Types of Reaction in Aqueous Solution, Grade 10
There are four types of reaction in aqueous solution, and three of them are the same thing.
Textbooks list them as four separate definitions to memorise: precipitation, gas forming, acid-base, redox. That is four things to forget. Ask one question instead and the whole topic collapses into one idea.
What left the solution?
The driving force, which is the actual answer
| Reaction type | What leaves the solution | What you see |
|---|---|---|
| Precipitation | A solid | A cloud, then a layer on the bottom |
| Gas forming | A gas | Bubbles |
| Acid-base | Water, as molecules | Nothing. The beaker just gets warm |
| Redox | Nothing leaves | A metal changes, or a gas comes off the metal |
In the first three, ions stop being free ions. They leave as a solid, as a gas, or as water molecules, and the only difference between the three types is the exit they use. That is why textbooks group them together as ion exchange reactions.
Redox is the odd one out, and it is odd in an interesting way. Nothing leaves at all. There are as many dissolved ions in the beaker at the end as there were at the start, just different ones. What moved was electrons.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 2 |
| Topic | Chemical change, reactions in aqueous solution |
| Status | Class experiment. Not a formal assessment |
| Marks | 60 on our worksheet. None is prescribed |
Six reactions, four answers
The practical is six pairs of solutions and one question asked six times. Nothing else is needed.
| # | Mix | What happens | Type |
|---|---|---|---|
| 1 | Silver nitrate + sodium bromide | Cream precipitate | Precipitation |
| 2 | Sodium carbonate + copper(II) sulfate | Blue-green precipitate | Precipitation |
| 3 | Sodium carbonate + hydrochloric acid | Vigorous bubbling | Gas forming |
| 4 | Sodium hydroxide + hydrochloric acid | Nothing visible. The beaker warms | Acid-base |
| 5 | Zinc + hydrochloric acid | Steady bubbling from the metal | Redox |
| 6 | Zinc strip + copper(II) sulfate | The blue fades, the zinc goes brown | Redox |
Record four things before you classify anything
- Was there a temperature change?
- Was there a colour change?
- Did a precipitate form?
- Was there any bubbling?
Learners who decide the type first and observe afterwards get reaction 4 wrong, every year. There is nothing to see, so they write "no reaction" and move on.
Reaction 4 is the one that teaches
Sodium hydroxide and hydrochloric acid produce no precipitate, no gas and no colour change. And the beaker gets warm.
Hand it to the class. That warmth is the only evidence in the room, and it is enough.
H+(aq) + OH-(aq) → H2O(l)
Something did leave the solution. Hydrogen ions and hydroxide ions joined into water molecules, and a water molecule is not an ion. Two invisible things became a third invisible thing, and the energy came out as heat.
A class that reasons that out from a warm beaker has understood what a net ionic equation is for, which is more than most of them get from a page of equations.
If you want it visible, put two drops of an indicator into the sodium hydroxide first. Run it once without, though. Watching a colour change is not the same as working out that something happened when nothing appeared to.
Reaction 6 is the one they remember
A grey zinc strip goes into a clear blue solution. By the end of the period the strip is furred with dark brown copper and the blue is fading.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Net ionic: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Count the ions. There were dissolved positive ions before and there are dissolved positive ions after. Nothing came out of solution and nothing bubbled off. Two electrons moved from the zinc atom to the copper ion, and that is the entire event.
| What happened | Name for it | |
|---|---|---|
| Zinc | Lost electrons and went into solution | Oxidised |
| Copper ions | Gained electrons and became metal | Reduced |
Zinc is more reactive than copper, so it takes copper's place in the solution. That is what displacement means, and this one beaker is the whole of it. Put a copper strip into zinc sulfate and nothing at all happens, which is the same fact read backwards.
Leave a strip in overnight. By morning the solution is close to colourless.
Do not put copper in nitric acid
The standard version of this practical asks a Grade 10 class to drop thin copper wire into dilute nitric acid in an open beaker. Do not run it.
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
2NO(g) + O2(g) → 2NO2(g)
The nitrogen monoxide turns brown the moment it meets air, as nitrogen dioxide. It is toxic by inhalation, its worst effects are delayed by hours, and it does not feel dangerous while you are breathing it. The instruction comes with no fume cupboard, no ventilation note and no mention of the gas at all.
Reaction 6, zinc in copper sulfate, replaces it completely. Same reaction type, same electron transfer, more visible, and no gas of any kind.
Three more reagents that stay on paper
| Refused | Why | Use instead |
|---|---|---|
| Sodium sulfide and iron(II) sulfide | Both give hydrogen sulfide with acid, which is acutely toxic and deadens the sense of smell at the concentrations that matter | Classify them on paper. They never come off the page |
| Lead(II) nitrate | Cumulative toxin, restricted in school laboratories, and the waste cannot go to a drain. It is the standard worked example for precipitation | Barium nitrate and sodium sulfate |
| Ammonium hydroxide | Releases ammonia into a closed classroom, and it demonstrates nothing sodium hydroxide does not | Sodium hydroxide |
Writing the equations
Every reaction gets three lines, and the third one is the point.
Precipitation
| Balanced | AgNO3(aq) + NaBr(aq) → AgBr(s) + NaNO3(aq) |
| Full ionic | Ag+ + NO3- + Na+ + Br- → AgBr(s) + Na+ + NO3- |
| Net ionic | Ag+(aq) + Br-(aq) → AgBr(s) |
Gas forming
| Balanced | Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l) |
| Net ionic | CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l) |
Acid-base
| Balanced | NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) |
| Net ionic | H+(aq) + OH-(aq) → H2O(l) |
Redox
| Balanced | Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) |
| Net ionic | Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g) |
Line those four net ionic equations up next to each other. Three of them produce something that is not an ion: a solid, a gas, a molecule. The fourth swaps electrons and everything stays charged.
That is the lesson, in four lines, and it is worth writing on the board.
One thing textbooks get wrong here, and it is worth knowing
You will see MgCO3(aq) written in the gas-forming example. Magnesium carbonate is not aqueous. The same books also state that carbonates are insoluble except for Group 1 and ammonium, and magnesium is Group 2.
Use calcium carbonate instead and write the state symbol that is true:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
That is limestone, marble chips and the fizzing on a school bench, and it is the reaction the demonstration is actually doing.
A related contradiction turns up wherever the solubility rules are taught. The rules list calcium sulfate as fairly soluble, and then the exercises that follow have it precipitating out as CaSO4(s). The exercises are right. Calcium sulfate dissolves to about 0,2 g per 100 ml, which is gypsum, and for the purpose of predicting a precipitate it counts as insoluble.
What you need
| Item | Qty | Why |
|---|---|---|
| Beaker, borosilicate low form, 250 ml | 4 | One per reaction, run one at a time |
| Dropper bottles, 5 ml, pack of 10 | 1 pk | One labelled bottle per solution. See the note below |
| Test tubes, borosilicate 3.3, rimmed | 12 | For running reactions 5 and 6 in parallel across groups |
| Test tube rack, 6 hole, with pegs | 2 | |
| Measuring cylinder, glass class B, 100 ml | 2 | 10 ml of acid for the two redox reactions |
| Stirring rod, borosilicate | 2 | |
| Spatula, stainless steel, spoon end | 1 | |
| Zinc strip, 200 x 12 mm | 1 pk | Reactions 5 and 6. Sand the surface before use |
The substitution that halves the glassware bill
Most versions of this practical ask for six measuring cylinders, one per solution, so that nothing gets cross-contaminated.
| Cost | Does it solve cross-contamination? | |
|---|---|---|
| Six measuring cylinders | About R400 | Yes |
| Two cylinders plus ten labelled dropper bottles | About R150 | Better |
A labelled bottle keeps its own top. A cylinder has to be rinsed and returned to the right place by thirty learners in a hurry, which is where the contamination comes from in the first place.
The two cylinders stay, because the redox reactions measure 10 ml of acid rather than counting drops.
The solutions you supply yourself
All six at 0,1 mol·dm-3: hydrochloric acid, sodium hydroxide, sodium carbonate, silver nitrate, sodium bromide, copper(II) sulfate.
Bicarbonate of soda works for the gas-forming reaction if you have no sodium carbonate. A spatula of the kitchen powder in a little water does the same job, and every school has some.
If it does not work
| What happens | What caused it |
|---|---|
| Reaction 4 gets written up as "no reaction" | Nothing is visible. Make them touch the beaker. The warmth is the whole result |
| The zinc does not bubble in the acid | The grey surface layer is zinc oxide and it blocks the reaction. Rub the strip with sandpaper first |
| Nothing happens to the zinc in copper sulfate | Same oxide layer, or not enough time. Sandpaper, and leave it standing for the period. Overnight is better |
| Every beaker goes cloudy | Cross-contaminated droppers. One labelled bottle per solution, and the tops stay with their own bottles |
| The carbonate and acid fizz over the top | Too much of both. A quarter of a beaker each is plenty |
| An expected precipitate does not appear | A bottle has water in it, or the solutions are too dilute. Check the labels before blaming the chemistry |
| Learners classify before observing | The commonest failure of the practical. Insist on the four observations in writing first |
Safety
- Goggles on. Silver nitrate, plus dilute solutions of a strong acid and a strong base
- No copper and nitric acid. The reason is above, in full
- Silver nitrate stains skin black for about a week. Harmless, alarming, and worth announcing before the lesson
- Hydrochloric acid and sodium hydroxide at 0,1 mol·dm-3 are irritants rather than corrosives. They still go in eyes
- Reaction 5 gives off hydrogen. No flames anywhere near it
- Never test a gas by smell, and never sniff a beaker
- A labelled residue bottle for the silver waste. It does not go down the sink
Disposal: silver waste to its bottle. The copper sulfate and the zinc into a collection jar. Everything else down the sink with plenty of water.
How the 60 marks are made up
| Section | Marks |
|---|---|
| Observations, recorded before classifying | 12 |
| Classifying the six reactions | 6 |
| The reaction with nothing to see | 7 |
| The driving force of each type | 8 |
| Equations, including net ionic | 9 |
| The zinc strip, oxidation and reduction | 8 |
| Classifying six more on paper | 6 |
| Conclusion | 4 |
No mark allocation is prescribed for this practical. The worksheet and this split are ours.
The most valuable eight marks on the sheet are the driving-force questions. A learner who can say what leaves the solution in each type can classify any reaction they are ever shown, without memorising a list.
The mark most often dropped is reaction 4, written up as no reaction because nothing appeared.
If you have time
Give each group two unlabelled solutions and nothing else. They mix them, record the four observations and name the reaction type from what they saw. It takes ten minutes and it is the only part of this topic that is genuinely a test.
Stand a copper strip in zinc sulfate solution next to the zinc-in-copper-sulfate beaker. Nothing happens, all period. The comparison teaches reactivity better than a reactivity series printed on a page.
Ask why a galvanised roof sheet does not rust through. The zinc corrodes instead of the iron, for the same reason the zinc dissolved instead of the copper, and there is a whole industry built on it.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 60 marks, with the six-reaction observation table, the driving-force questions, four net ionic equations and six more reactions to classify on paper
- Marking memorandum, with the expected observations, how to mark a group whose zinc had not reacted by the bell, and the three answers that look right and score nothing
Related practicals
- Precipitation reactions and testing for ions, Grade 10. Two of the six reactions here are precipitations, and that page is where the solubility rules and the ion tests live
- Redox reactions and oxidation numbers, Grade 11. Where the electron transfer on this page goes next: oxidation numbers, half-reactions and the reduction potential table
- Solubility and dissolving, Grade 10. Why some products come out as solids and others stay dissolved
- Physical and chemical change, Grade 10. The signs of a chemical change, which is the observation discipline this practical runs on
- Titration, Grade 12. Where acid-base reactions become quantitative. Grade 10 needs no burette
Buy this experiment
We are putting together a Reaction Types Kit for Grade 10 with the beakers, dropper bottles, test tubes, racks, cylinders, rods, spatula and zinc strips in one box, plus a printed teacher guide and the marking memo. The six solutions are not in it yet, and we would rather say so than ship a box that cannot run the practical.
This is the cheapest practical in the Grade 10 range to set up. Four beakers and a handful of labelled dropper bottles run all six reactions, which is why the dropper bottles are the line worth buying first. They sit alongside the glass test tubes and the test tube racks.
The zinc is the item that turns it from a set of colour changes into a lesson about electrons, and a single strip standing in copper sulfate for a period is the best twenty minutes in the topic.