Coaching Centre
Mixtures
What a mixture is, how mixtures differ from compounds, homogeneous and heterogeneous mixtures, alloys, classifying matter using particle diagrams, and why Australian beaches have different coloured sand.
The air you are breathing right now is not a single substance. Every breath is a mixture of nitrogen, oxygen, argon, carbon dioxide, water vapour and traces of a dozen other gases, all jumbled together without being joined to each other.
What is a mixture?
A mixture contains two or more different substances (elements and/or compounds) that are not chemically bonded to each other. Each substance keeps its own properties, and the amounts can vary. Because nothing is bonded, mixtures can be separated by physical methods such as filtering, evaporating, distilling or using a magnet, which you met in Year 7.
| Compound | Mixture | |
|---|---|---|
| Made of | Two or more elements | Two or more elements and/or compounds |
| Bonded? | Yes, chemically bonded | No, just mixed together |
| Proportions | Fixed: water is always H2O | Can vary: a cordial can be weak or strong |
| Properties | Different from its elements | Each part keeps its own properties |
| Separating | Only by chemical reactions | By physical methods |
Here's a quick way to remember it: changing the ratio in a compound makes a different substance. O2 is the oxygen we breathe; O3 (ozone) is a toxic gas with a sharp smell. But changing the ratio in a mixture just gives a slightly different version of the same mixture. Grandma's chocolate cake might have more chocolate than yours, but it's still chocolate cake.
Two types of mixture
A homogeneous mixture looks the same all the way through, because the different particles are spread evenly. You can't see the separate parts, even with a microscope. Examples: air, seawater, cordial, vinegar, and alloys such as brass.
A heterogeneous mixture is not the same all the way through: you can see different parts, or different samples contain different amounts. Examples: sand, soil, granite, muesli, fruit cake and choc-chip cookie dough (one spoonful might have three chips and the next none).
Alloys
An alloy is a mixture of a metal with other elements (usually other metals, or carbon). Alloys are designed to have better properties than the pure metal:
| Alloy | Made of | Why it's useful |
|---|---|---|
| Steel | Iron + a little carbon (usually under 2%) | Much stronger and harder than pure iron |
| Stainless steel | Iron + chromium (at least 10.5%) + often nickel | Doesn't rust: cutlery, sinks, surgical tools |
| Bronze | Copper + tin | Harder than copper; resists corrosion: statues, bells, medals |
| Brass | Copper + zinc | Golden colour, easy to shape: musical instruments, door handles |
| Solder | Mainly tin (once tin + lead) | Low melting point: joins electronic parts |
| 18-carat gold | 75% gold + silver and copper | Harder-wearing than pure gold for jewellery |
- Look at each box carefully. How many kinds of particle are there?
- Inside each particle, are the atoms all the same or different?
- Try to score 10 in a row. Which type do you find hardest to spot?
Test yourself
Which is a heterogeneous mixture?
You can see the different parts in muesli.
Which statement about mixtures is true?
Nothing is bonded, so properties are kept.
Steel is
Bronze is copper and tin.
A box contains particles that are all identical, and each particle has one red atom bonded to two white atoms. It shows
One kind of particle made of different atoms: a compound (like H₂O).
Stainless steel doesn't rust because it contains
Chromium forms a protective oxide layer.
The red colour of Central Australian sand comes from
A thin coating of iron oxide on the grains.
Complete the classification of matter.
Classify each substance.
A mixture contains substances that are not chemically . A mixture that looks the same throughout is . Sand is a mixture. A mixture of a metal with other elements is an . Brass is made of copper and .
A student mixes iron filings with yellow sulfur powder. Then the student heats the mixture strongly and it glows, forming a black solid that a magnet does not attract. Explain which substance at each stage is a mixture and which is a compound, using evidence from the description.
Before heating, the iron and sulfur are a mixture. The two elements are just mixed, not bonded: each keeps its own properties, so the iron is still magnetic and the sulfur is still yellow. A magnet could pull the iron out and separate them.
When heated, the mixture glows, which shows a chemical reaction is giving out energy. The black solid is a new compound, iron sulfide, in which iron and sulfur atoms are chemically bonded in a fixed ratio. It has different properties from both elements: it is black and not attracted to a magnet, so the iron can no longer be separated by physical means.