What Is a Mixture?
A mixture contains two or more substances that are physically combined but not chemically bonded. The substances in a mixture keep their individual chemical properties and can be separated by physical methods.
Key features of mixtures:
- The substances are not in fixed proportions — you can have any amount of each.
- No chemical reaction takes place when the substances are mixed.
- Each component keeps its own properties.
- They can be separated by physical methods.
Types of Mixture
Homogeneous mixtures — look the same throughout (uniform composition). You cannot see the individual components. Example: salt water, air, brass (a metal alloy).
Heterogeneous mixtures — you can see different components or parts. Example: sand and water, salad, a mixture of iron filings and sulfur.
Air is one of the most important mixtures on Earth. It contains approximately 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, and small amounts of other gases. These gases are physically mixed — they can be separated by cooling air to very low temperatures (fractional distillation of liquid air).
Examples of Common Mixtures
| Mixture | Components |
|---|---|
| Seawater | Water + salts + dissolved gases |
| Air | Nitrogen + oxygen + argon + CO₂ |
| Soil | Rock particles + organic matter + water + air |
| Bronze | Copper + tin (metal alloy) |
| Lemonade | Water + citric acid + sugar + CO₂ |
| Blood | Water + cells + proteins + ions |
Mixtures vs Pure Substances
| Feature | Mixture | Pure Substance |
|---|---|---|
| Composition | Variable | Fixed |
| Properties | Mix of components' properties | Definite properties |
| Melting/boiling point | Broad range | Sharp, fixed temperature |
| Separation | By physical methods | Cannot be physically separated |
A pure substance has a sharp, defined melting point. A mixture melts over a range of temperatures. This property is used in chemistry to test purity — if a substance melts over a range of 5–10 degrees, it is likely impure (a mixture). If it melts sharply at one temperature, it is probably pure.
Why Separate Mixtures?
Separating mixtures is extremely important:
- Drinking water is purified by filtering and treating contaminated water.
- Crude oil is separated into petrol, diesel, and other fuels by distillation.
- Salt is recovered from sea water by evaporation.
- Gold nuggets are separated from rock and sand by panning.
Different separation techniques are used depending on the properties of the components in the mixture.
Real world: Air — separated industrially to make pure gases
Air is a mixture of nitrogen (78%), oxygen (21%), argon, CO₂ and others. Because it is a mixture (not a compound), its components can be separated by physical means. Industrial plants cool air to −200 °C until it liquefies, then fractionally distil the liquid air — nitrogen boils off first (at −196 °C), then argon, then oxygen (−183 °C). The separated pure gases are used in medicine (liquid oxygen for hospitals), food packaging (nitrogen to keep crisps fresh), and welding.
Real world: Blood — a complex heterogeneous mixture
Blood is a heterogeneous mixture containing red blood cells, white blood cells, and platelets suspended in plasma (a liquid containing dissolved proteins, hormones, ions and glucose). Because blood is a mixture, its components can be separated by centrifugation — spinning separates denser red cells from the lighter plasma. Hospitals routinely separate blood into components for transfusions: some patients only need plasma, others need red cells, others need platelets.
Real world: Alloys are designed mixtures with superior properties
Pure metals are rarely used in industry — instead, alloys (mixtures of metals) are engineered for specific properties. Stainless steel is iron mixed with chromium and nickel — harder than pure iron and resistant to rust. Bronze (copper + tin) was a revolutionary material in ancient history. 9-carat gold jewellery is gold mixed with silver and copper — harder and cheaper than pure 24-carat gold. Alloys are mixtures whose properties are the result of mixing, not chemical bonding.
Where do you see this in real life? Air in your lungs (mixture of gases separated industrially), blood in your veins (cells + plasma + dissolved substances), stainless steel cutlery (iron + chromium alloy), sea water (water + dissolved salts), concrete (mixture of sand, gravel, cement, water) — almost everything around you is a mixture, not a pure substance.
Practice Problems
Is seawater a pure substance or a mixture? How can you tell?
Show answer
What is the key difference between a mixture and a compound?