One candle, two kinds of change
Light a candle and watch closely. Near the wick, the wax melts into a liquid, and when it drips and cools it turns solid again. It’s still wax; only its state has changed. Meanwhile, liquid wax climbs the wick, turns into vapor, and burns, forming carbon dioxide and water. That wax has become different substances.
Two fundamentally different kinds of change are happening on the same candle at the same time.
Physical and chemical changes
- A physical change物理变化 forms no new substance. Examples: snow melting, glass breaking, rubbing alcohol evaporating.
- A chemical change化学变化 forms one or more new substances. It’s also called a chemical reaction化学反应. Examples: charcoal burning, iron rusting, food rotting.
| Physical change | Chemical change | |
|---|---|---|
| Key feature | No new substance forms | New substances form |
| Particles | The particles stay the same; only their spacing and arrangement change | Atoms rearrange into new particles |
| Common types | Changes of state, changes of shape, dissolving | Burning, rusting, rotting, fermenting |
| Often comes with | A change of shape or state | Light, heat, a color change, gas, a precipitate |
How the two are related: a chemical change always comes with physical changes too (the candle melts as it burns), but a physical change doesn’t have to involve any chemical change.
Both changes at the particle level
Pick an example below and compare what happens to the particles. Two standard gas tests appear in the examples: carbon dioxide turns limewaterlimewaterA clear solution of calcium hydroxide in water. Carbon dioxide bubbled through it forms white, insoluble calcium carbonate, which turns the solution cloudy. That makes limewater the standard test for carbon dioxide. cloudy, and oxygen relights a glowing splintrelighting a glowing splintA glowing splint is a wooden stick that was burning and has just been blown out, so its tip still glows red. Put it into oxygen and it bursts back into flame, which makes it the standard test for oxygen..
Macroscopic: what you can see
When iodine is heated, the shiny purple-black solid turns straight into a violet vapor without melting.
Particulate: what the particles do
Symbolic: how chemists write it
The iodine molecules themselves don't change. They just go from tightly packed to far apart. This is a physical change.
Macroscopic: what you can see
Charcoal burns brightly in oxygen, giving off heat and a gas that turns limewater cloudy.
Particulate: what the particles do
Symbolic: how chemists write it
Carbon and oxygen atoms recombine into carbon dioxide molecules, a new substance. This is a chemical change.
Macroscopic: what you can see
Add a pinch of manganese dioxide to hydrogen peroxide solution: it fizzes rapidly, and the gas relights a glowing splint.
Particulate: what the particles do
Symbolic: how chemists write it
Hydrogen peroxide molecules break apart, and their atoms recombine into water and oxygen molecules. This is a chemical change.
Compare them: in a physical change, the boxes before and after contain the same kinds of particles. After a chemical change, new kinds of particles appear that weren’t there before.
How to tell them apart
There is only one real test: did a new substance form? Light, heat, a color change, or bubbles can help you notice that a chemical change might have happened, but none of them is proof on its own:
| Observation | Example of a chemical change | But a physical change can do it too |
|---|---|---|
| Light and heat | A magnesium ribbon burning | A light bulb glowing when switched on |
| Gas or bubbles | Limestone fizzing in hydrochloric acid | Opening a can of soda, water boiling |
| Color change | Iron rusting from silver-gray to reddish brown | Oxygen gas cooled into a pale blue liquid |
| A solid appears | Limewater turning cloudy when carbon dioxide is bubbled through | Crystals formingcrystallizing outA dissolved substance coming back out of a solution as a solid, often as crystals. A hot saturated solution does this as it cools, because the cooler water can’t hold as much. as a hot, saturatedsaturatedA saturated solution holds as much of a substance as it can at that temperature: no more will dissolve. Change the temperature and the amount it can hold changes too. Lesson 1.7.4 covers this. potassium nitrate solution cools |
Example 1: Three things that happen to iron wire
Classify each change: (1) bending an iron wire into a coat hanger; (2) iron wire rusting in damp air; (3) melting scrap iron and casting it into a frying pan.
- (1) Bending. Only the shape changes. It’s still iron, and no new substance forms: physical change.
- (2) Rusting. Rust is a crumbly reddish-brown solid, mostly iron(III) oxide, which is a different substance from iron: chemical change.
- (3) Melting and casting. The iron goes from solid to liquid and back to solid, but it’s still iron: physical change.
- The method. Ask the same question each time: is it still the same substance afterward?
Example 2: A light bulb and a burning magnesium ribbon
A light bulb gives off light and heat when it’s on. A burning magnesium ribbon gives off a dazzling white light and heat too. Are they both chemical changes?
- The light bulb. When you switch it off, the filament is the same as before. No new substance formed: physical change.
- The magnesium. After burning, the shiny silver ribbon has turned into a white powder (magnesium oxide), a new substance: chemical change.
- Conclusion. Both give off light and heat, but only the one that forms a new substance is a chemical change. The same observation doesn’t mean the same kind of change.
- A glass shatters on the floor.
Only the shape changes. It's still glass.
- Leftover food goes bad.
As food spoils, new substances with sour or foul smells form.
- Gasoline left in an open can slowly disappears.
The gasoline evaporates, going from liquid to gas. No new substance forms.
- Grapes are fermented to make wine.
Fermentation turns the sugars into new substances, including alcohol.
- A steel bike rim rusts.
Iron turns into rust, a new substance.
- Oranges are squeezed to make juice.
The juice is just separated from the pulp. No new substance forms.
- A candle burns.
Burning wax forms carbon dioxide and water.
- A bike tire bursts in the hot sun.
The air inside expands as it heats up and splits the tire. No new substance forms.
- Milk turns sour.
As milk spoils, new substances with a sour taste form.
Common mistakes
Mistake 1: “If it gives off light and heat, it’s a chemical change.” A light bulb gives off light and heat, but that’s a physical change.
Mistake 2: “If gas is produced, it’s a chemical change.” Boiling water produces water vapor, and an opened soda releases bubbles of carbon dioxide that were dissolved in it. Both are physical changes.
Mistake 3: “Every explosion is a chemical change.” Fireworks exploding form new substances, so that’s a chemical change. A tire blowing out or a balloon popping forms no new substance, so those are physical changes.
Quick check
Which of these is a chemical change?
Burning paper forms new substances such as carbon dioxide and water. None of the others forms a new substance.
What is the one sure sign that a change is chemical?
The defining feature of a chemical change is that a new substance forms. A, B, and C are common clues, but physical changes can show them too.
Summary
- A physical change forms no new substance. A chemical change, also called a chemical reaction, forms new substances.
- At the particle level, a physical change leaves the particles themselves unchanged, while a chemical change rearranges atoms into new particles.
- Light, heat, color changes, gas, and precipitates are clues. The real test is whether a new substance forms.
- A chemical change always comes with physical changes, but not the other way around.
On to AP (for later)
AP topic 4.4 distinguishes the two in terms of chemical bonds: a chemical change breaks and forms bonds and produces new substances. AP also points out that some processes are hard to classify. Dissolving table salt in water breaks the attractions between ions, yet it’s usually still called a physical change. Exam questions often show particle diagrams from before and after a change and ask which kind it is, using exactly the comparison you made today.