What Is a Pure Substance?
In everyday language, "pure" often means clean or natural. In chemistry, a pure substance has a precise meaning: it contains only one type of substance — a single element or a single compound — with nothing else mixed in.
Examples of pure substances:
- Pure water (H₂O only — no dissolved salts or minerals)
- Pure gold (Au only)
- Pure sucrose (table sugar, C₁₂H₂₂O₁₁)
- Pure oxygen gas (O₂ only)
How to Test for Purity
The most reliable way to test purity in chemistry is to measure the melting point and boiling point:
Pure substances:
- Have a sharp, fixed melting point — they melt at exactly one temperature.
- Have a sharp, fixed boiling point.
- The temperature stays constant during melting or boiling.
Impure substances (mixtures):
- Melt over a range of temperatures (e.g. 98–105 °C instead of a sharp 100 °C).
- The melting point is lower than the pure substance's melting point.
- The boiling point is higher than the pure substance's boiling point.
This is why pure water boils at exactly 100 °C, but salt water boils above 100 °C. The dissolved salt raises the boiling point and lowers the freezing point (which is why salted roads in winter don't freeze as quickly). The effect is called "boiling point elevation" and "freezing point depression."
Chromatography and Purity
Paper chromatography can also be used to test purity:
- A pure substance gives a single spot on chromatography (one Rf value).
- A mixture gives multiple spots as different components separate.
This is especially useful for testing whether a pharmaceutical drug is pure or contains unwanted contaminants.
Pure vs Impure in Context
| Substance | Pure? | Why |
|---|---|---|
| Distilled water | Yes | Only H₂O molecules |
| Tap water | No | Contains dissolved minerals, chlorine |
| Pure gold (24 carat) | Yes | Only gold atoms |
| 9-carat gold jewellery | No | Gold + silver, copper, and other metals alloyed together |
| Pharmaceutical aspirin tablet | No | Aspirin + binders, fillers |
| Laboratory-grade aspirin | Yes (very nearly) | Extremely high purity for research |
In pharmaceutical chemistry, purity is critical. A medicine that is 99% pure might sound excellent, but the 1% impurity could be toxic. This is why rigorous purity testing (using melting points, chromatography, and spectroscopy) is essential before medicines can be approved for use.
Why Purity Matters
- Medicine: Impurities in drugs can have serious health effects.
- Electronics: Silicon in computer chips must be almost perfectly pure — trace impurities affect electrical properties.
- Food: Pure water is essential for safe drinking.
- Research: Chemical reactions give unreliable results if reagents contain impurities.
Real world: Drug purity in pharmaceuticals — 1% impurity can be toxic
In pharmaceutical manufacturing, drug purity is critical. Thalidomide, prescribed in the 1950s as a pure racemate, caused birth defects because one molecular form (enantiomer) of the mixture was toxic. The "pure" drug was actually a mixture of two mirror-image molecules with different biological effects. Today, pharmaceutical companies use melting point testing, chromatography, and spectroscopy to verify that each batch of medicine is chemically pure and contains the correct molecular form. Regulatory authorities like the FDA require purity above 99.9% for many medicines.
Real world: Silicon for computer chips must be ultra-pure
Computer processors are made from silicon that is purified to 99.9999999% (nine nines pure) — less than 1 atom in a billion can be a different element. This is called "electronic grade" silicon. Even tiny impurities change how electrons move through the material, which would cause circuits to fail. The Czochralski process grows huge pure silicon crystals by pulling a seed crystal slowly out of molten silicon. The purity required for silicon chips vastly exceeds anything achievable by simple distillation or crystallisation alone.
Real world: Gold purity — carats measure how pure the alloy is
24-carat gold is pure gold (Au, ~100%). But pure gold is too soft for jewellery — it scratches and bends easily. 9-carat gold is 37.5% gold mixed with silver, copper, and zinc (a mixture/alloy). 18-carat is 75% gold. The lower the carat, the more impurities, but the harder and cheaper the alloy. Testing gold purity: a pure gold sample melts at exactly 1064 °C; alloyed gold melts over a range at a lower temperature. Jewellers use acid tests and fire assay to verify the purity of gold.
Where do you see this in real life? Pharmaceutical drugs tested for purity before sale (impurities can be toxic), ultra-pure silicon in computer chips (impurities disrupt circuits), 24-carat vs 9-carat gold jewellery (pure vs alloy/mixture), distilled water in car batteries (tap water minerals would corrode them), laboratory-grade chemicals vs industrial-grade — purity matters enormously in medicine, electronics, and materials science.
Practice Problems
A sample of aspirin is heated and found to melt between 134 °C and 140 °C. Pure aspirin melts at exactly 135 °C. What does this tell you about the sample?
Show answer
Why does sea water boil at a higher temperature than pure water?