Almost nothing around us is a pure substance: seawater, ink, air and soil are all mixtures. To use or study one component, it must first be separated out. Every separation technique in this topic works by exploiting a difference in physical properties between the components, and the chemical identity of each substance is never changed.
Sec 2 IP Science covers five named techniques: magnetic separation, filtration, evaporation to dryness, simple distillation and paper chromatography. Examiners award marks for identifying the property difference first and then naming the technique. Application questions link the same ideas to food safety, recycling and Singapore's water supply.
What these notes cover
- •Why we separate mixtures
- •Matching the technique to the property difference
- •Magnetic separation
- •Filtration and decanting
- •Evaporation to dryness
- •Simple distillation
- •Paper chromatography and Rf values
- •Food safety, recycling and water treatment
- •Desalination and reverse osmosis
- •NEWater and the Four National Taps
- •Common mistakes and exam technique
- •One-look revision summary
Key definitions
- Separation technique
- A physical method that divides a mixture into its components without changing the chemical identity of any component.
- Residue
- The insoluble solid trapped on the filter paper.
- Filtrate
- The liquid that passes through the filter paper and is collected below. It is not necessarily pure, because dissolved substances pass through with it.
- Distillate
- The liquid collected in the receiving beaker after the vapour has been condensed in the condenser during distillation.
- Solvent front
- The furthest level reached by the solvent on the chromatography paper. It is marked in pencil immediately when the paper is removed, before it evaporates.
Key formulas
Retention factor
Rf = distance travelled by substance ÷ distance travelled by solvent front
Both distances are measured from the start line. Rf is always between 0 and 1 and has no units, because a substance can never overtake the solvent carrying it. It is used to compare chromatograms run under different conditions.
Worked example
A beaker contains a dry mixture of iron filings, table salt and sand. Describe how you would obtain each component as a separate, dry solid. Justify every step.
- 1Pass a bar magnet wrapped in a plastic sheet slowly over the dry mixture several times. Iron is magnetic while salt and sand are not, so the iron filings are attracted to the magnet, and peeling away the sheet releases them into a separate dish.
- 2Add the remaining salt–sand mixture to a beaker of water and stir well. Salt is soluble in water but sand is insoluble, so the salt forms a solution while the sand stays as suspended particles that can be separated by particle size.
- 3Pour the mixture through filter paper in a filter funnel. The insoluble sand particles are too large to pass through the pores and are trapped as the residue, while the dissolved salt passes through with the water as the filtrate.
- 4Rinse the sand residue with a little distilled water to wash off any salt solution clinging to it, then leave it to dry.
- 5Heat the filtrate in an evaporating dish until all the water has evaporated. Water has a much lower boiling point than salt, so the water escapes as vapour and dry salt crystals are left behind.
Watch out: The order matters. Use the magnet first while the mixture is dry, and add water before filtering, because filtration cannot separate two dry solids or a dissolved solid from its solvent.
Common mistakes
Swapping residue and filtrate
Wrong: The residue is the liquid that drips through the filter paper.
Right: The residue is the insoluble solid trapped on the filter paper; the filtrate is the liquid that passes through it.
Thinking evaporation gives back pure water
Wrong: Evaporating salt water to dryness produces both the salt and pure water.
Right: In evaporation to dryness the water escapes into the air as vapour and is lost, so only the salt is recovered. To collect pure water, use simple distillation, where the vapour is condensed back into liquid in the condenser.
Assuming a magnet attracts all metals
Wrong: Magnetic separation works on any mixture containing a metal.
Right: Only iron, steel, nickel and cobalt are attracted to a magnet. Aluminium, copper and gold are non-magnetic, so a mixture of aluminium foil and sand cannot be separated with a magnet.
Questions students ask
What is the difference between evaporation and distillation?
Evaporation to dryness recovers the solute: the solvent escapes into the air as vapour and is lost. Simple distillation recovers the solvent, because the vapour is condensed in a condenser and collected as the distillate. Distillation is, in essence, evaporation plus condensation.
Why can't filtration remove salt from seawater?
The salts in seawater are dissolved, and their particles are far too small to be trapped by the pores of filter paper. The salt passes through with the water no matter how many times it is filtered. Drinking water can be obtained from seawater by distillation or reverse osmosis instead.
Why is the start line in paper chromatography drawn in pencil?
Pencil is made of graphite, which is insoluble in the solvent, so the line stays fixed in place throughout the run. A pen line is itself made of ink. It would dissolve, travel up the paper and produce extra spots that make the chromatogram impossible to interpret.
Why does Singapore use reverse osmosis instead of distillation for desalination?
Simple distillation requires seawater to be boiled, and boiling millions of litres daily needs an enormous input of heat energy, which makes it far too costly on a national scale. Reverse osmosis forces water through an ultra-fine membrane under high pressure and uses much less energy per litre. It underpins two of the Four National Taps: NEWater and desalinated water.
In the full PDF
- •Labelled diagrams of the simple distillation set-up and of chromatograms comparing an unknown ink with pure dyes
- •Further worked examples on choosing evaporation or distillation and on reading a chromatogram, plus three application examples on seawater, desalination and factory contamination
- •Full procedures and limitations for each technique, including decanting, thermometer position and condenser water flow
- •The four remaining misconceptions: filtering out a dissolved solute, thermometer bulb position, pen start lines and solvent level
- •Full-mark answer templates, a technical-vocabulary table and a final exam checklist
- •A one-look revision table and summary, with notes on crystallisation and sublimation as extensions