S2 Science | Separating Mixtures: a Tutor Prep Guide | Filtration, distillation, fractional distillation, chromatography

2026-11-09

Tutor lesson-prep guides · Secondary Biology

The moment a student sees the word separate they write distillation, because that apparatus looks impressive. But the question is really asking which property of the two substances differs; choose the wrong property and the best glassware is useless.

Objectives — By the end the student can: (1) choose filtration, distillation, fractional distillation or chromatography from the property that differs, and justify the choice; (2) work out the mass of salt and water from a salt solution and its percentage, then the recovery percentage; (3) calculate Rf from the spot and solvent front distances and explain why the larger Rf travelled further.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Dissolving	Which of sand and salt dissolves in water	salt dissolves, sand does not, so water is the tool
Percentage	A 250 g solution is 20 percent salt; mass of salt	50 g, from 250 times 0.2
Boiling point	Which boils first, water or ethanol	ethanol, so distillation can separate them

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	One beaker	Show sand, salt and water and ask how many things must be separated	List three: sand, salt, water
8-20 min	Choosing the order	Ask about solubility, then boiling point, then apparatus	Turn the three questions into a flow
20-34 min	Filtration and distillation	Demonstrate filtration and link pore size to particle size	Draw the set-up and label the filtrate
34-46 min	Mass and recovery	250 g of solution at 20 percent salt; how much salt and water	Get 50 g and 200 g, then a recovery figure
46-56 min	Rf values	Solvent front 8.0 cm, spot 6.0 cm; find Rf	Get 0.75 and compare with 0.40
56-60 min	Exit check	Collect three questions and mark at once	Work independently

03 Teaching script (read it out)

Scene 1: Ask what differs first

Every separation method starts from a difference in properties: solubility for filtration, boiling point for distillation or fractional distillation, and speed of movement in a solvent for chromatography.
Put three questions on the board: do both dissolve in water, are the boiling points far apart, does one travel with the solvent. Answer those and the method appears.
Usual misconception: distillation is best because it is cleanest. In fact it uses the most energy and only suits liquids with well-separated boiling points.

Ask the student to repeat: name the property that differs before naming the method.

Scene 2: The boundary between filtration and distillation

Filtration removes only insoluble solids: sand grains are larger than the paper pores and stay behind, while dissolved salt passes through with the water.
To recover the salt you must remove the water: distillation recovers the water as well, evaporation keeps only the salt.
Make this boundary explicit, otherwise students expect salt on the filter paper.

Ask the student to repeat: filtration only removes undissolved solids; dissolved salt needs the water boiled away.

Scene 3: Rf values and fractional distillation

Rf is the distance moved by the spot divided by the distance moved by the solvent front, so it always lies between 0 and 1. With a front at 8.0 cm and a spot at 6.0 cm, Rf is 0.75.
In the same solvent, the substance with the larger Rf is held less strongly by the paper and travels faster, so a mixture splits into separate spots.
Fractional distillation also uses boiling point, but for liquids with close boiling points: in a fractionating column the higher boiler condenses lower down and the lower boiler last.

Ask the student to repeat: Rf is spot distance over solvent front distance and is always below 1.

04 Question types and worked steps

Type	Example	Steps	Answer
Choose a method	A mixture of sand and salt water; you want dry sand	Ask about solubility: sand is insoluble, so filter	Filtration; sand stays on the paper
Mass from percentage	250 g of solution, 20 percent salt	Salt is 250 times 0.2; water is 250 minus the salt	50 g salt, 200 g water
Recovery	200 g of water yields 180 cubic centimetres of distillate	Recovered amount over original water, times 100	90 percent
Rf value	Solvent front 8.0 cm, spot 6.0 cm	Spot over front, no unit	0.75

Sanity check (teach this): Sanity checks: Rf always lies between 0 and 1, so a value above 1 means the ratio is inverted; a recovery can never exceed 100 percent, so more means the base is wrong; the masses of all parts must add to the total; if you cannot name the property that differs, the chosen method is a guess.

05 Independent practice (3 questions)

Question	Answer	What to watch
A 250 g solution is 20 percent salt; mass of salt after the water is removed	50 g	Whether 250 was multiplied by 0.2 and the unit written
Solvent front 8.0 cm, another spot at 3.2 cm; Rf	0.40	Whether 8.0 was divided by 3.2, and whether the answer is below 1
To separate ethanol and water, filtration, distillation or chromatography	Distillation (fractional)	Whether the boiling point difference is stated, not just the apparatus

06 Common mistakes and what to say

Mistake	Student writes	Say this
Expecting salt on the filter paper	The salt stayed on the paper after filtering	Salt is dissolved, so it passes through with the water. To get the salt, boil the water away; filtration only gives you the sand.
Treating distillation as universal	Distillation for every mixture	Distillation needs a boiling point difference. Two dissolved solids will both stay in the water and come out mixed.
Inverting Rf	Rf is 8.0 divided by 6.0, so 1.33	Rf must be below 1: the spot goes on top, the front underneath, so 6.0 over 8.0 is 0.75.
Using the wrong base for recovery	Recovered 180 cubic centimetres, divided by 250 g	The base is the water present, 200 g, so 180 over 200 is 90 percent; over 250 it would read 72 percent.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — All chapters in the subject
Acids, alkalis and neutralisation — Another chapter built on careful observation
Redo this chapter — Recalculate salt mass and recovery for three concentrations

08 Exit check (last 5 minutes)

Question	Answer	Decision
Mass of salt in 250 g of 20 percent solution	50 g	Answering 200 means water was calculated; retake the fourth stage
Rf for a front at 8.0 cm and a spot at 6.0 cm	0.75	Answering 1.33 means the ratio is inverted; retake the third act
Method to separate sand from salt water	Filtration	Answering distillation means solubility was not checked; retake the first act

09 Tutor FAQ

Can filtration and evaporation be combined?

Yes, and it is the usual sequence: filter off the sand, then evaporate the salt water. The order cannot be reversed, because sand would slow evaporation and contaminate the salt.

How does fractional distillation differ from distillation?

The principle is the same; simple distillation takes off one liquid at a time, while a fractionating column separates several liquids with close boiling points, such as crude oil.

Must chromatography use filter paper?

No, silica plates and thin-layer chromatography are also used; what matters is a stationary phase and a mobile phase, with substances partitioning differently between them.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.