S2 Science | Separating Mixtures: Filtration, Evaporation, Distillation and Chromatography

S2 Science | Separating Mixtures: Filtration, Evaporation, Distillation and Chromatography — Tutor Prep Guide

2026-11-15

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Tutor lesson-prep guides · Junior Secondary Science

Students can name four separation methods but choose the wrong one for salt water and sand. This lesson teaches one selection rule: decide what must be kept, then find the property that differs.

Objectives — Students can: (1) choose a separation method by asking what must be kept and which property differs; (2) explain what property each of filtration, evaporation, distillation, chromatography and magnetism relies on; (3) calculate the mass, volume and percentage recovered and explain why recovery is never 100 per cent.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Mixtures and pure substances	Is salt water a mixture or a pure substance?	A mixture, separable by physical methods
Dissolving	Where does the salt go when it dissolves?	It spreads as particles among the water particles; it does not disappear
Boiling point	What is the boiling point of water?	100 degrees Celsius at normal pressure

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Start with a dilemma	Write salt, sand and water and ask how to recover all three	Realise at least two steps are needed
5-16 min	The selection rule	Teach three questions: what to keep, which property differs, will the method destroy it	Apply the three questions to four examples
16-28 min	Filtration and evaporation	Filter the sand and salt water, then evaporate the filtrate	State that sand stays on the paper, salt remains, water leaves
28-37 min	Distillation and chromatography	Distillation uses boiling point, chromatography uses solubility and adsorption	Calculate Rf as 0.75 and explain that distillation recovers water
37-42 min	Recovery rate	Only 170 of 200 cm3 is collected; ask where the rest went	Name evaporation, holdup in apparatus and incomplete transfer
42-45 min	Exit check	Ask how to separate ethanol from water	Fractional distillation, because the boiling points differ

03 Teaching script (read it out)

Scene 1: Choosing a method is comparing properties, not reciting names

Give students a repeatable sequence. First question: what am I keeping? Second: which property of the two substances differs – particle size, boiling point, solubility, magnetism, or affinity for the paper? Third: will this method destroy what I want to keep? Run the sequence over four cases. Sand and water differ in particle size, so filter. Salt and water differ in boiling point, so evaporate or distil, and distil if the water must be kept. Iron and sulfur differ in magnetism, so use a magnet. Dyes in ink differ in solubility and adsorption, so use chromatography. Once the three questions are automatic, unfamiliar mixtures become manageable instead of guesswork.

Ask students to run the three questions out loud on sand, salt and water and name the two steps

Scene 2: Filtration keeps the solid, evaporation loses the liquid

Teach filtration with three details. The paper pores are smaller than sand grains but larger than water particles, so sand stays and water passes. The funnel stem should touch the beaker wall so the filtrate does not splash out. The words matter: filtrate is what passes through, residue is what stays on the paper. Then evaporate the filtrate: water leaves as vapour, salt stays and crystallises. Have students state the difference themselves, since this is where marks are lost. The common misconception is that evaporation gives pure water; correct it immediately, because recovering water needs distillation and condensation.

Ask what stays on the paper, what the filtrate contains, and what evaporation leaves behind

Scene 3: Distillation and chromatography rely on boiling point and affinity

Distillation is one sentence: heat until the lower boiling component vaporises, then condense and collect it. Ask why distilled water is purer than the original: water vaporises at 100 degrees while the dissolved salt does not, so the condensate is mostly water. It is not absolutely pure, because volatile impurities can travel with the vapour. Chromatography needs numbers to be understood. A dye spot climbs the paper; the faster dye is less strongly adsorbed and more soluble in the solvent. Rf is the distance travelled by the dye divided by the distance travelled by the solvent front, so 4.5 cm over 6.0 cm gives 0.75. Rf is characteristic under fixed conditions, which is why it can identify a substance.

Ask what travelling further means in chromatography and calculate Rf as 0.75

04 Question types and worked steps

Type	Example	Steps	Answer
Choosing a method	How would you separate sand from salt water?	1) The particle sizes differ. 2) Sand is insoluble while salt is dissolved. 3) Filter: sand stays on the paper and salt water becomes the filtrate	Filtration
Percentage by mass	A 250 g mixture contains 40 g of salt and 210 g of sand. What percentage is salt?	1) Salt mass over total mass. 2) 40 over 250. 3) Multiply by 100	16 per cent
Recovery rate	Distilling 200 cm3 of salt water gives 170 cm3 of distillate. What is the recovery rate?	1) Collected volume over original volume. 2) 170 over 200. 3) Multiply by 100	85 per cent
Chromatography Rf	A dye travels 4.5 cm while the solvent front travels 6.0 cm. What is the Rf value?	1) Rf is dye distance over solvent front distance. 2) 4.5 over 6.0. 3) Give two decimal places	0.75

Sanity check (teach this): Three checks when choosing a method: state what must be kept, name the property that differs, and confirm the method will not destroy what is being kept. For calculations check three things: the parts add up to the original total, the percentage denominator is the total, and recovery is below 100 per cent with a stated reason.

05 Independent practice (3 questions)

Question	Answer	What to watch
How would you separate iron powder from sulfur powder?	Use a magnet, because magnetism differs	Check the property is named
What percentage of a 250 g mixture is 40 g of salt?	16 per cent	Check 250 is the denominator
What does an Rf of 0.75 mean?	The dye travelled 0.75 of the solvent front distance	Check the ratio is not inverted

06 Common mistakes and what to say

Mistake	Student writes	Say this
Evaporation as a way to get pure water	Evaporating salt water gives pure water.	Evaporation drives the water off and leaves the salt. To keep the water, distil and condense the vapour.
Filtering dissolved salt	Filter paper will trap the salt from salt water.	Dissolved salt particles are smaller than the paper pores, so they pass through with the water.
Reporting 100 per cent recovery	All 200 cm3 should be recovered.	Some always escapes as vapour or stays in the apparatus, so recovery must be below 100 per cent.
Inverting the Rf ratio	Rf is solvent distance divided by dye distance.	The dye distance is always on top. Inverted values come out above 1, which is impossible.

07 Homework (paper, section and question numbers)

Density, floating and the particle model, question 2 — Dissolving and filtering are both explained with particle size
The water cycle and water treatment, question 2 — Sewage treatment combines filtration, sedimentation and distillation ideas
Sound waves and hearing, question 1 — Both topics use the idea of a medium carrying something

08 Exit check (last 5 minutes)

Question	Answer	Decision
How do you separate sand from salt water, and what does evaporating the filtrate give?	Filter first, then evaporate the filtrate to recover salt	Answering water means solute and solvent are confused
What is the main difference between distillation and evaporation?	Distillation condenses and collects the vapour; evaporation lets the liquid escape	Answering that they are the same means the purpose was missed
How is an Rf of 0.75 calculated?	4.5 cm of dye travel divided by 6.0 cm of solvent travel	Answering 1.33 means the ratio is inverted

09 Tutor FAQ

Should this chapter include a real chromatography experiment?

Yes. Put a dot of black ink on filter paper with a shallow layer of water and the dyes separate within ten minutes. Students then calculate Rf themselves.

Why not aim for 100 per cent recovery?

Real procedures always lose material. Naming where it went is better science than chasing an impossible number.

Is distilled water completely pure?

No. Volatile impurities can carry over, so laboratories use repeated distillation or deionisation for higher purity.

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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.