Mixtures vs Compounds

Mixtures and compounds are two of the most important ideas in chemistry, and people often confuse them. The difference is simple but important. A compound is a substance where atoms of two or more elements are chemically bonded together to form a brand new substance. A mixture is when two or more substances are just mixed together physically, without forming any chemical bonds. The substances in a mixture keep their original properties and can usually be separated again, while a compound has totally different properties from its ingredients and is much harder to take apart.

  • CompoundAtoms chemically joinedFixed ratio, new substance
  • MixtureSubstances physically combinedVariable ratio, originals preserved
  • Compound exampleWater (H2O)Always 2H per O
  • Mixture exampleSalt waterCan be any salt-to-water ratio
  • Separating a compoundChemical reactionHard work, breaks bonds
  • Separating a mixturePhysical methodsFilter, distill, evaporate

The key differences

  • Compounds have a fixed ratio of ingredients. Water is always 2 hydrogen atoms to 1 oxygen atom (H2O), no exceptions.
  • Mixtures have a variable ratio. Salt water can be slightly salty (like rain) or very salty (like the Dead Sea), and it is still salt water.
  • Compounds have completely new properties. Sodium is a soft metal that explodes in water. Chlorine is a poisonous green gas. Combine them and you get harmless table salt.
  • Mixtures keep the original properties of their ingredients. Salty water still tastes salty AND wet.
  • Compounds can only be separated by chemical reactions, breaking the bonds between atoms.
  • Mixtures can be separated by physical methods: filtering, evaporation, distillation, magnets, sieving.

How to tell which is which

If you have a substance and you are not sure whether it is a compound or a mixture, ask yourself:

  • Can the ratio change? Yes = mixture. No = compound.
  • Do the ingredients keep their original properties? Yes = mixture. No = compound.
  • Can you separate it using simple physical methods? Yes = mixture. No = compound.
  • Is the new substance very different from its ingredients? Yes = probably a compound.

Common examples

Compounds:

  • Water (H2O)
  • Salt (NaCl)
  • Sugar (C12H22O11)
  • Carbon dioxide (CO2)
  • Most medicines
  • Plastics

Mixtures:

  • Air (nitrogen + oxygen + others)
  • Sea water (water + salts)
  • Milk (water + fats + proteins + sugar)
  • Salad (lettuce + tomato + cucumber)
  • Most rocks (different minerals)
  • Blood (water + cells + proteins + nutrients)
Fact The same atoms can form completely different substances depending on whether they are mixed or chemically combined. Take two glass tubes: fill one with hydrogen and oxygen gas mixed together (a mixture), and the other with water (the compound H2O). They contain the same elements, but the mixture will explode if you light a match near it, while the water will put out the match. That is the difference between mixing and bonding.

How chemists tell the difference

In a chemistry lab, scientists have several precise tools for telling mixtures from compounds.

  • Melting and boiling points: a pure compound has a single sharp temperature where it melts or boils. A mixture melts and boils over a range of temperatures.
  • Chemical analysis: tells you exactly what is in the substance.
  • Spectroscopy: each compound absorbs and emits light in unique patterns, like a fingerprint.
  • Chromatography: separates the parts of a mixture so you can see how many different substances are in it.
Did you know? Some substances look like compounds but are actually mixtures. Steel, for example, is a mixture of iron with small amounts of carbon (and sometimes other metals like nickel or chromium). It looks uniform, but the ingredients are not chemically bonded into a new molecule; the iron and carbon atoms just sit together in the same metal. Mixtures of metals like this are called alloys.
Deeper dive: why some "compounds" used to fool scientists

The distinction between mixtures and compounds was actually quite difficult for early chemists to work out. For centuries, people thought all kinds of substances were "elements" that turned out to be either compounds or mixtures, and vice versa. Water was thought to be an element for over 2,000 years; only in the 1780s did Henry Cavendish and Antoine Lavoisier prove it was actually a compound of hydrogen and oxygen.

The key tool was the careful measurement of mass. The French chemist Joseph Proust noticed in the 1790s that every sample of a particular compound always contained the same ratio of ingredients by mass. Pure copper carbonate always had the same percentage of copper, the same percentage of carbon and the same percentage of oxygen. He called this the law of definite proportions. It only works for compounds; mixtures have variable proportions.

The same idea explained why early chemists kept finding "compounds" that gave different analyses each time. They were actually working with mixtures, not pure substances. Once chemists realised that pure compounds have fixed ratios, they could systematically distinguish them from mixtures and the whole science of chemistry became much more rigorous.

For more, see solutions, separating mixtures and common compounds.