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Pupils can name solid, liquid and gas, but cannot say where the sugar goes or why ice floats. This lesson makes them explain with particles and separates dissolving from melting.
Objectives — Pupils can: (1) describe the three states by particle spacing and arrangement, and state that the particles themselves do not change; (2) give the melting point of water as 0 degrees and the boiling point as 100, and explain why ice floats because water expands about 9 percent on freezing; (3) explain a saturated solution using 36 g of salt in 100 g of water at 20 degrees and compute a concentration of 26.5 percent; (4) separate dissolving from melting and choose filtration or evaporation for a given mixture.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Classifying states | Which state are a stone, milk and air | Solid, liquid and gas |
| Mass and volume units | How many kilograms is 250 grams | 0.25 kilograms, moving the point three places |
| Percentages | Ten grams of sugar in 50 grams is what percentage | 10 divided by 50 times 100, which is 20 percent |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0 to 5 minutes | Two opening questions | Show an ice cube and a glass of sugar water, asking where the ice goes and where the sugar goes, writing answers on the board | Give their guess and say whether the two answers are the same question |
| 5 to 15 minutes | Three states by particles | Draw three boxes: particles packed, packed but sliding, and far apart, matching solid, liquid and gas | Note that the particles are identical and only spacing and arrangement differ |
| 15 to 25 minutes | Melting and boiling points | Use water: melts at 0, boils at 100, and stress these belong to the substance itself | Say ice starts melting at 0 and water boils at 100, with the temperature pausing |
| 25 to 32 minutes | Freezing expands | Show about 109 millilitres of ice from 100 millilitres of water, and density falling from 1 to 0.92 | Explain why ice floats and why a full bottle cracks in the freezer |
| 32 to 40 minutes | Dissolving and saturation | Weigh 100 g of water, stir in salt until no more dissolves, record 36 g, then compute 36 over 136 | Compute the concentration and say that saturated means no more will dissolve |
| 40 to 45 minutes | Two separation methods | Filter muddy water through paper, then evaporate salt water to dryness, demonstrating both | Say which mixture needs which method and why |
03 Teaching script (read it out)
Scene 1: The states differ in spacing, not in particles
Put a cup of water, a block of ice and some steam side by side. All three are water and the particles are the same; only the spacing and order differ. In ice the particles are locked in place and only vibrate, so the shape is fixed. In water they still touch but slide past each other, so it flows and takes the shape of its container. In steam they are far apart and spread out to fill the room. When a pupil says the ice disappears, not one particle has gone; only the arrangement changed.
Pupils restate that the three states differ in particle spacing and arrangement, so mass is unchanged.
Scene 2: Melting and boiling points are identity cards
Water melts at 0 and boils at 100. Those numbers are not invented by us; they belong to water. So as ice is heated, the temperature stops at 0 until all the ice has melted, and while water boils the temperature stays at 100 however fierce the flame. That gives a practical rule: if heating is applied and the temperature does not rise, a change of state is happening.
Pupils restate that the temperature is steady during a change of state because the energy goes into rearranging particles.
Scene 3: Dissolving is spreading out, not vanishing
Sugar stirred into water spreads between the water particles. It cannot be seen, but it is still there, which is why the water tastes sweet and why the sugar returns when you evaporate the water. Dissolving has a limit: at 20 degrees, 100 g of water holds about 36 g of salt, and any extra sinks to the bottom, which is a saturated solution. Concentration is the solute as a share of the whole solution, so 36 g of salt in 100 g of water makes 136 g, and the salt is 26.5 percent. Note that ice turning to water is melting, a change of state, not dissolving.
Pupils restate that dissolving spreads a solute with a saturation limit, while melting changes the state of one substance.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Separate dissolving from melting | Which is dissolving, ice turning to water or sugar vanishing in water | (1) ask whether the substance changed state; (2) ice to water is a state change, so melting; (3) sugar spread through water is dissolving | The sugar is dissolving; the ice is melting |
| Compute a saturated concentration | At 20 degrees, 36 g of salt dissolves in 100 g of water. What is the concentration | (1) total mass is 100 plus 36 equals 136; (2) divide 36 by 136; (3) times 100 for a percentage | 26.5 percent, since the denominator is the whole solution |
| Judge whether more will dissolve | At 20 degrees, 200 g of water already holds 70 g of salt. Can more dissolve | (1) the limit for 200 g is 36 times 2 equals 72 g; (2) compare 70 with 72; (3) conclude | Yes, up to 72 g, so 2 g more will dissolve |
| Choose a separation method | How do you get salt from salty water mixed with sand | (1) filter out the insoluble sand first; (2) evaporate the water; (3) state what each step removes | Filter first, then evaporate, leaving the salt behind |
Sanity check (teach this): Check the arithmetic: a concentration must be under 100 percent, and the denominator must exceed the solute mass, so an answer of 36 percent means the water mass was used alone; a saturation limit scales with the water, so 200 g of water doubles it; during a change of state the temperature must hold at the melting or boiling point, so an answer climbing to 200 degrees has confused heating with melting.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| At 25 degrees, 50 g of water holds 18 g of salt. What is the concentration, and what is the limit after adding 50 g more water | Concentration is 18 over 68, about 26.5 percent; with 100 g of water the limit is 36 g | Whether the denominator was the whole solution and the limit scaled with the water |
| A 250 g cup of salt water leaves 45 g of salt when evaporated. What was the concentration | 45 divided by 250 times 100, which is 18 percent | Whether they used the solution mass, not just the water |
| Why does a glass bottle of water crack in the freezer? Use volume and density | Freezing adds about 9 percent in volume and drops density to 0.92, so the bottle bursts | Whether both volume increase and density decrease were mentioned |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Thinking dissolving means vanishing | The sugar dissolved, so the sugar is gone | The particles are spread through the water and still there; evaporate the water and the sugar returns. |
| Confusing dissolving with melting | Ice dissolving in a glass as an example of dissolving | Ice turning to water is melting, one substance changing state; sugar in water is dissolving, two substances mixing. |
| Using the water mass as the denominator | 36 g of salt in 100 g of water gives 36 percent | The denominator is the 136 g of solution, so the answer is 26.5 percent. |
| Saying ice floats because it is light, with no reason | Ice floats because ice is light | Give the volume and density: the same mass takes about 9 percent more volume, and 0.92 is less than 1. |
07 Homework (paper, section and question numbers)
- Primary 5 General Studies paper, Section C Science and Technology, item 6 — Section C item 6: the three states of water and changes of state; ask for one sentence on why the temperature pauses at 0 and 100
- Primary 5 General Studies paper, Section C Science and Technology, item 8 — Section C item 8: the three parts of a circuit, used to compare with needing an energy source for heating
- Primary General Studies chapter index — Link from the matter and energy chapter to the water cycle and air
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| What are the melting and boiling points of water | 0 degrees and 100 degrees | Answering 100 for melting means the two points must be taught again, with the pupil restating them |
| At 20 degrees, how much salt does 100 g of water hold | About 36 g | If missing, redo the spoon-by-spoon demonstration |
| 36 g of salt in 100 g of water is what concentration | 26.5 percent, from 36 over 136 | Answering 36 percent means the solution-mass denominator must be retaught |
09 Tutor FAQ
Which year is this for
Written for Primary 5. For Primary 4, teach the states and the two temperatures only and leave concentration sums for later.
Must pupils memorise solubility figures
Not all of them, but 36 g of salt per 100 g of water at 20 degrees should be known, since the lesson builds its concentration sum on it.
Is there a matching paper
The Primary 5 General Studies paper has matter items in Section C with an answer sheet, and the homework block names the section and item.
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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.