Solubility and Magnetism in Materials

Solubility is the ability of a substance to dissolve in a liquid, and magnetism is the ability of a material to be attracted to a magnet. Both are physical properties that can be used to sort and identify materials. In brief: sugar and salt dissolve in water but sand does not, and iron is attracted to a magnet but copper and aluminum are not.

These two properties are simple to test and surprisingly useful. Solubility helps to separate mixtures and make solutions, and magnetism lets us pick out iron-containing objects from waste. This article describes both properties, explains how to test them, compares common materials in tables and shows a worked example of separating a mixture.

What is solubility?

A substance that dissolves is called soluble, and one that does not is called insoluble. When sugar is stirred into water, it disappears into the liquid and forms a clear solution. Sand stays at the bottom as separate grains, so it is insoluble in water.

Solubility depends on both the substance and the liquid. Salt dissolves well in water but hardly at all in cooking oil. Oil dissolves in other oils but not in water. A useful rule of thumb is that a substance often dissolves in a similar kind of liquid.

Temperature matters too. Most solids dissolve more in warm water than in cold water, while gases such as carbon dioxide dissolve less in warm liquids. That is why a warm soda goes flat faster than a cold one.

What is magnetism in materials?

A magnet attracts some materials strongly and ignores others. Materials that are strongly attracted are called magnetic (or ferromagnetic). The common examples are iron, nickel, cobalt and steel, which is mostly iron.

Most other materials are not noticeably attracted. Copper, aluminum, gold, wood, plastic and glass do not stick to an ordinary magnet. Some stainless steels are also non-magnetic or only weakly magnetic, so a magnet is not a perfect test for steel.

How can you test these properties?

Testing solubility

1. Measure 100 mL of water into a clear beaker or glass.
2. Add one level teaspoon of the sample and stir for 30 seconds.
3. Observe whether the solid disappears and the liquid is clear (soluble), or whether solid remains or the liquid is cloudy (insoluble or only slightly soluble).

Keep the amount, temperature and stirring time the same for each sample, otherwise the comparison is not fair.

Testing magnetism

Hold a magnet close to each object, then touch it. Record whether the object jumps toward the magnet, sticks to it or shows no reaction. Do not use strong magnets near phones, credit cards or medical devices.

Solubility and magnetism of common materials

Worked example: separating a mixture

A sample contains iron filings, sand and salt. The task is to separate all three using only these two properties and simple equipment.

1. Remove the iron filings. Pass a magnet over the dry mixture. The filings stick to the magnet and are lifted away.
2. Dissolve the salt. Add water to the remaining sand and salt and stir. The salt dissolves, and the sand does not.
3. Remove the sand. Pour the mixture through filter paper. The sand stays on the paper, and the salt solution passes through.
4. Recover the salt. Gently heat the solution until the water evaporates, leaving the salt crystals.

Magnetism handled the first step, and solubility handled the last three. Each substance was recovered in its original form, which shows that the changes were physical.

Where are these properties used?

• Recycling: large magnets pull steel cans out of mixed waste.
• Food and drink: instant coffee and drink powders are chosen for their solubility.
• Medicine: the solubility of a drug influences how fast the body absorbs it.
• Mining: magnetic separators extract iron ores from crushed rock.

How were magnets discovered?

The natural magnetic rock lodestone, a form of magnetite, was known in ancient Greece and in China. Early writers described its attraction to iron. By around the 11th and 12th centuries, magnetic compasses were being used for navigation in China and, later, in Europe. In 1600 William Gilbert published De Magnete, in which he argued that the Earth itself behaves like a giant magnet.

Which mistakes are common?

Mistake 1: Confusing melting with dissolving

Melting is a change of state caused by heat. Dissolving is the mixing of one substance with a liquid to form a solution.

Mistake 2: Thinking that all metals are magnetic

Only a few metals, notably iron, nickel and cobalt, are strongly attracted to a magnet. Aluminum, copper and gold are not.

Mistake 3: Believing that a cloudy liquid means a substance dissolved

A true solution is clear, although it may be colored. A cloudy liquid usually contains tiny undissolved particles.

Mistake 4: Assuming that a substance that disappears is destroyed

Dissolved sugar is still present, spread out among the water particles. Evaporating the water recovers it.