Magnetism

Introduction

Hold a magnet near a paper clip and it jumps across the gap. Hold it near a copper coin, an aluminium can or a gold ring and nothing happens. Being attracted to a magnet is a physical property that only a few materials show strongly. It provides a quick, non-destructive test and a practical way of separating mixtures.

Core explanation

The magnetic elements. Only three common elements are strongly attracted to magnets at room temperature: iron , nickel and cobalt . They are called ferromagnetic materials. Many alloys containing them, such as most steels, are also magnetic. (Gadolinium becomes ferromagnetic just below room temperature, but it is rarely met.)

Most metals are not magnetic. Copper, aluminium, zinc, gold, silver, lead and tin are not attracted noticeably by an ordinary magnet. This surprises many people, who assume "metal" means "magnetic". Some types of stainless steel are also non-magnetic because of how their atoms are arranged.

Non-metals are not magnetic. Wood, plastic, glass, sulfur and carbon are not attracted by magnets.

Permanent and temporary magnets. A permanent magnet keeps its magnetism — for example, a fridge magnet or a compass needle. Iron can be magnetised temporarily: a nail held against a magnet can pick up other pins, but loses most of its magnetism when removed. Steel keeps magnetism longer and is used to make permanent magnets.

Magnetism can be lost. Heating a magnet strongly or hammering it can destroy its magnetism. Above a certain temperature (770 °C for iron), even iron stops being ferromagnetic.

Uses of magnetism. Magnets separate iron and steel from mixtures — for example, in recycling plants, cans made of steel are lifted out with large electromagnets, while aluminium cans are left behind. Magnetism is also used in compasses, electric motors, loudspeakers and computer hard drives.

Step-by-step reasoning

To separate a mixture of iron filings and sulfur powder:

1. Spread the mixture thinly on paper. 2. Wrap a magnet in cling film or place it inside a plastic bag (so the filings can be released easily). 3. Pass the magnet over the mixture: the iron filings stick to it. 4. Remove the magnet over a separate sheet of paper and pull off the covering to release the iron. 5. The sulfur remains behind. This works because iron is magnetic and sulfur is not.

Visual explanation

A table of samples with a tick or cross for "attracted by magnet": iron nail ✓, steel paper clip ✓, nickel coin ✓, copper wire ✗, aluminium foil ✗, brass key ✗, plastic ruler ✗. A second picture shows iron filings lining up along the field lines around a bar magnet.

Real-world analogy

Magnetism is like a club that only admits three members — iron, nickel and cobalt — and a few of their relatives (alloys). Everyone else, however "metallic" they look, is left outside the door.

Real-world example

Waste-sorting plants use powerful magnets to pull steel cans out of mixed recycling. Aluminium cans, which are not magnetic, pass by, and are then removed by a different technique that induces currents in them. Separating metals this way allows both to be recycled efficiently.

Why?

Why are only a few metals magnetic? Each atom behaves like a tiny magnet because of its electrons. In most materials these tiny magnets point in random directions and cancel out. In iron, nickel and cobalt, neighbouring atoms line up their tiny magnets in the same direction in regions called domains, so the effects add up and the material is strongly attracted by a magnet.

Common misconception

"All metals are magnetic." Most metals are not. A magnet test can therefore help identify metals: a silvery metal attracted by a magnet is likely to be iron, nickel, cobalt or a steel; one that is not attracted could be aluminium, zinc, tin or several others.

Worked example

Question: A student has two silvery coins of similar density. One is attracted to a magnet and the other is not. What can be concluded?

Reasoning: The attracted coin must contain a significant amount of iron, nickel or cobalt. The other coin contains none of these in magnetic form.

Answer: The first coin contains a magnetic metal (such as nickel or steel); the second is made of non-magnetic metals.

Quick check

1. Name the three common magnetic elements. Answer: Iron, nickel and cobalt.

Exam focus

Remember that only iron, nickel and cobalt (and alloys such as steel) are magnetic among common metals. Questions on separation expect you to choose magnetic separation for mixtures containing iron and to explain why it works. Magnetism is a physical property: testing it does not change the substance.

Advanced insight

In the chemistry of iron and sulfur, magnetism shows the difference between a mixture and a compound. In an iron–sulfur mixture the iron can be pulled out with a magnet. After heating, the iron and sulfur combine to form iron sulfide, a compound that is not attracted to a magnet in the same way — evidence that a new substance has formed.

Summary

Magnetism is a physical property shown strongly by iron, nickel, cobalt and their alloys such as steel. Most metals, and all common non-metals, are not magnetic. Magnets are used to test materials and to separate magnetic substances from mixtures, as in recycling and in separating iron from sulfur.

Practice questions

1. Which would a magnet pick up: a steel spoon, an aluminium can, a nickel coin, a copper pipe? Answer: The steel spoon and the nickel coin. 2. How could you separate iron filings from sand? Answer: Use a magnet (wrapped in plastic); the iron filings stick to it and the sand stays behind. 3. Why is being magnetic considered a physical property? Answer: It can be tested without changing the substance into a new substance. 4. How do recycling plants separate steel cans from aluminium cans? Answer: Large magnets attract the steel cans; aluminium, which is not magnetic, is left behind.