In chemistry, a pure substance is a material with a constant composition. In other words, it is homogeneous no matter when you sample it. A pure substance consists entirely of one type of atom or compound. It participates predictably in a chemical reaction.
Examples of Pure Substances
The best examples of pure substances are pure elements, molecules, and compounds:
- Hydrogen gas
- Gold metal
- Sugar (sucrose)
- Baking soda (sodium bicarbonate)
- Copper wire
- Silicon chip
- Copper sulfate
- Ethanol (pure grain alcohol)
The Gray Area
Some people consider any homogeneous mixture or alloy to be an example of a pure substance. This is the usual definition, but won’t get you the grade in a chemistry class. These examples of pure substances don’t consist of one type of atom or compound, but they do have a uniform structure. This type of pure substance needs to be free of contaminants or impurities:
- Sterling silver (an alloy of silver and copper)
- Air (a homogeneous mixture of gases)
- Vegetable oil
- Stainless steel
- Tap water
- Rubbing alcohol
Examples of Impure Substances
Some specimens are clear examples of impure substances:
- Mixture of sand and salt
- A bouquet of different flowers
- A cake
How to Identify a Pure Substance
The only way to know for certain if a sample is a pure substance is to perform a chemical analysis. However, there are other clues that can help you make a determination:
- Is is crystalline? Crystals are typically relatively pure compounds.
- A pure substance is usually all one color. The exception is if the matter has unusual optical properties, such that it reflects or refracts light.
- A pure substance has the same appearance and properties, no matter where you sample it.
- If you magnify the matter, it should appear the same everywhere in its composition. The exception is when matter exists in multiple phases (e.g., melting ice).
- If you can write the element symbol or chemical formula for a sample, it’s a pure substance.