Substance
Definition and meaning of Substance in chemistry.
A chemical substance is a pure material with a definite and strictly fixed chemical composition. It displays distinct physical and chemical properties that remain completely constant regardless of the sample size. Every sample of a specific substance will always contain the exact same elements in identical proportions.
In more detail
In chemistry, substances differ fundamentally from everyday mixtures because they have uniform composition throughout their structure. You can separate a mixture like saltwater into different parts using basic physical methods like boiling. You cannot separate a pure substance into different parts without breaking actual chemical bonds.
A substance can be a single pure element like oxygen gas or a block of solid iron. It can also be a compound like pure water made of multiple elements in a fixed ratio. The measurable properties of a pure chemical substance are always characteristic and perfectly reproducible.
Pure liquid water will always freeze at precisely zero degrees Celsius under standard atmospheric pressure. It will always have a density of exactly one gram per milliliter at four degrees Celsius. Once chemists identify a pure substance, they can reliably predict its exact behavior under specific conditions. This absolute consistency allows scientists to use known substances to create precise and repeatable chemical reactions.
Key facts
| Field | General Chemistry |
|---|---|
| Composition | Fixed and definite throughout every part of the entire sample. |
| Types | Can be either pure individual elements or chemically bonded compounds. |
| Key Characteristic | Exhibits constant and reproducible physical and chemical properties. |
| Separation | Cannot be separated into simpler components by physical means like filtering. |
| Common Misconception | People often call mixtures like air or wood substances, but chemically they are not. |
Table salt, officially known as sodium chloride (NaCl), is a classic example of a pure chemical substance. Every pure crystal of table salt contains exactly one sodium ion for every single chloride ion. This fixed atomic ratio means sodium chloride is always 39 percent sodium and 61 percent chlorine by mass. It does not matter if the salt came from a deep underground mine or an evaporated ocean. The resulting pure substance will always form identical cubic crystals and melt at exactly 801 degrees Celsius.
Frequently asked questions
How does a pure substance differ from a chemical mixture?
A substance has a fixed composition throughout, while a mixture has variable composition. You can separate mixtures using physical methods, but substances require chemical reactions.
Can a substance be made of multiple different elements?
Yes, compounds are pure substances made of two or more elements chemically bonded together. Sodium chloride (NaCl) and pure water (H2O) are excellent examples of compound substances.
Is tap water considered a pure chemical substance?
No, tap water is actually a mixture. It contains the pure substance water (H2O) along with dissolved minerals, gases, and treatment chemicals like fluoride or chlorine.