Physical vs Chemical Changes: What's the Difference?

Melting ice, burning toast, dissolving sugar, rusting iron — all are "changes," but chemists split them into two big families. Knowing which is which is a classic exam skill, and the test for telling them apart is wonderfully simple.

The short answer: a physical change alters how a substance looks or its state, but it's still the same substance underneath (melting ice is still water). A chemical change creates one or more brand-new substances (burning wood turns it into ash, smoke, and gas — you can't get the wood back).

Quick comparison at a glance

FeaturePhysical changeChemical change
New substance formed?NoYes
Chemical identityStays the sameChanges
Easy to reverse?Usually yesUsually no
What changesShape, size, or stateThe actual substance
ExamplesMelting, boiling, dissolving, cuttingBurning, rusting, cooking, digesting

What is a physical change?

In a physical change, the molecules stay exactly the same — only their arrangement or form changes. Ice, liquid water, and steam are all still H₂O; you've just changed the state.

Common physical changes:

  • Melting, freezing, boiling, condensing (all changes of state)
  • Dissolving sugar or salt in water
  • Cutting, crushing, or tearing something
  • Mixing things that don't react

These are often easy to reverse — freeze the water back, or evaporate the solution to get your salt again.

What is a chemical change?

In a chemical change (also called a chemical reaction), bonds break and form, rearranging atoms into new substances with new properties. The original material is gone.

Common chemical changes:

  • Burning (combustion)
  • Rusting and other corrosion
  • Cooking or baking
  • Digestion
  • Photosynthesis

These are usually very hard to reverse — you can't "un-bake" a cake.

Signs that a chemical change has happened

Look for these clues — one or more usually means a reaction took place:

  • A color change that isn't just mixing pigments
  • Bubbles or gas produced (not from boiling)
  • A temperature change (it gets hot or cold on its own)
  • Light or a flame
  • A solid (precipitate) forming in a liquid
  • A new smell
  • It's difficult to reverse

How to tell them apart

Ask one question: "Is it still the same substance?"

  • If yes (just a different shape or state) → physical.
  • If a new substance formed → chemical.

Common mistakes to avoid

  • Dissolving looks dramatic but is physical. Salt dissolving in water is still salt and water — you can evaporate it back.
  • Boiling is physical, not chemical. Steam is still water.
  • Don't rely on "reversible" alone. It's a helpful hint, but the real test is whether a new substance formed.

FAQ

Is dissolving salt in water a physical or chemical change?
Physical — the salt and water are unchanged, and evaporating the water gives the salt back.

Is boiling water a chemical change?
No. Boiling turns liquid water into steam, but it's still H₂O — a physical change.

Is cooking an egg reversible?
No — it's a chemical change. The proteins permanently change, so you can't turn it back into a raw egg.

Are all changes of state physical?
Yes. Melting, freezing, boiling, and condensing are all physical changes.

The takeaway

Physical changes rearrange a substance without changing what it is; chemical changes create new substances. When in doubt, look for the tell-tale signs of a reaction — and ask whether you could easily get the original substance back.

Next: Endothermic vs Exothermic Reactions, which looks at the energy behind chemical changes.

Comments

Post a Comment

Popular posts from this blog

What is Pepsin?

Image
In Your Body... Pepsin is a digestive enzyme that performed the best in the acidic condition (pH 2) produced by the gastric juice. It digests and breaks down protein (e.g. meat, seeds, eggs, dairy products, etc.). Keep in mind that pepsin digests proteins (not carbohydrates, not fat) into peptides. These peptides are then either absorbed in the intestine or further break down by enzymes from pancreas. In small intestine (pH around 7), pepsin is no longer effective and no longer digests proteins. Hormones Hormones that regulate digestion and hunger such as gastrin and secretin stimulates the production of pepsinogen, the proenzyme of pepsin. In other words, when pepsinogen is activated, it becomes pepsin.
Read more

The structure of Chimeric Antigen Receptor (CAR) III - Transmembrane Domains

Image
Transmembrane Domains Among the four components of a chimeric antigen receptor (CAR), the transmembrane domain is the least studied one. How different transmembrane domains affect and contribute to each other's functions is also not well studied. That being said, we will go through the function of a transmembrane domain of a CAR, the CD3ζ transmembrane domain, and the CD8α transmembrane domain.  Introduction of the transmembrane domain The transmembrane domain is where the transmembrane proteins are located. In a CAR, the transmembrane domain has the function to anchor the CAR to the T cell membrane. How strong the CAR is anchored to the T cell membrane affects the CAR expression level and stability.  Currently, the commonly used transmembrane domain is derived from proteins such as CD3ζ, CD4, CD8α, and CD28. The CD3ζ transmembrane domain Among the transmembrane domains mentioned, the CD3ζ transmembrane may improve CAR-mediated T cell action as the CD3ζ transmembrane medi...
Read more

Current Limitations of CAR-T Cell Therapy I - Antigen Escape

Image
Antigen Escape Antigen Escape is one of the most concerning challenges during the clinical development of CAR-T cell therapy. A robust initial response is often followed by a relapse in many trials. This is often due to the downregulation or the complete loss of target antigens on the tumor cells. Introduction Antigen escape describes the development of tumor resistance to a single antigen targeting CAR construct. After the initial encounter with the CAR-T cells, the malignant cells mutate and show partial or complete loss of the target antigen expression.  In patients with ALL treated with CD19-targeted CAR-T cell therapy, a downregulation or loss of CD19 antigens was observed.  In patients with myeloma treated with BCMA-targeted CAR-T cell therapy, a downregulation or loss of BCMA expression was observed.  In patients with glioblastoma, a solid tumor treated with IL13Ra2-targeted CAR-T cell therapy, a downregulation of IL13Ra2 expression was observed.  Improve...
Read more