Compounds Versus Mixtures

Introduction

Both air and water contain oxygen, yet they are very different kinds of material. Air is a mixture : its oxygen is simply mixed with nitrogen and other gases, and it still behaves like oxygen. Water is a compound : its oxygen is chemically bonded to hydrogen, and the result behaves nothing like oxygen. Telling compounds and mixtures apart is one of the most important skills in early chemistry, and it depends on a handful of clear differences.

Core explanation

The key difference. In a mixture, the substances are only physically mixed; each keeps its own particles and its own properties. In a compound, the elements are chemically combined by bonds, forming a new substance with new particles and new properties.

Five differences.

Feature Mixture Compound --- --- --- Composition Variable — any proportions Fixed — always the same ratio Properties Those of the separate substances New properties, different from the elements Separation Physical methods (filtering, magnet, distillation) Only chemical reactions Formation No chemical reaction; little or no energy change Chemical reaction; energy usually given out or taken in Melting/boiling Usually over a range of temperatures Sharp, fixed temperatures

The iron and sulfur example. Iron filings are grey and magnetic; sulfur is a yellow powder that is not magnetic.

- Mixture of iron and sulfur: stirring the two powders together gives a speckled grey-yellow mixture. You can use any proportions. A magnet pulls out the iron, leaving the sulfur behind. Both still have their own properties. - Compound of iron and sulfur: when the mixture is heated, the elements react and glow, giving out energy, to form a dark grey solid, iron(II) sulfide, FeS. This compound is not attracted to a magnet the way iron is, and it is not yellow. Iron and sulfur always combine in the same mass ratio, about 7 g of iron to 4 g of sulfur.

Why the classification matters. Many everyday materials are mixtures: air, sea water, alloys such as brass, and most foods. Pure water, salt and sugar are compounds. Knowing which is which tells you how the material can be separated and whether its composition can vary.

Step-by-step reasoning

To decide whether a material is a mixture or a compound:

1. Ask whether its composition can vary. If it can, it is a mixture. 2. Ask whether its components keep their own properties. If they do, it is a mixture. 3. Ask whether it can be separated by a physical method. If so, it is a mixture. 4. If it has a fixed composition, new properties, and needs a reaction to separate it, it is a compound.

Visual explanation

In a particle diagram of a mixture of iron and sulfur, grey circles and yellow circles are scattered separately among each other. In a diagram of iron(II) sulfide, every grey circle is bonded to a yellow circle, and the pairs are packed in a regular pattern. The mixture has two kinds of separate particle; the compound has joined units.

Real-world analogy

A mixture is like a bowl of fruit salad: you can put in any amount of each fruit, and you can pick out the grapes with a fork. A compound is like a boiled egg: the white and yolk have been changed by a chemical process, and you cannot turn it back into a raw egg by any sorting method.

Real-world example

Air is a mixture of about 78% nitrogen and 21% oxygen, and industry separates it by cooling it until it liquefies and then distilling it, a purely physical process. Carbon dioxide in that air, however, is a compound: the carbon and oxygen in it cannot be separated by distillation.

Why?

Why does a compound have a fixed composition when a mixture does not? In a compound, each particle is made of atoms bonded in a definite pattern, such as one iron atom to one sulfur atom in FeS. Every particle is the same, so the ratio is fixed. In a mixture, the separate particles can be added in any numbers, so the ratio can be anything.

Common misconception

"A mixture of two elements is a compound because it contains two elements." Containing two elements is not enough. The elements must be chemically bonded. A mixture of iron and sulfur powders contains two elements but is still a mixture until a chemical reaction takes place.

Worked example

Question: A solid is made by heating shiny magnesium ribbon in oxygen. It is white, is not shiny, and always contains 3 g of magnesium for every 2 g of oxygen. Is it a mixture or a compound?

Reasoning: It was formed by a chemical reaction, it has new properties (white and dull, unlike shiny magnesium), and its composition is fixed.

Answer: It is a compound, magnesium oxide.

Quick check

1. How could you separate a mixture of iron filings and sulfur powder? Answer: Use a magnet to attract the iron filings.

Exam focus

The iron and sulfur comparison is a classic exam question. Be ready to give at least three differences between the mixture and the compound, including appearance, magnetism, energy change on forming and method of separation. Always mention "fixed composition" for compounds.

Advanced insight

Alloys blur the boundary a little. Brass, a mixture of copper and zinc, can be made in a wide range of compositions, so it is classed as a mixture. Some alloys, however, contain small regions where the metals combine in fixed ratios, forming what are called intermetallic compounds. Careful analysis of composition and structure is needed to classify such materials.

Summary

In a mixture, substances are physically mixed, keep their own properties, can be in any proportions, and can be separated by physical methods. In a compound, elements are chemically bonded in a fixed ratio, have new properties, form in a reaction with an energy change, and can only be separated chemically. The iron and sulfur example shows all of these differences clearly.

Practice questions

1. Give two differences between a mixture and a compound. Answer: A mixture has variable composition and can be separated physically; a compound has fixed composition and can only be separated by chemical reactions. 2. Why is sea water classed as a mixture? Answer: Its salt content can vary, and the water and salts can be separated by physical methods such as evaporation or distillation. 3. What evidence shows that a chemical reaction happens when iron and sulfur are heated together? Answer: The mixture glows as energy is released, and a new dark grey solid forms that is not attracted to a magnet like iron. 4. Classify each as an element, compound or mixture: air, carbon dioxide, copper, brass. Answer: Air — mixture; carbon dioxide — compound; copper — element; brass — mixture.