8.4 Compounds
We split water into hydrogen and oxygen — but why couldn’t we separate them by a physical method (filtering, evaporating)? Because in water, the elements are chemically joined. That makes water a compound.
What Is a Compound? A compound forms when different elements combine chemically in a fixed ratio to make something entirely new. Two key facts:
- A compound’s properties are completely different from the elements in it.
- Its elements cannot be separated by any physical method — only a chemical change will do it.
In water, hydrogen and oxygen atoms combine in a fixed 2 : 1 ratio. And the properties flip entirely: hydrogen is a fuel, oxygen supports burning, yet water puts out fire!
Salt: From Dangerous to Delicious Sodium is a soft metal that reacts violently with water; chlorine is a poisonous gas. Yet combined in a fixed 1 : 1 ratio they form sodium chloride — common salt, which is harmless and essential to life. A perfect illustration that a compound is nothing like its elements.
Sugar hides its elements
Activity 8.4 — Charring sugar Heat sugar gently in a boiling tube. It turns brown, then black, leaving charcoal (carbon) behind — and droplets of water appear near the tube’s mouth (from the dry sugar, not the air). Since water is itself hydrogen + oxygen, sugar must contain carbon, hydrogen and oxygen. So sugar is a compound, not an element.
Mixture or compound? The iron-and-sulfur test
Activity 8.5 — Same elements, two different things Mix iron filings and sulfur powder (Sample A). Heat half of it strongly until it forms a black mass (Sample B). Now compare:
| Test | Sample A (mixture) | Sample B (compound) |
|---|---|---|
| Look | Black & yellow bits visible | Uniform black throughout |
| Magnet | Iron is pulled out | Not attracted |
| With dilute acid | Hydrogen gas (no smell, “pop”) | Hydrogen sulfide (rotten-egg smell) |
The Verdict Sample A is a mixture — iron and sulfur keep their own properties and can be separated (a magnet pulls the iron out). Sample B is a compound (iron sulfide) — formed by a chemical change, uniform throughout, with brand-new properties; the iron is locked in, so a magnet does nothing and it cannot be separated physically. Same two elements, but mixing vs chemically combining gives utterly different results.
Now put it all together. Classify each substance as an Element, a Compound, or a Mixture.