Substance
Definition and meaning of Substance in chemistry.
A substance is a material with definite, fixed composition and distinct physical and chemical properties that remain constant regardless of the sample size. Every sample of a pure substance contains the same elements in identical proportions.
In more detail
Substances differ fundamentally from mixtures because they have uniform composition throughout. A substance can be either an element (like pure oxygen or iron) or a compound (like water or table salt) composed of two or more elements in fixed ratios. The properties of a substance are characteristic and reproducible: water always melts at 0°C and boils at 100°C at standard atmospheric pressure. Once identified, a substance's behavior in specific conditions can be reliably predicted.
Key facts
| Field | General Chemistry |
|---|---|
| Composition | Fixed and definite throughout; same elements in same proportions |
| Types | Elements or compounds |
| Key Characteristic | Constant, reproducible physical and chemical properties |
Water (H2O) is a substance because every sample, whether from a glacier or a tap, contains hydrogen and oxygen in a fixed 2:1 atomic ratio (about 11% hydrogen and 89% oxygen by mass) and exhibits identical physical properties (melting point 0°C, boiling point 100°C at 1 atm, density 1.00 g/mL at 4°C).
Frequently asked questions
How does a substance differ from a mixture?
A substance has a fixed composition and specific properties throughout, while a mixture has variable composition and can be separated by physical means without changing the chemical identity of its components.
Can a substance be made of multiple elements?
Yes. Compounds are substances made of two or more elements chemically bonded in fixed proportions, such as sodium chloride (NaCl) and glucose (C6H12O6).