Salt in seawater, colours in a black marker, gold in river sand — and the clever tricks that pull them apart.
Almost nothing in the real world is pure. Seawater is water + salt + minerals. A felt-tip pen's "black" ink is usually a secret blend of blues, purples and pinks. Even the air is a mixture of gases. In this lesson you'll learn the difference between a pure substance and a mixture, meet the chemist's toolbox for un-mixing things, and use the maths you already know — ratios and percentages — to say exactly how strong a mixture is.
Two key words:
A pure substance is made of only one kind of particle — nothing else mixed in (e.g. pure water, pure salt, pure gold).
A mixture is two or more substances jumbled together but not chemically joined — so they can be separated again using physical tricks, without any reaction.
A solution is a special mixture where one thing (the solute, e.g. salt) completely dissolves and disappears into a liquid (the solvent, e.g. water). It looks clear, but the salt is still there — hiding between the water particles.
🔺 The chemist's trick — picture the particles
A pure substance in the particle view is all the same circles. A mixture is different circles jumbled together. Because nothing is chemically bonded in a mixture, you can always sort the circles back out — that's the whole idea behind every separation method below. Whenever a question feels tricky, draw the particles and ask: same circles, or different circles?
In a mixture the different particles are not bonded — just sharing space — so they can be sorted out again.
The Separation Toolbox
Every method works by finding one property where the two substances differ — size, boiling point, solubility, or how strongly they stick to paper — and exploiting it:
Method
Splits…
Property it uses
Filtration
Insoluble solid from a liquid (sand from water)
Particle size — solid too big to pass the filter
Evaporation / Crystallisation
A dissolved solid from its solvent (salt from seawater)
Only the liquid evaporates; solid is left behind
Distillation
A pure liquid from a solution (fresh water from seawater)
Boiling point — the liquid boils off, then is cooled back
Chromatography
Different coloured dyes in an ink or dye
How strongly each colour sticks to the paper vs travels with water
Why does distillation give you pure water back? When seawater boils, only the water particles have enough energy to escape as gas — the salt particles are left stuck in the pan. Catch that steam, cool it down, and it condenses into salt-free water. That's how ships and desert cities turn seawater into drinking water.
Maths connection — concentration is a ratio: "How strong is this mixture?" is a maths question. Concentration is just the amount of solute per amount of solvent — a ratio you can scale up and down exactly like a recipe (Lesson 13). If 20 g of salt is dissolved in 100 g of water, the concentration is $\frac{20 \text{ g}}{100 \text{ g}} = 20\%$. Double the water and you halve the concentration; boil off half the water and you double it. Mixtures are proportional reasoning wearing a lab coat.
🔬 Try It at home — kitchen chemistry
A · Secret colours (paper chromatography)
Cut a strip from a coffee filter or kitchen paper. Draw a fat dot with a black or brown non-permanent felt-tip about 2 cm from the bottom.
Stand the strip in a glass with just 1 cm of water — the dot must sit above the water line.
Predict what will happen, then watch for 10 minutes. The water climbs the paper and drags the dyes with it. Colours that stick less travel further, so the "black" splits into a rainbow. Compare two different pens!
B · Grow salt crystals (evaporation)
Stir salt into warm water until no more will dissolve (a saturated solution). Note roughly how many spoons it took.
Pour a little into a shallow dish and leave it on a sunny windowsill for a few days.
As the water evaporates, the salt is left behind as crystals. You separated a solution — no filter needed.
Worked Examples
Worked Example 1 — choosing the right tool
Mia's family collects water from a stream in the Bavarian hills. It has bits of grit and dead leaves floating in it, and it also tastes faintly of dissolved minerals. How would you get clean, clear, mineral-free water?
Step 1 — remove the grit and leaves: they're insoluble solids, so filter the water (property used: particle size).
Step 2 — remove the dissolved minerals: filtering won't catch dissolved particles, so distil the filtered water — boil it and re-condense the pure steam (property used: boiling point). Order matters: filter first, then distil.
Worked Example 2 — concentration as a ratio
A recipe for salt dough uses 50 g of salt in 200 g of water. Mia only has 150 g of water. How much salt keeps the same concentration?
$$\frac{50 \text{ g salt}}{200 \text{ g water}} = \frac{x}{150 \text{ g water}}$$
$$x = \frac{50 \times 150}{200} = 37.5 \text{ g salt}$$
Same ratio, scaled down — identical to scaling a recipe. The concentration is $\frac{50}{200} = 25\%$ either way.
Warm-Up
Problem 1
Pure substance or mixture? (a) pure gold in a ring, (b) a smoothie, (c) tap water with fluoride and minerals, (d) a single sugar crystal, (e) sand from a beach in Mexico, (f) the air in the room.
pure / mixture for each →
Problem 2
Match the mixture to the best separation method (filtration, evaporation, distillation, chromatography): (a) sand mixed into water, (b) the colours in a green felt-tip pen, (c) getting the salt back from salty water, (d) getting drinkable water back from salty water.
match each →
Problem 3
In the words solute, solvent, solution: when Mia stirs hot chocolate powder into milk, which is the solute, which is the solvent, and what do we call the drink as a whole?
label all three →
Problem 4
30 g of sugar is dissolved in 120 g of water. What is the concentration as a fraction, and as a percentage?
$\dfrac{30}{120} = \; ?$
simplify the fraction → × 100 →
Core Problems
Problem 5
Mia does the marker chromatography experiment. On her paper strip, the black ink splits into three colours. Their distances travelled up the paper are: yellow 6 cm, red 4 cm, blue 1.5 cm. (a) Which dye sticks most strongly to the paper (travels least)? (b) Which sticks least? (c) Explain in one sentence why the dyes end up in different places.
compare distances → interpret sticking →
Problem 6
A cup holds 250 mL of seawater. When Mia evaporates all the water on a beach in Portugal, 8.75 g of salt is left. (a) What is the concentration in grams of salt per litre (g/L)? (b) The nearby Dead Sea is about 340 g/L. How many times saltier is the Dead Sea than this seawater?
$250 \text{ mL} = 0.25 \text{ L}$
g ÷ L → then divide the two concentrations →
Problem 7
Mia has 300 mL of salt solution at a concentration of 40 g/L. She boils it until only 120 mL of solution is left (no salt escapes). (a) How many grams of salt were in the solution to begin with? (b) What is the new concentration in g/L? (c) Explain, using particles, why boiling makes the solution more concentrated.
total salt = concentration × volume → new concentration = salt ÷ new volume →
Problem 8
Design a step-by-step method to recover both the sand and the salt, cleanly separated, from a beaker containing sand + salt + water all mixed together. List the steps in order and name the method used at each step. (Hint: salt dissolves; sand does not.)
order the steps → name each method →
Problem 9 Challenge
A painter mixes a custom green by combining a yellow paint and a blue paint in the ratio 3 : 2. She makes 200 mL of green in total. (a) How many mL of yellow and how many of blue did she use? (b) Is her green paint a pure substance or a mixture — and could chromatography, in principle, "un-mix" it back into yellow and blue? Explain your reasoning. (c) She now wants 500 mL of the same green. How much of each colour?
split by ratio → classify → scale up →
Problem 10 Open
You're given a muddy, salty puddle of water and challenged to make it as clean and drinkable as possible using only things you'd find in a kitchen (no lab equipment). Describe your plan, which properties each step exploits, and one thing you're not sure your method can remove. There's no single right answer — explain your thinking.
your kitchen purification plan →
Show answers
Problem 1
(a) pure · (b) mixture · (c) mixture · (d) pure · (e) mixture · (f) mixture.
Solute = the chocolate powder (it dissolves); solvent = the milk (does the dissolving); the whole drink is a solution.
Problem 4
$\frac{30}{120} = \frac{1}{4} = 25\%$.
Problem 5
(a) Blue sticks most strongly — it travelled the least (1.5 cm). (b) Yellow sticks least — it travelled furthest (6 cm). (c) Each dye is pulled up by the water but held back by the paper; dyes that grip the paper less are carried further, so they separate.
(a) Salt $= 40 \text{ g/L} \times 0.3 \text{ L} = 12$ g. (b) New concentration $= 12 \text{ g} \div 0.12 \text{ L} = 100$ g/L. (c) The salt particles don't leave when water boils off — only water particles escape as gas. Same salt in less water means the particles are more crowded, so the concentration rises.
Problem 8
1. Add water and stir so the salt dissolves (sand does not). 2. Filter — the sand stays in the filter paper; the salty water passes through. 3. Rinse and dry the sand. 4. Evaporate the filtered salt water — the water leaves, salt crystals remain. You now have clean sand and clean salt.
Problem 9
(a) 3 + 2 = 5 parts; each part = 200 ÷ 5 = 40 mL. Yellow = 3 × 40 = 120 mL, blue = 2 × 40 = 80 mL. (b) It's a mixture (the paints aren't chemically bonded), so in principle chromatography could separate the yellow and blue pigments again. (c) For 500 mL: each part = 500 ÷ 5 = 100 mL → yellow = 300 mL, blue = 200 mL.
Problem 10
Open answer. A strong plan: (1) let the mud settle, then pour off the top / filter through a coffee filter or cloth to remove the mud — uses particle size; (2) boil the filtered water and catch the steam on a cold lid, letting it drip into a clean cup (home distillation) to leave the salt behind — uses boiling point. Honest uncertainty: simple filtering won't remove dissolved salt or germs; only the boiling/distilling step does, and improvised gear catches little water.
Next up → s11: Acids, Bases & the pH Scale
You can now separate mixtures — but how do you tell what a liquid actually is? Is it a harmless drink or something that could burn you? Chemists use indicators that change colour, and a number line called the pH scale where every single step means ten times stronger. That "ten times per step" is a geometric sequence — exactly the pattern you learned in maths Lesson 09.