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IB DemystifiedMYP Sciences
Pure substances, mixtures and separation
Sea water, air, ink and orange juice are all mixtures. Chemists separate mixtures to get clean water, salt, medicines and pure metals, and to find out what things are made of.
Recommended for MYP 1 · About 3 lessons · Criteria A, B, C and D
Figure 1. Chromatography separates the dyes in an ink so they can be identified.
explain the difference between pure substances and mixtures
describe solutions using the words solute, solvent and soluble
choose and explain methods to separate mixtures
interpret chromatograms and calculate Rf values
plan fair tests and process simple data
discuss clean water and recycling
Before you start
You will use these skills. If any feel shaky, review them first.
solids, liquids and gases (see States of matter)
particles and changes of state
reading scales and tables
Key vocabulary
Pure substance
A material that contains only one substance.
Mixture
Two or more substances mixed but not chemically joined.
Solute
The substance that dissolves.
Solvent
The liquid that the solute dissolves in.
Filtration
Separating an insoluble solid from a liquid using filter paper.
Chromatography
Separating dissolved substances, such as dyes, as a solvent moves through paper.
Understanding the ideas
What is it?
A pure substance contains only one substance; a mixture contains two or more that are not chemically joined. Because the substances in a mixture keep their own properties, we can use those differences to separate them.
Why does it happen?
Filtration separates insoluble solids from liquids; evaporation and crystallisation recover dissolved solids; distillation separates a liquid from a solution using boiling points; chromatography separates dyes; magnets and sieves use magnetism and size.
How do we know?
Pure substances melt and boil at exact temperatures, so measuring a boiling point shows whether something is pure. Chromatography is used to test foods, medicines and inks.
Why does it matter?
Separation gives us drinking water, salt, fuels from crude oil and recycled materials. Many communities still need better ways to remove harmful substances from water.
What does it connect to?
Mixtures link to particles, states of matter and solutions in chemistry, to water treatment and the environment, and to crude oil and recycling later in the course.
Choosing a separation method
Insoluble solid + liquid (sand and water): filtration.
Dissolved solid from solution (salt from sea water): evaporation or crystallisation.
Liquid from a solution (pure water from sea water): distillation.
Different dyes (in ink): chromatography.
Magnetic metal (iron from sand): a magnet.
Different-sized solids (rice and salt): a sieve.
Separation in the real world
Salt is produced at salt works along the Sindh and Balochistan coasts and mined at Khewra in Punjab. Water works filter and treat river water. Recycling plants use magnets to pull steel cans out of mixed waste.
Worked examples
Example 1: filtering and evaporating
How can you get salt from a mixture of salt and sand?
Add water and stir: the salt dissolves; the sand does not.
Filter: the sand stays in the filter paper.
Evaporate the water from the salt solution to leave salt crystals.
Example 2: an Rf value
A dye travels 2.4 cm while the solvent travels 8.0 cm. Find its Rf value.
Rf = distance moved by dye ÷ distance moved by solvent.
Rf = 2.4 ÷ 8.0 = 0.30.
Assessment tips
Questions on mixtures often ask you to choose methods and read chromatograms. Expect to:
Choose the right separation method and explain why it works.
Describe methods in clear steps, with safety.
Interpret chromatograms and calculate Rf values.
Process results, including means and percentages.
Common mistakes: saying dissolved substances disappear; trying to filter out a dissolved solid; drawing the baseline in ink; and using the everyday meaning of "pure".
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: what colours are in black ink?
Partially guided investigation · about 40 minutes · pairs
Research question
Do different brands of black felt-tip pen contain the same coloured dyes?
Scientific background
Many inks are mixtures of dyes. As water soaks up chromatography paper, each dye moves at its own speed, so the dyes separate into different spots.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Chromatography or filter paper strips, several black felt-tip pens (water-based), pencil, ruler, beaker, water, paper clips or tape.
Method
Draw a pencil line 2 cm from the bottom of each strip and put a dot of one pen on it.
Hang the strips in a beaker with water about 1 cm deep, keeping the dots above the water.
Remove the strips when the water is near the top, and mark the solvent front in pencil.
Compare the colours and positions of the spots, and calculate Rf values.
Safety. Wipe up spills and keep water away from electrical sockets. Wash hands after handling ink.
Then evaluate: why do some black inks not separate at all in water?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
