Pure Substances vs Mixtures
A pure substance is matter with a fixed composition and definite properties, while a mixture is made of two or more substances that are physically combined and can be present in different proportions. In brief: distilled water and gold are pure substances, but salt water, air and a salad are mixtures; a pure substance has a fixed melting point, and a mixture melts over a range of temperatures.
Almost everything in daily life is a mixture, from the air we breathe to the soil in a garden. Pure substances are less common but essential in chemistry, because their properties are constant and can be used for identification. This article explains the difference, lists the key tests, gives examples in a table, works through a classification exercise and explains why purity matters.
What is a pure substance?
A pure substance consists of only one kind of particle. It can be an element, such as oxygen or copper, or a compound, such as water or table salt. Every sample of a pure substance has the same composition and the same properties, whatever its source. Pure water from a spring and pure water made in a laboratory both boil at 100 °C at normal pressure and have a density of about 1.00 g/cm3 at 4 °C.
A pure substance cannot be separated into other substances by physical methods such as filtering, sorting or evaporating. Only a chemical change can break a compound into its elements.
What is a mixture?
A mixture contains two or more substances that are not chemically combined. Each substance keeps its own properties, and the amounts can vary. Sweet tea can have a little sugar or a lot of sugar, and it is still tea with sugar.
The parts of a mixture can be separated by physical methods, such as filtering, evaporating, using a magnet or distilling. Later articles in this series explain these methods in detail.
Pure substances and mixtures compared
| Feature | Pure substance | Mixture |
|---|---|---|
| Composition | Fixed | Variable |
| Number of kinds of particle | One | Two or more |
| Melting and boiling points | Sharp, constant | Range of temperatures |
| Separation | Only by chemical change | By physical methods |
| Properties | Definite | Depend on the proportions |
| Examples | Water, iron, oxygen | Air, seawater, brass |
How can you tell them apart?
Several simple tests help.
- Melting point: a pure solid melts sharply at one temperature. A mixture melts gradually over a range and usually at a lower temperature.
- Boiling point: a pure liquid boils at a constant temperature, whereas a mixture of liquids boils over a range.
- Separation: if the sample can be split into parts by physical means, it is a mixture.
- Chromatography: a pure substance forms a single spot, and a mixture of dyes may form several spots.
Examples of pure substances and mixtures
| Material | Classification | Reason |
|---|---|---|
| Distilled water | Pure substance (compound) | Only H2O |
| Gold (24 carat) | Pure substance (element) | Only gold atoms |
| Seawater | Mixture | Water, salts and gases |
| Air | Mixture | Nitrogen, oxygen, argon and others |
| Brass | Mixture (alloy) | Copper and zinc |
| Table sugar | Pure substance (compound) | Only sucrose |
| Orange juice | Mixture | Water, sugars, acids and pulp |
Worked example: reading melting data
Two white solids, A and B, are heated slowly and their melting is observed.
- Sample A starts to melt at 80.0 °C and is fully liquid at 80.5 °C. The range is only 0.5 °C, so A is very likely pure. This matches the melting point of naphthalene, which is about 80 °C.
- Sample B starts to melt at 62 °C and is fully liquid at 74 °C. The range is 12 °C, so B is probably a mixture.
- Conclusion. If B were made of naphthalene with an impurity, it would melt below 80 °C and over a range, which fits the data.
Why does purity matter?
- Medicines: impurities in a drug can cause harm, so manufacturers test purity carefully.
- Electronics: computer chips require extremely pure silicon.
- Food and water: water supplies are tested for unwanted substances.
- Research: chemists use pure substances so that experimental results can be repeated.
The word “pure” in everyday language can be misleading. “Pure orange juice” on a label is still a mixture in chemical terms. Chemists use “pure” in a strict sense: only one substance is present.
How did the concept develop?
Early chemists separated materials by distillation, crystallization and filtering long before atoms were understood. In the 1700s, Antoine Lavoisier distinguished elements as substances that could not be broken down further by the methods of his time. In the late 1700s, Joseph Proust proposed the law of definite proportions, which states that a compound always contains the same elements in the same mass ratio. This law is one of the foundations of the distinction between compounds and mixtures.
Which mistakes are common?
Mistake 1: Thinking that clear means pure
Tap water and air are clear, yet both are mixtures.
Mistake 2: Saying that only elements are pure substances
Compounds such as water and carbon dioxide are also pure substances.
Mistake 3: Assuming a mixture always looks mixed
In some mixtures, such as salt water or air, you cannot see the different parts.
Mistake 4: Confusing a mixture with a compound
Iron filings and sulfur powder can be mixed in any ratio and separated with a magnet. Heating them forms iron sulfide, a compound, which is a new substance.