A bike left out all winter grows rust. Cake batter goes into the oven runny and comes out spongy. A sparkler throws off light and heat until nothing is left but a grey wire. In each case the substances you started with have turned into new substances with new properties. That is a chemical change, or chemical reaction. Chemists cannot watch atoms rearranging, so they rely on clues they can see, hear, feel and smell, and then confirm with tests. This lesson covers those clues and one of the most important laws in chemistry: in a reaction, mass is never created or destroyed.
Physical change or chemical change?
- The substance stays the same; only its form, size or state changes
- Examples: ice melting, sugar dissolving, paper being cut, water boiling
- Often easy to reverse (freeze the water again, evaporate the solution)
- No new substance, so the particles themselves are unchanged
- One or more new substances form, with different properties
- Examples: wood burning, iron rusting, an egg cooking, baking soda fizzing in vinegar
- Usually hard to reverse
- Atoms are rearranged: old bonds break and new bonds form
Five kinds of evidence
| Evidence | Chemical change example | A physical change that can fool you |
|---|---|---|
| Colour change | A copper roof turning green; a cut apple browning | Adding food colouring to water |
| Gas produced (bubbles) | Baking soda fizzing in vinegar | Water boiling; a soft drink going flat |
| Precipitate (a solid forming in a liquid) | Two clear solutions mixing to make a cloudy yellow solid | Crystals forming as a hot salt solution cools |
| Energy released or absorbed (heat, light) | A burning candle; a chemical hand warmer heating up | A light bulb glowing; a cold pack cooling as its salt dissolves |
| New odour | Food spoiling; a match being struck | Perfume evaporating across a room |
- Hydrogen gas: a burning splint held at the mouth of the test tube makes a squeaky "pop".
- Oxygen gas: a glowing splint (just blown out) bursts back into flame.
- Carbon dioxide gas: bubbled through limewater (calcium hydroxide solution), it turns the limewater milky. It also puts out a burning splint, but so do several other gases, so the limewater test is the confirming one.
Reactants, products and word equations
The substances present at the start of a reaction are the reactants; the new substances formed are the products. A word equation names them, with an arrow that reads "produces" or "yields" and plus signs between substances. For baking soda and vinegar: sodium hydrogen carbonate + acetic acid → sodium acetate + water + carbon dioxide. The carbon dioxide is the gas you see bubbling. In the next lessons you will replace the names with formulas and balance them.
The law of conservation of mass
In the 1770s and 1780s the French chemist Antoine Lavoisier weighed reactions very carefully in sealed containers and found that the total mass never changed. This is the law of conservation of mass: in a chemical reaction, the total mass of the products equals the total mass of the reactants. The reason is that atoms are only rearranged; none are created or destroyed. The law is easy to see in a closed system, where nothing can enter or leave. Many everyday reactions happen in an open system, where gases can escape into the air or be drawn in from it, and then a balance seems to show mass appearing or disappearing.
| Set-up | What the balance shows | Explanation |
|---|---|---|
| Baking soda and vinegar in a sealed bottle with a balloon | No change in mass | Closed system: the carbon dioxide is trapped and still weighed |
| Baking soda and vinegar in an open beaker | Mass decreases | Carbon dioxide gas escapes into the air |
| Steel wool burned in an open dish | Mass increases | Oxygen from the air joins the iron to form iron oxide |
| Wood burned in a campfire | The ash weighs far less than the wood | Carbon dioxide and water vapour float away as gases |