From what you see to what happened
Lesson 1.2.3 separated physical from chemical changes by whether a new substance forms. AP Unit 4 starts there but asks for more care: what you observe is only possible evidence, and the real test happens at the particle level.
Two kinds of change
- A physical change物理变化 changes a substance’s properties (its state, its shape, whether it is mixed with something) but not its composition. Changes of state, dissolving to form a solution, and separating mixtures (filtration, distillation, chromatography, U3.9) are common physical changes.
- A chemical change化学变化 rearranges atoms into new substances with different compositions: a chemical reaction化学反应.
At the particle level, each kind of particle (molecule, ion, or atom) is the same before and after a physical change; only the spacing, arrangement, or mixing changes. After a chemical change, particles appear that weren’t there before.
Possible evidence, and why it isn’t enough
Chemical changes often come with the observations below, but each of them can also happen in a physical change:
| Observation | A chemical change | But also a physical change |
|---|---|---|
| A gas forms (bubbles) | Magnesium in hydrochloric acid forms H₂ | Boiling water: the bubbles are water vapor |
| A precipitate forms | Mixing AgNO₃ and NaCl solutions forms AgCl | Cooling a saturated solution crystallizes solute (U3.10) |
| Color change | Heated copper turns black as CuO forms | Adding food coloring to water |
| Temperature change | A fuel burning releases heat | NH₄NO₃ dissolving makes the solution cold |
| Light | Magnesium burning gives a white flash | A light bulb glowing |
The reverse is also true: some chemical changes show no obvious sign. Mixing dilute hydrochloric acid with dilute NaOH solution forms water and NaCl, yet the solution looks unchanged; only its temperature rises slightly. So observations are clues, and the conclusion has to rest on whether new substances form.
Not evidence: two colorless solutions stay colorless when mixed; the total volume equals the sum of the two volumes; the mixture conducts electricity (the original solutions already did). None of these shows that a reaction occurred, or that one didn’t.
Decide first, then check your answer with the particle diagrams:
Boiling water
A pot of water is heated, and lots of bubbles form in it.
The bubbles are water vapor. The water molecules themselves are unchanged, just far apart: a physical change. Bubbles do not always mean a chemical change.
Electrolysis of water
An electric current is passed through water. Bubbles form at both electrodes, and the two gases collect in a volume ratio of about 2 : 1.
Water molecules are broken apart and the atoms recombine into H₂ and O₂ molecules, new substances: a chemical change.
Burning natural gas
A gas stove is lit, and its blue flame gives off a lot of heat and light.
The atoms of CH₄ and O₂ rearrange into CO₂ and H₂O: a chemical change.
Mixing two solutions
Colorless AgNO₃ solution is poured into colorless NaCl solution, and a white precipitate forms at once.
Ag⁺ and Cl⁻ combine into a new solid, AgCl, while Na⁺ and NO₃⁻ are unchanged: a chemical change.
Dissolving sugar
A spoonful of sugar is stirred into water. The sugar disappears, and the water tastes sweet.
The sucrose molecules simply spread out among the water molecules, unchanged, forming a mixture: a physical change.
A cold pack
Squeezing a cold pack lets ammonium nitrate dissolve in water inside it, and the pack quickly gets cold.
As NH₄NO₃ dissolves, NH₄⁺ and NO₃⁻ spread through the water; no new substance forms: a physical change (dissolving). A temperature change does not always mean a chemical change.
“Smoking” dry ice
Dry ice left on a table is surrounded by white fog and gets smaller and smaller.
CO₂ molecules go straight from solid to gas (the fog is water vapor from the air condensing into tiny droplets in the cold); the CO₂ molecules are unchanged: a physical change.
Magnesium in hydrochloric acid
A magnesium ribbon fizzes with bubbles, the test tube gets warm, and the ribbon gradually disappears.
Mg atoms lose electrons to become Mg²⁺, and H⁺ gains electrons to become H₂. New substances form: a chemical change.
Adding food coloring
A few drops of red food coloring are added to a glass of water, and all the water turns red.
No new substance forms, so there is no chemical equation to write.
The dye molecules just spread through the water, forming a mixture with no new substance: a physical change. A color change does not always mean a chemical change.
Heating copper
A shiny, reddish copper strip is heated in air, and its surface turns black.
Cu reacts with O₂ to form black CuO: a chemical change.
Physical changes have equations too
Equations aren’t only for chemical reactions. Changes of state and dissolving can also be written as balanced equations, with the same substance (or the ions it separates into) on both sides:
The second equation shows NaCl dissolving in water, a physical change. The next lesson shows how to write ionic equations for reactions in solution.
Testing with mass and recovery
When observations aren’t enough, measure mass or try to recover the substance:
- Mass gain: a magnesium ribbon heated in air gets heavier; the extra mass is oxygen that combined with the magnesium to form MgO, so a chemical change occurred.
- Recovering the substance: dissolving a crystal in water, then evaporating the water to get back the same mass of crystals with the same appearance and properties, shows that no new substance formed: dissolving was a physical change.
- Mass is conserved in a sealed container: in any physical or chemical change, the total mass in a sealed container stays the same, even when solid iodine turns completely into vapor.
So when an experiment gives only ambiguous evidence, such as a temperature change, the best follow-up test is one that tells “still the same substance” from “a new substance”: evaporating the solution to recover the solid and compare its properties, or testing for a gas that wasn’t there before.
Example 1: Deciding from particle diagrams
Box A starts with 4 N₂ molecules and 6 H₂ molecules; afterward it holds 2 N₂ molecules and 4 NH₃ molecules. Box B starts with 6 closely packed H₂O molecules; afterward the 6 H₂O molecules are spread throughout the box. Decide whether each process is a physical or a chemical change, and justify your answers.
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Identify: the test. A chemical change forms new particles (atoms rearrange); in a physical change the particles stay the same and only their arrangement, spacing, or motion changes.
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Box A: compare the particles. Before, only N₂ and H₂; afterward, NH₃ molecules appear that weren’t there before: the atoms have rearranged.
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Box A: check the atoms.
The atoms are conserved, just recombined: a chemical change.
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Box B: compare the particles. There are 6 H₂O molecules before and after; they’ve only gone from packed together to far apart. Liquid water became water vapor and the molecules themselves are unchanged: a physical change.
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State the basis. The conclusion rests on whether particles turned into new particles, not on whether a gas appeared or the temperature changed. Box B also produced a gas, and it is still a physical change.
Example 2: One substance, two changes
Sugar is heated slowly in a pan. It first melts into a colorless liquid; with more heating, the liquid turns brown-black and gives off water vapor. Identify the kind of change in each stage.
- Identify: judge each stage. One experiment can involve a physical change and then a chemical change, so ask for each stage: did the sucrose molecules change?
- First stage: melting. Solid sucrose becomes liquid sucrose; only the arrangement of the molecules changes, not the molecules themselves: a physical change. On cooling it solidifies back into sucrose.
- Second stage: browning and steam. A brown-black, carbon-rich substance forms and water is released, so the sucrose molecules have broken down and their atoms rearranged into new substances with different compositions: a chemical change. Cooling won’t turn it back into sugar.
- Separate clues from the basis. The color change and the water vapor are clues; the basis for the decision is that new substances with different compositions form.
- Iodine turning into a violet vapor when heated
The I₂ molecules are unchanged; they just go from solid to gas.
- An iron nail rusting in damp air
A new substance, rust, forms.
- Dissolving table salt in water
Na⁺ and Cl⁻ spread through the water; no new substance forms.
- Hydrogen peroxide fizzing with MnO₂
H₂O₂ decomposes into H₂O and O₂.
- Frost forming on a window in winter
Water vapor turns directly into solid ice.
- Frying an egg
The proteins change irreversibly into new substances.
- Distilling fairly pure ethanol from an ethanol–water mixture
Separating a mixture; the ethanol molecules are unchanged.
- Breathing into limewater, which turns cloudy
CO₂ reacts with Ca(OH)₂ to form a CaCO₃ precipitate.
- Mixing hydrochloric acid and NaOH solution, with no visible change
H⁺ and OH⁻ combine into H₂O, a new substance. No visible change doesn't mean no reaction.
- Crystals forming as a saturated KNO₃ solution cools
KNO₃ that no longer fits crystallizes out unchanged.
- Hammering a copper block into a thin sheet
Only the shape changes; it's still copper, with no new substance.
- A magnesium ribbon heated in air weighs more after cooling
Magnesium combined with O₂ to form MgO.
Common mistakes
Mistake 1: “Bubbles, a color change, or a temperature change always mean a chemical change.” These are only possible evidence. Boiling water bubbles, dye changes water’s color, and dissolving NH₄NO₃ gets cold, yet all are physical changes. Decide by whether new substances form.
Mistake 2: “If nothing visible happens, no chemical change occurred.” Dilute hydrochloric acid and dilute NaOH solution react to form water, yet the solution looks the same. Whether a reaction happened is decided at the particle level, with other measurements (temperature, pH) if needed.
Mistake 3: “Sugar ‘disappears’ in water, so dissolving is a chemical change.” The sucrose molecules are still there, just spread among the water molecules; evaporating the water gives the sugar back. Dissolving forms a mixture, a physical change.
Quick check
AgNO₃ solution and NaCl solution are mixed, and a white precipitate forms. Which statement is best?
A substance that wasn't present before, AgCl, formed, so the change is chemical. B describes crystallization on cooling, where the solid is the same substance that was already dissolved.
A student says, “Water boiling makes bubbles, so a new substance forms.” Which rebuttal is best?
Boiling only overcomes the attractions between water molecules; the H₂O molecules stay intact. A is a common error; neither B nor C says anything about whether a new substance formed.
Which observation is the strongest evidence that a chemical change occurred?
Detecting a new substance (CO₂) that wasn't there before is the most direct evidence. A is a change of state; C may just be dilution; D could come from dissolving.
A packet of NH₄NO₃ is added to water. The solid dissolves, and the temperature drops from 22 °C to 12 °C. What kind of change is this, and why?
Dissolving forms no new substance. A and C treat an observation as the conclusion; D has the right answer for the wrong reason, since a temperature change doesn't show whether a reaction occurred.
Which process does represent?
The same substance is on both sides, with only a change of state: a solid turning directly into a gas is sublimation. In C the gas was dissolved in the water, not a solid.
Which set contains only physical changes?
No new substance forms in any process in A. Each other set includes a chemical change: rusting, burning, rotting, or electrolysis.
Two solutions are mixed. Which observation is evidence that a chemical reaction occurred?
A precipitate shows that a new substance formed. B, C, and D would happen even with no reaction: the solutions were colorless already, volumes simply add, and the original solutions already conducted.
A magnesium ribbon is heated in a crucible and weighed after cooling; it now weighs more than before. Which explanation is best?
The extra mass is oxygen combined with the magnesium. Heat has no mass, and changes of volume or state don't change mass.
A student dissolves a white solid, X, in water, and the solution gets warm. Which follow-up test would best show whether a chemical change occurred?
Recovering the same mass of X with the same properties means it only dissolved; getting a different substance means a reaction occurred. None of the other tests can tell these apart.
A little iodine is sealed in a glass container, and the total mass is m. The container is heated until all the iodine sublimes and then weighed again. What is the total mass?
Conservation of mass holds for physical and chemical changes alike. Gases have mass, and a larger volume doesn't change the mass.
AP-style free response
A student carries out three experiments:
- Experiment 1: A magnesium ribbon is placed in dilute hydrochloric acid. Bubbles form on the ribbon, the test tube gets warm, and the ribbon gradually disappears.
- Experiment 2: A beaker of water is heated, and bubbles form in it.
- Experiment 3: KNO₃ crystals are stirred into water. The crystals disappear, and the solution gets cold.
(a) Experiments 1 and 2 both produce bubbles. Identify the gas in the bubbles in each experiment, and classify each experiment as a physical or a chemical change.
(b) Write the net ionic equation for Experiment 1. (Mg is a solid; hydrochloric acid contains H⁺ and Cl⁻.)
(c) A student claims, “The solution in Experiment 3 got cold, so a chemical reaction occurred.” Evaluate the claim, and use a particle-level description to identify the kind of change in Experiment 3.
(d) Draw (or describe) how the particles in the bubbles of Experiment 2 differ from those in the liquid water.
Model answer and scoring
(a) Experiment 1: the gas is H₂, a new substance, so it’s a chemical change. Experiment 2: the gas is water vapor (H₂O); the H₂O molecules are unchanged, so it’s a physical change.
(b)
(c) The claim is wrong. A temperature change is only possible evidence. As KNO₃ dissolves, K⁺ and NO₃⁻ leave the crystal and spread out among the water molecules; no new substance forms, so it’s a physical change (dissolving).
(d) Both contain intact H₂O molecules. In the liquid, the molecules are close together and attract each other; in the bubbles, the water molecules are far apart. There are no H₂ or O₂ molecules in the bubbles.
Scoring: (a) 1 point for each gas and 1 for each classification; (b) correct equation without Cl⁻; (c) states that a temperature change isn’t proof, and explains that the ions spread out and no new substance forms; (d) both are H₂O molecules, differing only in spacing.
Summary
- Physical change: composition stays the same (changes of state, dissolving, separating mixtures). Chemical change: atoms rearrange into new substances with different compositions.
- A gas or precipitate forming, a color change, a temperature change, or light is only possible evidence: physical changes can show these too, and some chemical changes show almost nothing.
- Decide at the particle level: do new particles appear after the change?
- Physical changes can be written as equations too, such as .
AP tip: When asked whether a change is physical or chemical, give a particle-level reason: “a new substance, X, forms” or “the X molecules (ions) are unchanged; they only …”. “It bubbled” or “the color changed” won’t earn the reasoning point.