ChemGem

How to Tell Physical and Chemical Changes Apart (Step-by-Step, with Practice)

A physical change alters form; a chemical change makes new substances. Heat is energy that flows from hotter to colder, changing how fast particles move. Scientists test ideas against evidence in a repeating cycle.

Spot it, solve it, practice it: 3 steps, 1 worked example, 6 practice problems. Free, no account.

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The route · 3 steps

  1. Ask: is it still the same substance afterward?
  2. Look for chemical-change clues: new gas, color change, solid forming, light or heat, new odor.
  3. Same identity: physical. New substance: chemical.
Try one now See a worked example
Spot it

How to recognize physical vs. chemical change questions

Look for "physical or chemical" change or property, heat and particle motion, or labeling a statement as observation, hypothesis, law, or theory.

Example wording, with the clue words marked:

Is boiling point a physical or a chemical property?

Not sure it’s this type? Compare the neighbors:

Step-by-step

Worked example: physical vs. chemical change

The idea

Physical or chemical change?
Physical change
Form or state changes; identity does not.
Melting ice, boiling water, cutting paper, dissolving sugar.
Chemical change
New substances with new properties form. Atoms rearrange into new particles.
Burning, rusting, cooking.
Clues for a chemical change
  • Bubbles of a new gas
  • A color change
  • A solid forming when two solutions are mixed
  • Light or heat given off
  • A new odor
Physical or chemical property?
Physical property
Observed without changing the substance.
Color, density, melting point, boiling point.
Chemical property
How a substance reacts.
Flammability, reactivity with acid, tendency to rust.
One change, three levels
MacroscopicWhat you seeParticleWhat particles doSymbolicHow we write itice meltsmolecules slide apartH₂O(s) → H₂O(l)
Chemists describe every change at all three levels.
Heat and particle motion
heatHotter: faster particlesColder: slower particles
Particles are always moving. Temperature measures their average kinetic energy (energy of motion): hotter means faster on average. Heat is energy flowing from hotter to colder.
  • Heat flowing in: particles speed up, or the substance changes state.
  • Heat flowing out: particles slow down.
  • Chapter 7 shows how to calculate heat.
Scientific methods: a cycle, not a fixed list
ObservationHypothesisExperimentAnalysisnot supported?revise and test again
Scientists test ideas against evidence and repeat as needed.
Words in the cycle
TermMeaning
ObservationNoticing something. Qualitative describes (“the solution turned blue”). Quantitative measures, with a number and unit (“the mass was 2.50 g”).
HypothesisA testable explanation or prediction
ExperimentA controlled test that changes one variable (one factor, such as temperature) at a time
AnalysisDo the data support the hypothesis? If not, revise it and test again.
Law or theory?
Scientific law
Describes what happens, often as an equation.
Mass is conserved in a chemical reaction.
Theory
Explains why it happens; supported by many experiments.
Atomic theory.

Worked example · 4 steps

Physical or chemical change? Ice melting, iron rusting, baking soda fizzing in vinegar, and salt dissolving in water.

  1. 1
    Ice melting

    It is still H2O; only the state changes.

    ClueH2O(s) → H2O(l): same substance
    ChangePhysical
  2. 2
    Iron rusting

    Iron becomes a new substance, iron oxide, with a new color.

    ClueNew substance, new color
    ChangeChemical
  3. 3
    Baking soda in vinegar

    Bubbles of a new gas, CO2, form.

    ClueBubbles of a new gas
    ChangeChemical
  4. 4
    Salt dissolving in water

    The salt is still there; evaporating the water gets it back.

    ClueSalt recovered by evaporation
    ChangePhysical
  5. ✓
    Answer

    Physical, chemical, chemical, physical

Your turn

Physical vs. chemical change practice problems

Answer first, then check. Hints open one at a time; the answer and full solution open after you try.

Problem 1 of 6 · warm-up

Is boiling point a physical or a chemical property?

Hint 1 · the clue

Does measuring it turn the substance into something new?

Hint 2 · the next move

"Physical or a chemical property": ask whether measuring it makes a new substance.

Your path:

  1. Ask: does measuring make something new?
  2. Boiling water is still H2O
  3. No new substance → physical
Answer

Answer: Physical

Full worked solution

What you know: Boiling point is a property. What you want: physical or chemical.

  1. Ask: does measuring it turn the substance into something new?
  2. When water boils, liquid water becomes water vapor. It is still H2O, only in a different state.
  3. A property you can measure without making a new substance is physical.

Why it matters: A chemical property, like flammability, can only be seen when the substance changes into something new.

Watch out: Bubbles in boiling water are water vapor, not a new gas. A change of state is a physical change.

Answer: Physical

Problem 2 of 6

Wood burning: physical or chemical change?

Hint 1 · the clue

Look for new substances, heat, and light.

Hint 2 · the next move

"Physical or chemical change": look for signs of a new substance like heat, light, smoke, or new smells.

Your path:

  1. Check for chemical-change clues
  2. Heat, light, smoke, ash appear
  3. New substances → chemical change
Answer

Answer: Chemical

Full worked solution

What you know: Wood is burning. What you want: physical or chemical change.

  1. Look for the clues of a chemical change: a new gas, a color change, light or heat given off, a new odor.
  2. Burning wood gives off heat and light, smoke and a new smell. It leaves ash, which is not wood.
  3. New substances form (carbon dioxide, water vapor and ash). That makes it a chemical change.

Why it matters: You cannot turn the ash and gases back into wood. Physical changes, like melting ice, are usually easy to reverse.

Answer: Chemical

Problem 3 of 6

A cup of hot cocoa cools on the table. What happens to the motion of its particles?

Hint 1 · the clue

Which way does heat flow: into the cocoa or out of it?

Hint 2 · the next move

Something "cools" and you're asked about "motion of its particles": link temperature to average particle motion.

Your path:

  1. Heat flows from hot cocoa outward
  2. Temperature tracks average particle motion
  3. Lower temperature → slower particles
Answer

Answer: They slow down on average.

Full worked solution

What you know: Hot cocoa cools on a table. What you want: what happens to its particles' motion.

  1. Heat flows from hotter to cooler. The cocoa is hotter than the room, so heat flows out of the cocoa.
  2. Temperature measures the average kinetic energy, the energy of motion, of the particles.
  3. A falling temperature means lower average kinetic energy. So the particles slow down on average.

Why it matters: Particles never stop moving. Cooling only lowers their average speed.

Watch out: "On average" matters. Some particles still move fast; the average speed drops.

Answer: They slow down on average.

Problem 4 of 6

“If I add salt, the water will boil at a higher temperature.” Is this an observation, a hypothesis, or a law?

Hint 1 · the clue

Has it been tested yet, or is it a prediction?

Hint 2 · the next move

A statement predicting what "will" happen ("If I add salt…") that is not yet tested: a testable prediction is a hypothesis.

Your path:

  1. Ask: has it been tested yet?
  2. It predicts what will happen
  3. Testable prediction → hypothesis
Answer

Answer: A hypothesis

Full worked solution

What you know: The statement "If I add salt, the water will boil at a higher temperature." What you want: observation, hypothesis, or law.

  1. Ask: has it been tested yet? No. It says what will happen.
  2. An observation records what you saw. A law describes a pattern confirmed many times. A hypothesis is a testable explanation or prediction.
  3. This is a testable prediction, so it is a hypothesis.

Why it matters: The next step is an experiment. If the data do not support it, revise the hypothesis and test again.

Watch out: Wording like "If I do this, that will happen" predicts an untested result. That is a strong clue for a hypothesis.

Answer: A hypothesis

Problem 5 of 6

Which explains why something happens: a scientific law or a theory?

Hint 1 · the clue

One describes; the other explains.

Hint 2 · the next move

"Explains why" signals the difference between a law (describes what) and a theory (explains why).

Your path:

  1. Law describes what happens
  2. Theory explains why it happens
  3. Pick the one that explains
Answer

Answer: A theory

Full worked solution

What you know: Two kinds of scientific statement, laws and theories. What you want: the one that explains why.

  1. A scientific law describes what happens, often as a pattern or equation.
  2. A theory explains why it happens, and it is supported by many tests.
  3. So the one that explains is a theory.

Why it matters: In science, "theory" does not mean a guess. It is a well-tested explanation.

Watch out: A theory does not "grow up" into a law. They do different jobs: one explains, the other describes.

Answer: A theory

Problem 6 of 6

A silver spoon darkens (tarnishes) in air. Is this a physical or a chemical change, and what clue tells you?

Hint 1 · the clue

Is the dark coating still silver?

Hint 2 · the next move

"Physical or a chemical change" plus a color change ("darkens"): a color change is a clue a new substance formed.

Your path:

  1. Ask: is the dark coating still silver?
  2. Silver reacts to make silver sulfide
  3. Color change, new substance → chemical
Answer

Answer: Chemical; the dark color is a clue that a new substance, silver sulfide, formed.

Full worked solution

What you know: A silver spoon darkens in air. What you want: physical or chemical change, and the clue.

  1. Ask: is the dark coating still silver? No. Silver reacts with sulfur compounds in the air to make silver sulfide, a black solid.
  2. A color change is one of the clues for a chemical change, because it often signals a new substance.
  3. A new substance formed, so the answer is chemical; the dark color is a clue that a new substance, silver sulfide, formed.

Watch out: Not every color change is chemical. Here the black coating has a different formula from silver, so a new substance really formed.

Answer: Chemical; the dark color is a clue that a new substance, silver sulfide, formed.

Watch out

Common physical vs. chemical change mistakes