (freezing water you just melted), but some chemical changes can be reversed too, and some physical changes, like mixing two gases, are hard to undo in practice. Focus on whether the composition changed.
Making predictions
You may be asked to predict what you'd observe if a reaction happens. Use the products. If a reaction makes a gas, predict bubbling. If it makes an insoluble solid, predict a cloudy precipitate. If a colored ion is used up, predict the color fading. Topic 4.4 explains the same distinction at the particle level, in terms of bonds versus intermolecular forces.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying observations
Classify each as a physical or chemical change and name the evidence: (a) clear solutions of AgNO₃ and NaCl are mixed and a white solid forms; (b) water is heated and bubbles form; (c) a magnesium strip burns with a bright white light, leaving a white powder.
Show the solutionHide the solution
- Step 1: (a) A solid forms that wasn't in either clear solution. It's AgCl, a new substance. Chemical change; evidence: precipitate.
- Step 2: (b) The bubbles are water vapor, still H₂O. No new substance forms. Physical change (boiling).
- Step 3: (c) Light is given off, and the white powder (MgO) has different properties from the shiny metal. Chemical change; evidence: light, heat and a new solid.
Answer: (a) chemical (precipitate); (b) physical (phase change); (c) chemical (light and a new substance)
- Example 2
A temperature change isn't enough (classic trap)
Solid ammonium nitrate dissolves in water and the solution gets much colder. A student concludes a chemical reaction took place. Is that justified?
Show the solutionHide the solution
- Step 1: A temperature change is possible evidence, but it can come from physical processes too.
- Step 2: When NH₄NO₃ dissolves, its ions separate and are surrounded by water: NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq). The NH₄⁺ and NO₃⁻ ions are the same ions that were in the solid.
- Step 3: No new substance forms, so dissolving is usually classified as physical, even though energy is absorbed.
- Step 4: To argue for a chemical change, you'd need evidence of a new substance.
Answer: No. The temperature change alone doesn't show a new substance formed; dissolving NH₄NO₃ just separates its existing ions.
Common mistakes
- Treating any bubbles as evidence of a chemical reaction. Boiling makes bubbles too.
- Calling dissolving sugar a chemical change. The sugar molecules stay intact.
- Assuming that a color change always means a new substance. Dilution or mixing two colored solutions changes color without a reaction.
- Forgetting that heat being absorbed (getting colder) is also possible evidence, not just heat being released.
On the exam
- Lab questions may ask what you'd observe when solutions are mixed, or what evidence supports a reaction. Name the specific observation (precipitate, gas, color, temperature) and connect it to a new substance.
- If asked whether an observation proves a chemical change, consider whether a physical process could cause the same thing.
Connected topics
Videos
Check yourself
5 questions on 4.1 Introduction for Reactions. Pick an answer to see if you got it, and why.
Which of the following observations provides the strongest evidence that a chemical reaction has occurred?
Boiling water produces a gas that condenses back into liquid water. Passing an electric current through water produces two gases: one burns with a pop, and the other makes a glowing splint burst into flame. Which of the following is the best evidence that the second process is a chemical change?
When solid ammonium nitrate, NH₄NO₃, dissolves in water, the solution becomes very cold. A student claims that a chemical reaction must have occurred, because the temperature changed. Which of the following best evaluates this claim?
| Trial | Solution 1 | Solution 2 | Observation |
|---|---|---|---|
| 1 | Na₂CO₃(aq) | HCl(aq) | Colorless gas bubbles form |
| 2 | NaCl(aq) | KNO₃(aq) | No visible change |
| 3 | Pb(NO₃)₂(aq) | KI(aq) | Bright yellow solid forms |
| 4 | NaOH(aq) | HCl(aq) | No visible change; the mixture gets warmer |
Hypothetical observations. A student mixes equal volumes of two 0.1 M solutions in each trial. Lead(II) iodide is insoluble in water.
In which trials is there evidence that a chemical reaction occurred?
Which gas is most likely produced in trial 1?
0 of 5 answered