中文（繁體）
中文（简体）
English
Primary 4 General Studies | Dissolving and Separation 備課指引（導師用）｜solubility limit, filtration, evaporation
Pupils think that what disappears from sight has gone for good, so salt left after evaporating looks like magic. This lesson fixes that with conservation of mass.
Objectives — By the end the pupil can: (1) state that dissolving has a limit and work out that 50 g of water holds at most 18 g of salt at 20 degrees Celsius; (2) choose a separation method from three properties, soluble or not, magnetic or not, boiling point; (3) use conservation of mass to say that 5 g of sand, 10 g of salt and 100 g of water give 5 g on the filter paper, 110 g of filtrate and 10 g of salt after evaporation.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Solids and liquids | Is salt a solid or a liquid | A solid that dissolves in water |
| Mass does not vanish | Where does evaporated water go | Into the air as water vapour |
| Filter paper | Can sand pass through filter paper | No, sand stays on the paper |
02 Lesson flow (40 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-4 min | Diagnose | Ask how much salt one cup of water can hold | Most answer unlimited |
| 4-14 min | The limit | Add 25 g at a time and record when it stops dissolving | Plot the result |
| 14-25 min | Filtration | Pour the mixture through filter paper | Say what stays and what passes |
| 25-34 min | Evaporation | Heat the filtrate in an evaporating dish | White crystals return |
| 34-40 min | Exit check | Mark three items live | Three out of three |
03 Teaching script (read it out)
Scene 1: Dissolving spreads particles, it does not destroy them
The core misconception is that dissolved means gone, so pupils expect nothing to be left after evaporation.
Start with particles: salt breaks into pieces that spread into the gaps between water particles. They are too small to see but they are still there. Two proofs: the water tastes salty, and evaporating the water brings the salt back with the same mass.
Run the mass check: 5 g of sand, 10 g of salt and 100 g of water weigh 115 g in total; after filtration the paper holds 5 g and the filtrate weighs 110 g, which adds back to 115 g. Nothing is created or lost.
Start with particles: salt breaks into pieces that spread into the gaps between water particles. They are too small to see but they are still there. Two proofs: the water tastes salty, and evaporating the water brings the salt back with the same mass.
Run the mass check: 5 g of sand, 10 g of salt and 100 g of water weigh 115 g in total; after filtration the paper holds 5 g and the filtrate weighs 110 g, which adds back to 115 g. Nothing is created or lost.
Ask the pupil: the salt is invisible in the water, so where is it and how do you prove it is still there?
Scene 2: Dissolving has a limit set by amount of water and temperature
Pupils believe water dissolves salt without limit, because at the start everything disappears.
Let them find the limit: 50 g of water holds at most 18 g of salt at 20 degrees. Add 25 g in total and 7 g is left undissolved at the bottom. That layer is the evidence that the water is full.
Two variables: more water holds more salt, warmer water holds more salt. So one cup is never a complete answer; always state the mass of water and the temperature. Train the full sentence: 50 g of water at 20 degrees holds at most 18 g of salt.
Let them find the limit: 50 g of water holds at most 18 g of salt at 20 degrees. Add 25 g in total and 7 g is left undissolved at the bottom. That layer is the evidence that the water is full.
Two variables: more water holds more salt, warmer water holds more salt. So one cup is never a complete answer; always state the mass of water and the temperature. Train the full sentence: 50 g of water at 20 degrees holds at most 18 g of salt.
Ask the pupil: give the full sentence with the mass of water and the temperature, not just one number.
Scene 3: Pick the method from the property, not from memory
Do not make pupils memorise method names. Make them compare three properties: soluble in water, magnetic or not, and boiling point.
Sand does not dissolve, so filter it. Salt dissolves and water evaporates, so evaporate the filtrate. Iron powder is magnetic and sand is not, so use a magnet.
The routine has 3 steps: list what is in the mixture, compare the properties, then choose the method. Separating sand and salt needs two methods in sequence, dissolving then filtering then evaporating.
Sand does not dissolve, so filter it. Salt dissolves and water evaporates, so evaporate the filtrate. Iron powder is magnetic and sand is not, so use a magnet.
The routine has 3 steps: list what is in the mixture, compare the properties, then choose the method. Separating sand and salt needs two methods in sequence, dissolving then filtering then evaporating.
Ask the pupil: restate the rule from this step in their own words and give one example.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Choose a method | Separate sand and salt | Dissolve, filter, evaporate | Two methods in sequence |
| Solubility arithmetic | How much salt does 50 g of water hold | 36 g per 100 g, so scale down | 18 g |
| Conservation of mass | What is the mass of the filtrate | Total mass minus the sand on the paper | 110 g |
| Design an experiment | Separate iron powder from sand | Magnet, several passes | Say how you check it is clean |
Sanity check (teach this): Reasonableness check: the lesson is 40 minutes and the five blocks add to 40; the limit of 18 g in 50 g of water is a 20 degree figure, so heating raises it and answers must state the temperature; the mass of salt recovered must equal the mass dissolved, and any answer where salt appears or vanishes is conceptually wrong.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Find the limit | Add 5 g of salt five times | Records the moment it stops dissolving |
| Three filter questions | Paper, funnel and stirring rod | Says the sand stays on the paper |
| Evaporation | Heat the dish to dryness | White crystals appear |
| Match four mixtures | Four mixtures, four methods | Every choice has a reason |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Dissolved means gone | The salt has disappeared | Evaporate it and the salt comes back, watch. |
| Water dissolves anything | It just keeps dissolving | There is solid left at the bottom, so the water is full. |
| Filtering used for dissolved salt | Use paper to catch the salt | The salt is already in the water, so the paper cannot catch it. |
| Boiling to dryness without stirring | The mixture spits and salt is lost | Heat gently and stir, stop when crystals appear. |
07 Homework (paper, section and question numbers)
- Primary General Studies index — Other science chapters in this subject
- All prep guides — Comparable chapters in other subjects
- Redo this chapter — Items 1 to 3 again, list the properties first
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| How much salt does 50 g of water hold at 20 degrees | 18 g | Answer 25 g means reteach act two |
| How do you separate sand and salt | Dissolve, filter, evaporate | Only filtering misses the evaporation |
| What happens to the mass of salt after evaporation | It stays the same as what dissolved | Answer vanished means reteach act one |
09 Tutor FAQ
Why does salt water look the same as water?
The salt particles spread into the gaps between water particles, too small to see, but they are still there, which is why it tastes salty.
Must I always evaporate?
It depends which part you want. Evaporate to keep the salt; distil to keep the water by condensing the vapour.
Can a magnet separate iron powder from sand?
Yes, and it is the simplest method. Pass the magnet several times and check nothing is left behind.
Find a tutor — freeTutor 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.