Pure substances and mixtures

What does "pure" mean?

A shop may say "pure milk", "pure water" or "pure honey". In daily life, "pure" means that nothing harmful has been added. In science, the word has a stricter meaning.

A pure substance is made of only one kind of matter. All its particles are the same. Distilled water is a pure substance. So is a piece of gold with nothing else in it.

Most things around us are not pure. Milk has water, fat, sugar and proteins. Air has several gases. Soil has sand, clay and many other things. These are mixtures.

What is a mixture?

A mixture has two or more pure substances, which are called its constituents (or components). The constituents are mixed, but they are not joined by a chemical change, and they can be in any amount.

Think of sugar and salt mixed in a bowl. We can add a little sugar and a lot of salt, or the other way. Both are still a mixture. We can often separate the parts by simple means.

In a mixture, each part keeps its own properties. In a mixture of iron filings and sulphur, the iron is still attracted by a magnet, and the sulphur still burns with a blue flame. If we heat them together, a chemical change happens, and a compound, iron sulphide, is formed. Its properties are new: it is not attracted by a magnet as before, and it does not burn like sulphur. So a mixture and a compound are very different, even when they have the same elements.

Pure substances: elements and compounds

A pure substance can be of two kinds.

• An element has one kind of atom, such as oxygen, iron or gold.
• A compound has two or more elements joined chemically in a fixed ratio, such as water, salt or carbon dioxide.

So the whole of matter can be arranged like this: matter is either a pure substance (an element or a compound) or a mixture (homogeneous or heterogeneous).

Two kinds of mixture

A homogeneous mixture has the same composition and the same properties everywhere in it. You cannot see the different parts. Salt dissolved in water is homogeneous. A spoonful from the top tastes as salty as a spoonful from the bottom. Air, sugar in water, and brass are other examples. A homogeneous mixture is also called a solution.

A heterogeneous mixture has parts that are different from place to place, and you can often see them. Sand in water, oil in water, and a salad are heterogeneous.

Alloys

An alloy is a mixture of two or more metals (or of a metal and another solid), made by melting them together. Brass is a mixture of copper and zinc. It is a homogeneous mixture of solids. An alloy has properties that are different from its parts: brass is harder than copper.

Other alloys are bronze (copper and tin), steel (iron with a little carbon), and solder (tin and lead). Alloys are mixtures, so the amounts of the metals can change. Gold of 22 carats is an alloy that has $22$ parts of gold in $24$ parts, with copper or silver for the rest, to make it harder.

How to tell a pure substance from a mixture

A practical test is the melting point. Pure ice melts at exactly $0\ ^\circ\text{C}$. Ice that is made from salt water melts at a lower temperature, and over a range. A pure substance boils at one temperature. A mixture boils over a range of temperatures.

You should find that sea water, with about $3.5\ \text{g}$ of salt in each $100\ \text{g}$ of water, freezes near $-2\ ^\circ\text{C}$, not at $0\ ^\circ\text{C}$.

Sorting some everyday things

Practise with new numbers

Try it in Python

The program decides which samples are pure from how sharply they melt.

Common mistakes

• Using "pure" in the everyday sense. Pure milk from a dairy is still a mixture in science.
• Thinking that a mixture is always easy to see. A homogeneous mixture looks like one substance.
• Thinking that a compound is the same as a mixture. In a compound, the elements are joined chemically in a fixed ratio.
• Calling an alloy a pure metal. An alloy is a mixture.

Remember these

• A pure substance has only one kind of matter. A mixture has two or more kinds.
• A homogeneous mixture has the same composition everywhere. A heterogeneous mixture does not.
• An alloy is a mixture of metals. Air and sea water are mixtures too.
• A pure substance has fixed properties, such as a fixed melting point.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Is a glass of fresh orange juice a pure substance in the scientific sense? Why?

2. Give one example each of a homogeneous mixture and a heterogeneous mixture from your kitchen.

3. Steel is iron with a little carbon. Is steel an element, a compound or a mixture?

4. A sample of ice melts between $-3\ ^\circ\text{C}$ and $-1\ ^\circ\text{C}$. Was it made from pure water?