Ages 10–13 · Fifth Class to Second Year · Science · 3 minutes

Chemical vs Physical Changes Explained

Why can you refreeze melted ice, but not unburn toast? And what turned the Statue of Liberty green? Learn the new-substance test, avoid the reversibility trap, and try a boiling-water question.

Key terms

Physical change
A change in form in which the substances stay the same.
Chemical change
A change in which new substances form.
Water molecule
A particle of water containing two hydrogen atoms and one oxygen atom.
Patina
The green surface layer of new substances containing copper that formed on the Statue of Liberty.

Transcript

0:00

This green statue used to be brown. Its green surface is not paint. So what changed? To find out, compare two things in your kitchen: melting ice and burned toast.

0:13

Start with an ice cube. Warm it, and you get a puddle. It looks completely different, but it is still water. Zoom in. Each tiny water molecule still has two hydrogen atoms and one oxygen atom. The molecules can move around each other now. They have not become a new substance. That is a physical change: a new form, the same substance. Freeze the water, and you can get ice again.

0:39

Imagine your toast has burned. You get blackened material, smoke, and gases. New substances have formed. Cooling it will not bring your breakfast back. Sorry, toast. Burning is a chemical change. Atoms join in new ways, making substances that behave differently. The important question is not just, does it look different? It is, did new substances form?

1:06

Here is the trap: you might think physical means easy to undo. Tear a sheet of paper into tiny pieces. Putting it back together is a nightmare, but the pieces are still paper. Tearing changes size and shape, not the substances in the paper. So, hard to undo does not automatically mean chemical.

1:28

Back to the Statue of Liberty. Its outer skin is copper. Over years, copper at the surface reacted with air and moisture. New substances containing copper formed a green layer called a patina. This is a chemical change, even though the statue kept its shape. And here is the surprise: that green layer helps protect the copper underneath.

1:52

Your turn. Water is boiling and bubbles appear. Chemical change? No. Those bubbles contain water as a gas. No new substance. Bubbles, a new smell, or a change in appearance can be clues. But clues are not proof by themselves. Ask whether a new substance has formed.

2:12

Remember three things. Physical change: same substances, different form. Chemical change: new substances form. And the test: did a new substance form? Melted ice is a physical change. Burned toast and the green statue show chemical changes. Now you can look beyond appearances.

Sources

Every fact in the video, and where it comes from.

  1. 1
    nps.gov/stli/faqs.htm
    • The Statue of Liberty has an outer copper skin; it was originally brown and developed a green patina through natural oxidation.
  2. 2
    nps.gov/teachers/classrooms/preserving-a-symbol.htm
    • The Statue of Liberty stands in New York Harbor; its copper surface reacts with air and moisture to form a protective green patina.
  3. 3
    learninglab.rmit.edu.au/chemistry/matter/physical-chemical-changes
    • A physical change alters appearance or state without changing chemical identity; melting and freezing water are examples.
    • Gas, a color change or a new smell can indicate chemical change, but no single such observation proves a chemical reaction; boiling water is physical.
  4. 4
    primaryconnections.org.au/science-content/chemical-science-big-ideas/cha…
    • Water molecules remain H2O during melting, freezing and boiling; chemical changes form new substances.
    • Burning toast forms new substances including char and gases; cooling it does not restore the original bread.
    • Tearing paper changes size and shape without making a new substance; reversibility is not a reliable definition of physical change.
  5. 5
    nps.gov/stli/planyourvisit/frequently-asked-questions-…
    • The green patina protects the copper underneath.
  6. 6
    mci.si.edu/node/1298146
    • The Statue of Liberty patina contains newly formed copper compounds, mainly basic copper sulfate and some basic copper chloride.
  7. 7
    acs.org/middleschoolchemistry/lessonplans/chapter2/les…
    • Liquid water molecules can move past each other; each molecule stays intact during melting.
  8. 8
    acs.org/middleschoolchemistry/lessonplans/chapter6/les…
    • Chemical reactions rearrange atoms into new substances.
  9. 9
    van.physics.illinois.edu/ask/listing/1436
    • Boiling water bubbles contain water vapor.
  10. 10
    scienceline.ucsb.edu/getkey.php
    • Bubbles and other visible clues alone do not prove that new substances formed.

Image and sound credits

  • “Statue of Liberty, NY” by William Warby, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) — https://commons.wikimedia.org/wiki/File:Statue_of_Liberty,_NY.jpg (cropped/reframed)
  • “Liberty Island photo Don Ramey Logan” by Don Ramey Logan, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) — https://commons.wikimedia.org/wiki/File:Liberty_Island_photo_Don_Ramey_Logan.jpg (cropped/reframed)
  • “Cloudy ice cubes” by liz west, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) — https://commons.wikimedia.org/wiki/File:Cloudy_ice_cubes.jpg (cropped/reframed)
  • “Two pieces of burnt toast” by Sheep purple, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) — https://commons.wikimedia.org/wiki/File:Two_pieces_of_burnt_toast.jpg (cropped/reframed)
  • “Bread slices” by Saral Shots (CC0) — https://commons.wikimedia.org/wiki/File:Bread_slices.jpg (cropped/reframed)
  • “Boiling water” by GRAN, CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) — https://commons.wikimedia.org/wiki/File:Boiling_water.jpg (cropped/reframed)
  • “Glass of water ouside” by Kurt Kaiser (CC0) — https://commons.wikimedia.org/wiki/File:Glass_of_water_ouside.jpg (cropped/reframed)
  • Copper (Element - 29) — James St. John, CC BY 2.0. https://commons.wikimedia.org/?curid=155677529 — https://creativecommons.org/licenses/by/2.0/ (foil detail crop)
  • Plain torn paper still life generated with GPT Image 2.6; illustrative image, not evidence.
  • Paper textures: ambientCG (CC0). Sound effects: Freesound contributors (CC0), see public/kit/sfx/CREDITS.txt.
  • Music generated with ACE-Step; voice generated with Gemini TTS. Thumbnails generated with GPT Image 2.6.

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