Chapter 4: Exploring Diversity of Matter using Separation Techniques (Summary Sheet)
Created by Miss Clarissa Ng | www.clartutors.com
Part A · Deciding How to Separate a Mixture
1 Is It a Pure Substance or a Mixture?
- A pure substance is one substance only; a mixture holds two or more substances together. Only a mixture can be separated into different parts.
- The two are told apart by their melting and boiling points: a pure substance has fixed ones, a mixture melts and boils over a range.
| Pure substance | Mixture | |
|---|---|---|
| Composition | Fixed — the same everywhere | Varies — the parts can be in any proportion |
| Melting and boiling point | Sharp and fixed | Melts and boils over a range |
| On a chromatogram | Turns up as a single spot | Turns up as more than one spot |
| Joined how? | Chemically combined or one type of particle | Not chemically combined — the parts just sit together |
2 Why a Mixture Can Be Taken Apart
- The constituents of a mixture are not chemically combined, so each one keeps its own properties: iron stays magnetic, salt still dissolves, water still boils at its own temperature.
- That is what makes separation possible — the parts can be separated based on their properties, using a difference between them.
- Separating is a physical process: no new substance is made, so the parts can be recovered unchanged.
- Nothing is separated by "the mixture" — always name the one property that differs, then pick the technique that uses it.
3 The One Question: Which Property Is Different?
- The five techniques you must know, and the property each one uses:
| Property that differs | Technique | Example |
|---|---|---|
| Magnetism | Magnetic attraction | Lifting the steel cans out of a recycling bin of mixed cans |
| Particle size (solid does not dissolve) | Filtration | Coffee grounds held back by the filter paper, coffee passing through |
| Boiling point — you want the solid | Evaporation | Brine left in a salt pan drying out to leave salt |
| Boiling point — you want the liquid | Distillation | Pure water recovered from sea water in a desalination plant |
| Solubility in a solvent (coloured substances) | Paper chromatography | A black pen mark on wet paper spreading into its separate dyes |
4 Two Things to Check Before Choosing
- Is the solid dissolved or just floating? A dissolved solid passes through a filter; only an undissolved one is held back.
- What does the question want you to collect? The solid, or the liquid? For salt and water: the salt means evaporation, the water means distillation.
- If one technique is not enough, use them in order — separate what you can, then treat the part that is left.
Part B · Magnetic Attraction
5 How Magnetic Attraction Works
- Principle: only some materials are attracted to a magnet. Held over the mixture, the magnet pulls the magnetic pieces out and leaves the rest behind.
- It separates the parts of a solid–solid mixture where one part is magnetic and the other is not.
- Nothing is heated, dissolved or reacted, so both parts come out unchanged.
- Wrap the magnet in a plastic bag or paper: the magnetic pieces drop off cleanly when the wrapping is removed.
6 Which Materials a Magnet Will Attract
| Attracted to a magnet | Not attracted |
|---|---|
| Iron, steel, nickel, cobalt (and things made from them, such as steel cans and paper clips) | Copper, aluminium, brass, plastic, glass, wood, sand, water, and most other substances |
- Steel is iron with a little carbon in it, so steel is magnetic too. A magnet cannot pull aluminium cans out of a bin — that is a common trap.
7 Where It Is Used
- At recycling plants, strong magnets lift iron and steel out of mixed waste — a fast way to recover metal and to clean up the rest.
- In scrap yards, a crane with a magnet unloads steel from a pile of mixed scrap.
- In the lab, separating iron filings from sulfur or sand to show the two parts are still themselves.
Part C · Filtration
8 How Filtration Works
- Filtration separates an insoluble solid from a liquid in a suspension, and uses the difference in particle size: the filter paper's tiny holes let the liquid through and hold the solid back.
- You can collect either part — the question decides which one you need.
| Word | What it means |
|---|---|
| Suspension | A mixture of an insoluble solid and a liquid; the solid is visible and settles if left to stand |
| Residue | The insoluble solid left on the filter paper |
| Filtrate | The liquid that has passed through the filter paper — it is clear if nothing is dissolved in it |
9 What a Filter Cannot Do
- A filter paper separates by particle size only. A soluble solid dissolves into particles small enough to pass straight through, so it comes out in the filtrate.
- That is why filtering salt water gives clear water that is still salty — no salt is removed at all.
- To separate a solution, you must use a difference in boiling point instead: evaporation for the solid, distillation for the liquid.
10 Using Filtration in a Sequence
- A mixture of salt and sand needs two techniques, because the two parts differ in different ways:
- 1. Add water and stir — the salt dissolves, the sand does not, so the mixture is now a suspension with a solution in it.
- 2. Filter — the sand stays on the paper as the residue, the salty solution passes through as the filtrate.
- 3. Evaporate the filtrate — the water leaves and the dry salt is left behind. Both parts recovered.
Part D · Evaporation and Distillation
11 Evaporation: Getting the Solid Back
- Principle: the solvent boils at a lower temperature than the solute, so heating the solution drives the solvent off as vapour first and leaves the solid behind.
- It recovers a soluble solid (the solute) from its solution, and the solid must be one that does not break down on heating.
- Heated gently until the solvent has gone — the solid left is the solute, dry and ready to be collected.
12 Where Evaporation Is Used
- Salt from sea water: shallow salt pans of sea water are left in the sun, the water leaves as vapour, and the salt is raked out.
- Anywhere a dissolved solid is wanted back in dry form — concentrating a solution down, or drying a sample for weighing.
- The water that leaves is lost to the air; if that water matters too, evaporation is the wrong choice.
13 Distillation: Getting the Liquid Back
- Principle: the solvent has the lower boiling point. Heat the solution and the solvent boils off first as vapour, leaving the dissolved solute in the flask.
- The vapour is then condensed back into liquid in the condenser, cooled by cold water running around it, and collected as the distillate — a pure liquid.
- So distillation gives you both parts: the pure solvent as the distillate, and the solute left in the flask.
- It is the choice when you want the liquid — for example pure water from sea water.
14 Choosing Between Them
| What you want | Technique | What you collect and why |
|---|---|---|
| The dissolved solid | Evaporation | The solute stays in the dish as the solvent escapes into the air — the liquid is given up |
| The pure liquid | Distillation | The solvent is boiled off and condensed back into a separate flask as the distillate — nothing is lost |
- Both use the same difference — their boiling points are not the same — so the question is never "which property?" but "which part do you want?"
Part E · Paper Chromatography
15 How Paper Chromatography Works
- A spot of the mixture is placed on the paper, and the bottom edge of the paper is dipped into a solvent — the spot itself must stay above the solvent.
- The solvent soaks up the paper and carries the dissolved substances with it. Each substance travels at its own speed, because each has a different solubility in that solvent.
- The more soluble a substance is, the further it travels up the paper. That is the principle that separates a mixture into separate spots.
16 Reading a Chromatogram
| Part | What it is |
|---|---|
| Start line | Where the spots were placed — drawn in pencil, because pencil does not dissolve in the solvent and so cannot spoil the result |
| Solvent front | The highest point the solvent reached before the paper was taken out — mark it at once, as it keeps rising |
| Spot | Where a substance ended up. One spot means a pure substance; more than one spot means a mixture |
| Chromatogram | The finished paper, holding the whole pattern of spots |
17 The Rf Value
- The Rf value turns a spot into a number: distance travelled by the substance divided by distance travelled by the solvent, both measured from the start line.
- It is always less than 1, because a substance can never travel further than the solvent that carries it.
- For the same paper, solvent and temperature, a substance always gives the same Rf value — so an unknown spot can be identified by comparing its value with the value of a known substance.
- Colourless substances can still be found: a locating agent reacts with them to give a coloured product, or the chromatogram is viewed under ultraviolet light.
18 Where It Is Used
- Food safety: checking the colourings and additives in a packet drink or a sauce against what the label claims.
- Telling whether a substance is pure — a single spot is needed — and finding unauthorised or banned substances in a sample.
- Separating the dyes in a pen or a food colouring, and separating DNA fragments in a forensic sample.
Part F · Separation in Real Life: Water, Food and Waste
19 Water Treatment
- Water treatment is where these techniques work at industrial scale, on water that people will drink.
| What is treated | Techniques used |
|---|---|
| Sea water (desalination) | Distillation, or reverse osmosis — pressure pushes the water through a partially permeable membrane that lets water particles through but holds back dissolved salts |
| Treated used water | Filtration to take out the solid bits, then reverse osmosis through the membrane to remove what is dissolved |
- Reverse osmosis is filtration taken to its finest: the membrane's pores are small enough to hold back not just sand but salts, bacteria and dyes.
20 Why Water Is Precious, and How Singapore Keeps It
- Water has no substitute and Singapore has very little of its own, so every drop has to be collected, cleaned and used again.
- NEWater: treated used water is put through filtration and reverse osmosis to give water clean enough to be added back to the reservoirs.
- Desalination turns sea water into drinking water; local reservoirs and imported water make up the rest.
- So separation techniques are what make a sustainable supply of potable water possible — and that is why using less water matters as much as cleaning it.
21 Food Safety and Waste Management
- Food safety: chromatography checks that a food's colourings and additives are the ones allowed, and that nothing banned is present.
- Waste management: magnets pull iron and steel out of household waste, and paper chromatography can check that a recyclable plastic is the type it claims to be.
- Sorting waste this way lets materials be recycled or reused instead of being thrown away — the same property-based thinking, used on a city-sized mixture.
Part G · Exam Tips
22 Exam Tips: What Each Syllabus Pointer Asks For
| The pointer | What earns the mark |
|---|---|
| Explain how the constituents of a mixture can be separated based on their properties, using magnetic attraction, filtration, evaporation, distillation and paper chromatography | Name the property that differs first, then the technique, then what each part becomes — "the iron is magnetic, so magnetic attraction lifts it out" |
| State some examples of the applications of the various separation techniques in everyday life and industries | Give a named real example for each: desalination plants and treated used water for water treatment, chromatography for food safety, magnets at a recycling plant for waste management |
| Investigate the separation of constituents of mixtures based on the basic principles involved | State the principle before the method — magnetism, particle size, boiling point, or solubility in a solvent — then say what happens to each part as the method runs |
| Show an appreciation of why water is a precious resource and the need to conserve it | Say water has no substitute and Singapore has little of its own, so it is cleaned and reused rather than used once |
| Show an appreciation of how Singapore uses separation techniques to ensure a sustainable source of potable water | Name the techniques in the chain — filtration, reverse osmosis, distillation — and what each one removes, as in NEWater and desalination |
If a question asks for both parts of a mixture, it wants a sequence: separate the insoluble part first, then treat what is left. Name each technique in order and say what you collect at each step.