Examples of Pure Substances

In chemistry, a pure substance or chemical substance is a material with a uniform and 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.

Key Takeaways: Examples of Pure Substances

• A pure substance has a constant composition and uniform properties throughout a sample.
• Pure substances include elements (like gold and hydrogen) and compounds (like water and salt).
• Some definitions allow homogeneous mixtures (e.g., air, steel) to count as pure substances in practical contexts.
• Impure substances are mixtures with non-uniform composition or multiple visible components.
• Pure substances can be identified by chemical formulas, uniformity, and sometimes by crystallinity.
• Common misconceptions arise from confusing homogeneous mixtures with true pure substances.

Examples of Pure Substances

The best examples of pure substances are pure elements, molecules, and compounds:

• Hydrogen gas (H₂)
• Gold metal (Au)
• Sugar (sucrose, C₁₂H₂₂O₁₁)
• Baking soda (sodium bicarbonate, NaHCO₃)
• Ammonia (NH₃)
• Diamond (C)
• Copper wire (Cu)
• Silicon chip (Si)
• Copper sulfate (CuSO₄)
• Ethanol (C₂H₅OH)

Note that in each case, there is either an element symbol or chemical formula for a pure substance.

What Is the Difference Between a Pure Substance and a Mixture?

Pure substances and mixtures are both forms of matter, but they differ in composition and properties. A pure substance consists of only one type of atom or compound with a fixed chemical makeup, such as water (H₂O) or gold (Au). In contrast, a mixture contains two or more different substances physically combined, like salt water or trail mix. Mixtures may be homogeneous (uniform throughout) or heterogeneous (visibly different components), while pure substances are always chemically uniform. The table below summarizes the key differences:

The Gray Area

Some people consider any homogeneous mixture or alloy to be an example of a pure substance. This is the usual definition outside of science, 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)
• Honey
• Air (a homogeneous mixture of gases)
• Vegetable oil
• Brass
• Stainless steel
• Tap water
• Rubbing alcohol

Examples of Impure Substances

Some specimens are clear examples of impure substances:

• Mixture of sand and salt
• Mixture of sand and salt
• Chocolate chip cookie
• Concrete
• Granite
• Salad dressing
• Soil
• A bowl of cereal with milk
• A slice of pizza

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:

• Uniform appearance: The sample looks the same throughout, without visible separation.
• Same properties throughout: It has consistent boiling/melting points and reacts uniformly.
• Crystalline structure: Crystals often indicate a pure compound, though impurities may be present.
• Chemical formula: If you can write a single chemical formula (e.g., H₂O, NaCl), the substance is pure.
• Microscopic uniformity: Magnification reveals a consistent composition (unless it’s in two phases, such as ice and water).

References

• Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. (2005). General Chemistry (4th ed.). New Jersey: Pearson Prentice Hall.
• Hunter, Lawrence E. (2012). The Processes of Life: An Introduction to Molecular Biology. MIT Press. ISBN 9780262299947.
• IUPAC (2025). “Chemical Substance”. Compendium of Chemical Terminology (the “Gold Book”) (5th ed.). doi:10.1351/goldbook.C01039