What Is a Mixture?

Introduction

Almost nothing you touch in daily life is a single substance. The air you breathe, the sea, a cup of tea, soil, blood, concrete and a fizzy drink are all mixtures . Chemists spend a great deal of effort taking mixtures apart to obtain the substances they want, so the first step in this unit is to be clear about exactly what a mixture is and how it differs from a single substance.

Core explanation

Definition. A mixture contains two or more different substances that are mingled together but not chemically joined . Each substance in a mixture is called a component .

The components keep their own properties. Because no chemical reaction has taken place, every component behaves as it did before mixing. Stir iron filings into sand and the iron is still magnetic, still grey and still rusts in damp air; the sand is still hard and yellowish. Dissolve sugar in water and the solution still tastes sweet, because the sugar particles are still sugar.

The composition can vary. You can make salty water with a pinch of salt or with several spoonfuls. Both are salt-water mixtures, but they contain different proportions. A mixture has no fixed recipe, which is one of the clearest ways to tell it apart from a compound, where the elements are always combined in the same ratio.

Mixtures can be separated by physical methods. Since the components are not bonded to one another, they can be pulled apart by using differences in their physical properties: particle size, magnetism, solubility, boiling point, density and so on. Methods such as filtration, evaporation, distillation and chromatography do this without changing any substance into a new one. Later pages in this unit look at each method in turn.

Mixing is a physical change. Making a mixture does not usually give out or take in much energy, no new substance forms, and the change can be reversed by separation. Compare this with burning magnesium, where a new substance (magnesium oxide) forms and the change cannot be undone by simple physical means.

Mixtures can involve any state. A mixture might be solid in solid (a metal alloy or a bag of mixed nuts), solid in liquid (sea water, muddy water), liquid in liquid (vinegar), gas in liquid (fizzy water) or gas in gas (air).

Step-by-step reasoning

To decide whether a material is a mixture:

1. Ask whether it contains more than one substance. If yes, it could be a mixture or a compound. 2. Ask whether the substances are chemically joined. If they can be separated by a physical method, they are not joined. 3. Check whether the properties of each part are still present and whether the proportions can vary. 4. If the parts keep their properties and the proportions can change, it is a mixture.

Visual explanation

Imagine a jar containing red beads and blue beads shaken together. The beads are jumbled, but every red bead is still a red bead and every blue bead is still a blue bead. You could add more red beads, and you could pick them apart again by colour. That jar is a picture of a mixture: different particles side by side, not stuck together.

Real-world analogy

A fruit salad is a good analogy. Chunks of apple, grape and banana sit together in one bowl, but each piece keeps its own taste and texture. You can make it with more grapes or fewer, and you could pick out every piece of apple with a fork. Nothing new has been created.

Real-world example

Sea water is a mixture of water with dissolved salts, mainly sodium chloride, plus small amounts of dissolved gases. Its composition varies from place to place: the Dead Sea is far saltier than the Baltic Sea. Salt-making works by letting the water evaporate, a physical process that leaves the salt behind unchanged.

Why?

Why do the components of a mixture keep their properties? Properties come from the particles of a substance and the way they are bonded. In a mixture the particles of each component are unchanged; they have simply been spread among particles of another substance. With no new bonds formed, there is no new substance and so no new properties.

Common misconception

"When you mix two things they become a new substance." Mixing on its own does not create a new substance. If a new substance with different properties appears, a chemical reaction has happened and the result is not just a mixture.

Worked example

Question: Sand and salt are stirred together. Water is then added and stirred until the salt dissolves. Is the final material a mixture? Give two reasons.

Reasoning: The salt dissolves but is still salt: the liquid tastes salty, and evaporating the water gives salt crystals back. The sand is unchanged and can be filtered out. The amount of salt could be varied.

Answer: Yes. The components keep their own properties and can be separated by physical methods such as filtration and evaporation.

Quick check

1. Give one property of a mixture that shows its components are not chemically joined. Answer: The components keep their own properties, so they can be separated by physical methods.

Exam focus

A full-mark definition must include both ideas: two or more substances and not chemically combined. Many answers lose a mark by saying only "two or more things mixed together". Be ready to give examples of mixtures in different states, such as air, sea water and alloys.

Advanced insight

Some mixing does involve weak interactions. When ethanol and water are mixed, the final volume is slightly less than the sum of the two volumes, because the molecules pack together more closely through hydrogen bonding. No new substance forms, though, and distillation can still separate the two liquids, so the result is still a mixture.

Summary

A mixture contains two or more substances that are mingled but not chemically joined. Each component keeps its own properties, the composition can vary, and the components can be separated by physical methods that use differences in their physical properties. Mixing is a physical change, and mixtures can involve solids, liquids and gases.

Practice questions

1. Define the term mixture. Answer: Two or more different substances mingled together but not chemically joined. 2. Iron filings and sulfur powder are stirred together. Describe a simple way to show that the result is a mixture. Answer: Pass a magnet over it; the iron filings are attracted and removed, showing the iron keeps its magnetic property and is not chemically joined to the sulfur. 3. Explain why two samples of lemonade can both be mixtures even though one is sweeter than the other. Answer: Mixtures have no fixed composition, so the proportion of sugar can vary while the drink is still a mixture of the same components. 4. Name a mixture that is a gas mixed with a gas, and one that is a gas mixed with a liquid. Answer: Air is a gas in a gas; fizzy water (carbon dioxide dissolved in water) is a gas in a liquid.