Comparing Compounds and Mixtures
A compound is a pure substance in which different elements are chemically bonded in a fixed ratio, while a mixture contains substances that are only physically combined and can be present in any proportion. In brief: iron filings and sulfur powder form a mixture that a magnet can separate, but heating them together produces iron sulfide, a compound with new properties that a magnet no longer separates.
The difference is one of the most useful ideas in chemistry because it tells us which separation methods can work. This article compares compounds and mixtures in detail, describes the classic iron and sulfur experiment, lists the criteria that distinguish them, works through a mass example and gives common errors.
What are the key differences?
- Bonding: in a compound, atoms of different elements are held together by chemical bonds. In a mixture there are no new bonds between the different substances.
- Composition: a compound has a fixed composition and a formula. A mixture can have any composition.
- Properties: a compound has properties that differ from those of its elements. A mixture shows the properties of its parts.
- Separation: a compound can only be broken down by a chemical reaction. A mixture can be separated by physical methods.
- Energy: forming a compound usually releases or absorbs a noticeable amount of energy. Mixing usually involves little or no energy change.
Compounds and mixtures compared
| Feature | Compound | Mixture |
|---|---|---|
| Components bonded? | Yes | No |
| Fixed composition? | Yes | No |
| Properties | New, different from elements | Those of the components |
| Separation method | Chemical reaction | Physical methods |
| Melting and boiling point | Sharp | A range |
| Written as | A formula (H2O) | A description (air, salt water) |
The iron and sulfur experiment
This standard experiment shows both a mixture and a compound made from the same elements.
- Make the mixture. Mix iron filings and yellow sulfur powder. The mixture is grayish-yellow and the two components are visible with a hand lens.
- Test the mixture. A magnet pulls out the iron, and sulfur stays behind. Adding water, the sulfur floats and the iron sinks. The properties of each element are still present, so it is a mixture.
- Heat the mixture. Heat it strongly in a test tube. The mixture glows red as a reaction occurs and releases heat.
- Test the product. The black solid, iron(II) sulfide (FeS), is not attracted to a magnet, and it does not float on water. It has new properties, so it is a compound.
This is carried out by teachers in a fume cupboard, since sulfur dioxide may form. The reaction is: Fe + S → FeS.
Classifying common materials
| Material | Class | Reason |
|---|---|---|
| Water (H2O) | Compound | Fixed ratio of H and O, bonded |
| Sea water | Mixture | Water, salts and gases in variable amounts |
| Carbon dioxide | Compound | C and O in a 1 : 2 atom ratio |
| Air | Mixture | Gases that are not bonded |
| Rust (iron oxide) | Compound | Iron and oxygen chemically combined |
| Brass | Mixture (alloy) | Copper and zinc, variable proportions |
| Baking soda (NaHCO3) | Compound | Na, H, C and O in a fixed ratio |
Worked example: mixture or compound?
A student has two samples that both contain only iron and sulfur.
- Sample A: 14.0 g of iron and 8.0 g of sulfur, mixed by shaking. A magnet removes 14.0 g of iron. The components are separated physically, so A is a mixture.
- Sample B: the same quantities heated until the reaction is over. The black solid has a mass of 22.0 g, is not magnetic and releases hydrogen sulfide gas with acid, which smells of rotten eggs. B is a compound, iron(II) sulfide.
- Mass check. 14.0 + 8.0 = 22.0 g, which agrees with the conservation of mass.
- Mass ratio. In FeS, the ratio is Fe : S = 55.85 : 32.07 ≈ 1.74 : 1. With 14.0 g of iron, the sulfur needed is 14.0 ÷ 1.74 = 8.04 g, which matches the 8.0 g used. Both reactants were used up.
If 14.0 g of iron had been heated with 12.0 g of sulfur, the extra 4.0 g of sulfur (about) would remain unreacted. This shows another way that a compound differs from a mixture: only a specific ratio reacts.
How are they separated?
Mixtures can be separated by methods such as filtration, magnetism, distillation and chromatography. Separating a compound requires chemical reactions, for example heating mercury(II) oxide to give mercury and oxygen, or passing a current through molten sodium chloride to obtain sodium and chlorine. Such methods are used to extract metals from their ores.
How did chemists learn the difference?
In the late 1700s, Joseph Proust measured the compositions of many compounds and found fixed ratios. He argued with Claude Louis Berthollet about whether compounds could have variable composition, and by about 1808 the matter was settled in Proust’s favor for most compounds. John Dalton’s atomic theory explained why: atoms combine in whole-number ratios. Later it was found that some solids have slightly variable composition, which are called nonstoichiometric compounds, but the rule remains true for most substances at school level.
Which mistakes are common?
Mistake 1: Judging by appearance alone
A uniform-looking substance may be a mixture, such as salt water, or a compound, such as water.
Mistake 2: Thinking that mixing releases a compound
Stirring iron and sulfur together does not make iron sulfide. A reaction is needed.
Mistake 3: Assuming that heating a mixture always makes a compound
Heating can also just melt or evaporate parts of a mixture without any reaction.
Mistake 4: Saying that a compound is a mixture of elements
The elements in a compound are bonded and have lost their individual properties, so it is a different type of substance.