Mixtures in Everyday Life
In 2015, CSIRO's air quality monitoring confirmed Sydney's atmosphere is 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% CO₂, plus dozens of trace gases — a mixture, never a pure substance, in every breath you take.
Printable Worksheets
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Q1 · List three things in your kitchen right now that you think are mixtures. What clue made you decide?
Q2 · A 10-cent coin looks like one shiny silver-coloured metal. Could it actually be a mixture? Why might mixing two metals be better than using a pure metal?
● Know
- The rough composition of air (78% N₂, 21% O₂, 1% other)
- That seawater is a solution of salt and minerals in water
- That an alloy is a mixture of metals (steel, bronze, brass)
● Understand
- Why mixtures (not pure substances) dominate the natural world
- Why an alloy is still classed as a mixture even though it's solid
- Why engineers often choose an alloy over a pure metal
● Can do
- Identify whether a real-world material is a pure substance or a mixture
- Name the main components of air, seawater and three common alloys
- Give one practical advantage of using an alloy
- Mixture
- Alloy
- Solution
- Pure substance
- Solid solution
- A mixture where one substance dissolves fully into another
- A solid where one solid is dissolved evenly into another
- Two or more substances physically combined
- Only one type of particle — an element or a compound
- A solid mixture of metals like steel or bronze
When we say "I'm breathing oxygen", we're being a bit sloppy. The air around you is a mixture of several gases, and oxygen is only the second most common one.
| Gas | Symbol | Approx. % |
|---|---|---|
| Nitrogen | N₂ | 78% |
| Oxygen | O₂ | 21% |
| Argon | Ar | ~0.9% |
| Carbon dioxide | CO₂ | ~0.04% |
| Water vapour + traces | — | varies |
Air is a homogeneous mixture — the gases are evenly mixed and you cannot see the separate components. Because it's a mixture, the proportions can change: a sealed room with lots of people will slowly have more CO₂ and less O₂ as everyone breathes.
Air is a mixture. The most common gas is at about %. The second most common is at about 21%. The rest is mostly and traces of carbon dioxide.
Seawater looks like water, but every litre of it contains about 35 grams of dissolved minerals. The biggest one is sodium chloride (table salt). It also contains magnesium, calcium, sulfate and tiny amounts of almost every element on Earth.
Seawater is a solution: the salts have dissolved completely into the water so you cannot see them as separate grains. If you boil seawater dry, the water evaporates and the salt is left behind — proof that it was a mixture all along.
- Pure water (H₂O) = a compound (one substance).
- Seawater = water + dissolved salts = a mixture.
- Drinking seawater dehydrates you because your kidneys can't process that much salt.
An alloy is a solid mixture made by melting two or more metals together and letting them cool. Even though it looks like a single substance, the atoms of each metal stay separate — they are not chemically joined, so an alloy is still a mixture.
| Alloy | Made from | Why it's used |
|---|---|---|
| Steel | Iron (Fe) + a small amount of Carbon (C) | Much stronger and harder than pure iron. Used in buildings, cars, rails. |
| Bronze | Copper (Cu) + Tin (Sn) | Harder and more rust-resistant than copper. Used in statues, medals, ship parts. |
| Brass | Copper (Cu) + Zinc (Zn) | Bright gold colour, easy to shape. Used in instruments (trumpets, door handles). |
Most alloys are solid solutions — the atoms of one metal slip in between the atoms of the other. Engineers love alloys because you can tune the properties (hardness, colour, rust-resistance) by changing the mix.
Wrong: "If it looks like one substance, it must be pure." Air looks like nothing. Seawater looks like water. Steel looks like iron. Looking uniform doesn't make it pure — many mixtures are perfectly even.
Right: A mixture can look uniform (homogeneous) and still be a mixture. The test is the particles: more than one type of particle = mixture.
Wrong: "An alloy is a compound because the metals are joined." Melting two metals together doesn't make a chemical bond between their atoms. They just sit mixed together when the liquid cools.
Right: An alloy is a mixture. The metals are physically mixed, not chemically joined. That's why alloys don't have a fixed formula like compounds do.
Wrong: "Pure water is what you get out of the tap." Tap water contains dissolved minerals (calcium, fluoride, chlorine) and is a mixture. "Pure" water is distilled water with nothing dissolved in it.
Right: Pure water is only H₂O. Tap water, mineral water and seawater are all mixtures because they contain dissolved substances.
If you walked outside and tried to find a sample of something perfectly pure, you'd struggle. Soil is a mixture. Rocks are mixtures of minerals. The oceans are mixtures. The atmosphere is a mixture. Even the gold in jewellery is usually mixed with silver or copper to make it harder.
Pure substances usually only exist because someone has worked hard to separate them from a mixture in a lab or a factory. That's why the next few lessons focus on how scientists do that.
- Pure copper for electrical wires → extracted and refined from ore.
- Pure aluminium for cans → extracted from bauxite ore.
- Pure salt for chemistry → evaporated from seawater or mined and purified.
So when you see a label saying "pure" something, it usually means a human has done a lot of work to make it that way.
Pure iron is fairly soft and bends easily. Add about 1% carbon and you get steel, which is much stronger and harder. Predict: why does adding a tiny bit of a non-metal (carbon) make iron so much tougher?
How close was your prediction?
Great — you saw that the extra atoms physically block the iron rows from sliding.
Good — alloys are a clever case of the particle model in action. Tiny "wrong-sized" atoms lock the metal in place.
At the start of this lesson you were asked: The air in your lungs is not "oxygen" — it's a cocktail of at least 5 different gases. What are they? Can you name them all now?
Write a fuller answer listing the gases in air, then explain what makes air a mixture (not a compound) using the words homogeneous mixture, nitrogen, oxygen and properties.
Q1. Name the two main gases in air and give their approximate percentages. Name one other gas in air and its approximate percentage. (3 marks)
Q2. For each material, state whether it is a pure substance or a mixture, and justify your answer in one sentence: (a) tap water, (b) distilled water, (c) bronze medal, (d) a copper wire. (4 marks)
Q3. An engineer is designing a new coin. Should it be made of pure copper or an alloy like brass? Justify your answer using at least two properties. (4 marks)
Answers
▾MCQ 1
C — Nitrogen is about 78% of air. Oxygen is the second most common at ~21%. Argon and CO₂ make up the rest.
MCQ 2
A — Steel is iron plus a small amount of carbon. Bronze is copper + tin; brass is copper + zinc.
MCQ 3
D — Distilled water is only H₂O, so it is a pure compound. Air, seawater and brass are all mixtures.
MCQ 4
B — In an alloy, the different metal atoms are physically mixed but not chemically bonded, and the ratio can change. Compounds have chemical bonds and fixed ratios.
MCQ 5
C — Seawater contains about 35 g of dissolved minerals per litre, mostly sodium chloride.
Short Answer 1
Model answer: Nitrogen (~78%) and oxygen (~21%) are the two main gases in air. Other gases include argon (~0.9%) and carbon dioxide (~0.04%). Air also contains water vapour, which varies day-to-day.
Short Answer 2
Model answer: (a) Tap water = mixture (it contains dissolved fluoride, chlorine and minerals). (b) Distilled water = pure substance (it is only H₂O). (c) Bronze medal = mixture (an alloy of copper and tin, not chemically joined). (d) Copper wire = pure substance (just one element, Cu).
Short Answer 3
Model answer: The coin should be made of an alloy like brass, not pure copper. Pure copper is too soft and would dent and wear out quickly under daily handling — the design and number details would fade. Brass (copper + zinc) is much harder and more rust-resistant, so the coin keeps its shape and shine for years. Brass is also cheaper than pure copper because zinc is added. (Accept any two valid properties: hardness, durability, rust resistance, cost.)