Solubility and Dissolving: How Much, and How Fast
Stirring makes sugar dissolve faster. It does not make more of it dissolve.
That one sentence is the whole of this topic, and it is where most of the marks go missing. Solubility is how much dissolves. Rate of dissolving is how fast it gets there. They are two different measurements, they answer two different questions, and the things that change one very often leave the other exactly where it was.
How much against how fast
| What you do | Does it change the RATE? | Does it change the SOLUBILITY? |
|---|---|---|
| Stir it | Yes, faster | No |
| Crush it first | Yes, faster | No |
| Use warmer water | Yes, faster | Yes, more dissolves |
Only the temperature column has two ticks in it. Stirring and crushing move the solute through the water and expose more surface to it, so the same amount of sugar gets there sooner. Neither one raises the ceiling.
Three learners each drop a sugar cube into 100 ml of water. One stirs, one crushes the cube first, one uses hot water. Ask which of them ends up with the most dissolved sugar and most of a class will say the one who stirred. It is the one with the hot water, and the other two finish sooner with exactly as much sugar in the glass as they started with.
The definitions, briefly
| Solubility | The amount of a substance that dissolves in a given amount of solvent at a particular temperature |
|---|---|
| Rate of dissolving | How quickly the solute disappears into the solvent |
| Solute | The substance that dissolves. The sugar |
| Solvent | The substance doing the dissolving. The water |
| Saturated solution | One that cannot hold any more solute at that temperature. Add more and it sits on the bottom |
A saturated solution is saturated at a stated temperature and at no other. Warm it and it stops being saturated, which is the reason the temperature has to be part of the sentence.
Why water dissolves so much
A water molecule is bent, at roughly 104 degrees, and that bend is the reason for everything that follows.
Oxygen pulls the shared electrons harder than hydrogen does, so the oxygen end of the molecule carries a small negative charge and the two hydrogen ends carry small positive ones. If the molecule were straight, those two pulls would cancel out. Because it is bent, they do not, and water ends up with a negative end and a positive end. That is what a dipole means.
Solid table salt is a lattice of Na+ and Cl- ions locked together by electrostatic attraction. Drop it in water and the negative oxygen ends crowd round the Na+ ions while the positive hydrogen ends crowd round the Cl- ions. Enough water molecules pulling in the same direction beats the attraction holding the lattice together, and the ions come away one at a time, each wrapped in its own shell of water.
That wrapping is called hydration, and it matters more than it sounds, because hydration is where the heat in the next section comes from.
NaCl(s) → Na+(aq) + Cl-(aq)
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 2 |
| Topic | Chemical change, reactions in aqueous solution |
| Status | Class experiments. Not a formal assessment |
| Marks | 46 on our worksheet. None is prescribed |
The practical: four white powders, four different answers
Half a gram each of sugar, table salt, calcium chloride and ammonium chloride, one per test tube, 1 ml of water on top, and a thermometer in the tube while it happens.
They look identical going in. They do not behave identically.
| Substance | What the thermometer does | Which way |
|---|---|---|
| Sugar | Drops by about a degree | Endothermic, barely |
| Sodium chloride | Drops by about a degree | Endothermic, barely |
| Ammonium chloride | Falls, typically by 5 degrees | Strongly endothermic |
| Calcium chloride | Rises, typically by 9 degrees, and you feel it through the glass | Strongly exothermic |
Two things are happening at once every time a solid dissolves. Energy goes in to break the lattice apart. Energy comes out when the water molecules hydrate the freed ions. Whichever of those two is bigger decides which way the thermometer moves.
For calcium chloride, hydration wins comfortably and the tube warms. For ammonium chloride, breaking the lattice costs more than hydration returns, and the tube pulls the missing energy out of its surroundings, which includes the thermometer bulb.
Your learners have already held both of these
| In the test tube | In real life |
|---|---|
| Calcium chloride warming up | The hot pack in a first aid kit, or the sachet in a self-heating tin |
| Ammonium chloride cooling down | The instant cold pack a physio snaps and puts on a sprain |
An instant cold pack is a salt and a sealed sachet of water in one bag. Squeeze it, the sachet bursts, the salt dissolves, and the pack drops well below room temperature within seconds. No refrigeration, no chemical reaction, nothing but a lattice coming apart.
A learner who has just watched a thermometer fall in a tube of ammonium chloride and is then handed a cold pack does not forget what endothermic means.
What you need
| Item | Qty | Why |
|---|---|---|
| Educational glass thermometer, -10 to 250 °C | 3 | The instrument the whole practical runs on |
| Test tubes, borosilicate 3.3, rimmed | 6 | One per substance, plus spares |
| Test tube rack, 6 hole, with pegs | 1 | Six tubes standing still while you read them |
| Test tube holder, wire | 1 | For handling the warm tubes |
| Beaker, borosilicate low form, 250 ml | 3 | Cold, room temperature and hot water for the rate comparison |
| Measuring cylinder, glass class B, 100 ml | 1 | Equal volumes, or the comparison is not fair |
| Stirring rod, borosilicate | 3 | One per beaker. Never stir with the thermometer |
| Digital stopwatch | 1 | Timing the rate half of the practical |
| Spatula, stainless steel, spoon end | 1 | Half a gram is about a level quarter teaspoon |
| Glass dropping pipette, 125 mm | 2 | 1 ml of water into a narrow tube |
| Watch glass | 2 | Evaporating the solutions to get the solids back |
Sugar and table salt are not on the list on purpose. Every school kitchen has both, and posting them adds weight and cost for nothing.
Method
Part A: does it get hot or cold?
- Weigh about 0,5 g of each solid into its own labelled test tube and stand the four in the rack
- Rest the thermometer on the dry solid in the first tube and read it. That is your "before" temperature, and all four tubes must start at the same one
- Add 1 ml of water. Not 10. The effect gets diluted away in a bigger volume and the thermometer barely moves
- Move the thermometer gently to help the solid dissolve, and watch the reading
- Record the temperature when it stops changing, not the instant the water goes in
- Repeat for the other three tubes
- Pour each solution onto a watch glass and leave it somewhere warm. Record what is left the next day
Part B: how fast, and how much
- Put the same volume of water into three beakers: one cold, one at room temperature, one hot
- Measure and record each temperature
- For rate: add the same measured amount of sugar to each, start the stopwatch, stir all three the same way, and stop the clock when the last grain disappears
- For solubility: keep adding equal small spatulas of sugar and stirring until no more will dissolve and some stays on the bottom. Record how much went in
Part B is two experiments that look like one, and separating them is the point. The stopwatch answers "how fast". The running total answers "how much". Learners who record both in the same table and then talk about "the result" have missed what they just did.
What you should see
- All four solids come back as solids when the water evaporates, looking much as they did going in
- Sugar dissolves fastest in the hot beaker, slowest in the cold one
- More sugar goes into the hot beaker before it saturates
- Only the calcium chloride tube gets warmer. Only one of the four moves upwards
The evaporation result is the important one
Dissolving salt looks irreversible and it feels like the salt has been destroyed. It has not. Take the water away and every gram of it is sitting on the watch glass.
Nothing was created and nothing new was made. The ions were pulled apart and surrounded by water, and when the water leaves they find each other again. That is a physical change demonstrated rather than asserted, and it settles the question learners get wrong most often: the calcium chloride got hot, so surely a reaction happened. It did not. A temperature change on its own has never been proof of a chemical reaction, and the watch glass is the evidence.
The conductivity demonstration, and the result nobody predicts
A bulb, a holder, a battery, three leads with crocodile clips, and two graphite rods standing in a beaker. The liquid in the beaker either completes the circuit or it does not.
| In the beaker | Bulb | Why |
|---|---|---|
| Distilled water | Off | Water is molecular. Almost no free ions |
| Tap water | On, faintly | Dissolved mineral ions from the ground and the pipes |
| Salt solution | On, brightly | Na+ and Cl- free to move |
| Sugar solution | Off | Sugar dissolves as whole, uncharged molecules |
Sugar dissolves better in water than salt does, and it will not light the bulb.
Ask the class to commit to a prediction on that one before you run it. Most of them will say the bulb comes on, because they have quietly decided that dissolving and conducting are the same event. They are not. A current needs charged particles that can move. Sugar goes into solution as intact neutral molecules and carries no charge anywhere, so there is nothing for the circuit to work with.
Use graphite rods, not copper wire. Copper corrodes in salt solution inside a single period and turns the water blue-green, which learners read as a chemical reaction and then write up as one.
The solubility rules
These are the tool for predicting whether two solutions will produce a precipitate, so they are worth the twenty minutes it takes to learn them.
| Ion | Rule |
|---|---|
| Na+, K+ and NH4+ salts | Always soluble |
| Nitrates and acetates | Always soluble |
| Chlorides, bromides and iodides | Soluble, except silver, lead and mercury(I) |
| Sulfates | Soluble, except barium, lead and strontium |
| Hydroxides | Mostly insoluble. Group 1 dissolves, and barium and calcium are moderately soluble |
| Carbonates | Mostly insoluble, except Group 1 and ammonium |
| Sulfides | Mostly insoluble, except Groups 1 and 2 and ammonium |
The first two rules do most of the work. If a compound has sodium, potassium, ammonium or nitrate in it, it dissolves, and you can stop reading the table.
If it does not work
| What happens | What caused it |
|---|---|
| The calcium chloride barely warms | The salt has absorbed water from the air. This is the commonest failure by a long way. Use fresh salt from a sealed jar |
| Nothing moves on any of the four | Too much water. 1 ml, not 10 |
| Every tube reads the same | Read too early or too late. Wait for the reading to settle, and do not read while you are still moving the thermometer |
| A tube cracks | The thermometer was used as a stirring rod. It is not one |
| The solid does not all dissolve | 0,5 g in 1 ml is near saturation for some of these. That is a result, not a failure. Write it down |
| Hot water dissolves the sugar no faster than cold | The three beakers are closer in temperature than you think. Measure them rather than judging by hand |
| The bulb stays off for salt solution too | Weak battery, a loose crocodile clip, or the rods are not deep enough in the liquid |
| The sugar test lights the bulb | Salt contamination. Rinse the rods and the beaker between every liquid, and run distilled water first while everything is clean |
| A learner writes that the salt disappeared | Point at the watch glass |
Safety
This is the mildest chemical practical in the Grade 10 range and it should still be run properly. Nothing here is corrosive, flammable or toxic, and no part of it needs a fume cupboard.
- Goggles on. Not because these four solids are dangerous, but because the habit is built in Grade 10 and used in Grade 12
- Calcium chloride pulls water out of the air and can irritate damp skin. Wash hands afterwards
- Ammonium chloride is a mild irritant. Nobody tastes anything, whatever a learner tells you about salt substitute
- Hot water for Part B, not boiling. A kettle and a few minutes standing is enough
- The thermometer is glass and it is going into a narrow tube. Move it gently
Disposal: all four solutions go down the sink with plenty of water.
How the 46 marks are made up
| Section | Marks |
|---|---|
| Predictions, made before the practical | 5 |
| The results table | 8 |
| Exothermic and endothermic, including the cold pack | 9 |
| Physical or chemical, justified from their own evaporation | 6 |
| How much against how fast | 6 |
| Conductivity | 6 |
| Water as a solvent, and a dissolution equation | 3 |
| Conclusion | 3 |
No mark allocation is prescribed for this practical. The worksheet and this split are ours.
The prediction section is marked on being done, not on being right. Most learners predict "stays the same" for all four, and that wrong prediction is exactly what makes the calcium chloride result land. Punishing it would defeat the purpose.
The mark most often dropped is the sugar cube question. Learners pick the one who stirred, because stirring feels like effort and effort feels like it should produce more.
If you have time
Put the ammonium chloride tube back on the rack and leave it. It climbs slowly back to room temperature, because the energy it borrowed is coming back in from the air. Nothing was lost.
Run the same 0,5 g of calcium chloride into 10 ml instead of 1 ml. The same energy is released into ten times the water, so the rise is roughly a tenth of the size. It is a clean way to show why the instruction said 1 ml.
Ask why a bag of ice melts faster when you throw salt on it. Same topic, opposite direction, and it is the reason road salt exists.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 46 marks, with the prediction table, the four-substance results table, the hot and cold pack question and the conductivity results
- Marking memorandum, with a sample data set, what to do about a calcium chloride result that barely moved, and the answers that look right and score nothing
Related practicals
- Physical and chemical change, Grade 10. The evaporation result on this page is the proof that belongs to that one
- Heating and cooling curves, Grade 10. Same term, same thermometer, energy going in with no temperature change
- Endothermic and exothermic reactions, Grade 11. Where the energy half of this practical goes next year
Buy this experiment
We are putting together a Solubility and Dissolving Kit for Grade 10 with the thermometers, test tubes, rack, holder, beakers, measuring cylinder, stirring rods, stopwatch, spatula, pipettes and watch glasses in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The thermometer is the item this practical cannot do without, and it has to be a real one with a readable scale rather than a strip. Ours are in the thermometers and temperature probes range.
The main practical runs on sugar or table salt, which every school already has. The four-salt version needs calcium chloride and ammonium chloride as well, and the contents card says exactly which ones you need to supply.