What is a compound?
A compound is a pure substance formed when two or more elements join together chemically in a fixed ratio. Once elements react to form a compound, they lose their own properties and the compound has completely new properties. A compound can only be broken apart by a chemical reaction, not by simple physical methods.
Key idea
A compound = elements chemically combined in a fixed ratio, with new properties. Example: water is H₂O (2 hydrogen atoms + 1 oxygen atom).
Common compounds
- Water (H₂O) – hydrogen and oxygen combined.
- Carbon dioxide (CO₂) – carbon and oxygen combined.
- Common salt (sodium chloride, NaCl) – sodium and chlorine combined.
Sodium is a soft, reactive metal and chlorine is a poisonous green gas, yet when they react they form ordinary table salt, which is safe to eat. This shows a compound has new properties.
Compound vs mixture
| Compound | Mixture | |
|---|---|---|
| Joined chemically? | Yes | No |
| Ratio of parts | Fixed | Any proportion |
| Properties | New properties | Keep original properties |
| Separation | Chemical reaction | Physical methods |
Example
Water (H₂O) is a compound: it always has 2 hydrogen atoms to 1 oxygen atom. It cannot be separated by filtering; only electricity (electrolysis) can split it into hydrogen and oxygen.
Remember
- A compound is made of elements chemically joined in a fixed ratio.
- A compound has new properties, different from its elements.
- A compound can only be separated by chemical means.
- Water, carbon dioxide and common salt are compounds.