Reactions of Acids with Bases, Carbonates and Metals, Grade 9
An acid always gives up its hydrogen to something. What that something is decides what else comes out.
Grade 9 Natural Sciences asks you to learn four reactions of acids. Most classes memorise four separate equations and forget three of them by the exam. They are one pattern, and once you see it you do not have to memorise anything.
This page covers all four, both gas tests, and a reaction that appears in a current textbook and does not happen.
The pattern
| An acid reacts with | Products |
|---|---|
| A metal oxide | salt + water |
| A metal hydroxide | salt + water |
| A metal carbonate | salt + water + carbon dioxide |
| A metal | salt + hydrogen |
Every one makes a salt. Two make water alongside it. One adds a gas you can catch with limewater, and one makes a different gas that pops.
Learn the pattern, not the four equations. The pattern tells you the products for combinations you have never seen.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 2 |
| Strand | Matter and materials |
| Topic | Topic 9, Reactions of acids with bases and metals |
| Status | Class activities. Not a formal practical task |
| Marks | 62 on our worksheet. The book's own revision is 15 |
Reaction 1: an acid and a hydroxide
This is neutralisation, and it is the first rough titration a learner ever does.
NaOH + HCl → NaCl + H2O
- 10 ml of sodium hydroxide solution in a test tube
- A drop of universal indicator. Purple or dark blue, strongly basic
- Add hydrochloric acid drop by drop, swirling every few drops
- When it turns light blue, slow to one drop at a time. Stop at green
Green is pH 7. The acid and the base have exactly cancelled.
The step that makes it land
Pour the neutral liquid onto a watch glass and leave it in the sun for a day or two.
What is left is table salt. A corrosive base and a corrosive acid both disappeared, and something completely ordinary came out. That is neutralisation demonstrated rather than asserted.
Nobody tastes it. It is not food grade, the neutralisation is rarely exact, and both reagents came out of laboratory bottles. Textbooks ask for the "everyday name" of the product and routinely forget to say this.
Reaction 2: an acid and a carbonate
CaCO3 + 2HCl → CaCl2 + H2O + CO2
- Half a teaspoon of calcium carbonate, or dust from old blackboard chalk, in a small beaker
- A few drops of water, swirl
- A drop of universal indicator. Read the pH
- Add ACID drop by drop, swirling as you go, until the indicator turns green
The fizzing is the point. Carbonates are the only one of the four that gives off a gas you can see immediately, and that gas is carbon dioxide.
A warning about the instructions. A widely used textbook tells you at this step to "add water to the reaction mixture", and then in the very next step refers to "the acid" you were never told to add. It means acid. Follow it literally and you add water forever and neutralise nothing. The book's own photograph caption on the facing page says to add the acid drop by drop.
Reaction 3: proving the gas is carbon dioxide
Limewater is a solution of calcium hydroxide. Carbon dioxide turns it milky.
- A quarter teaspoon of calcium carbonate in one test tube
- Half fill a second tube with clear limewater
- Add acid to the first tube until it is a third full, and stopper it at once
- Get the free end of the delivery tube under the limewater as fast as you can
- Watch it go milky
The carbon dioxide reacts with the calcium hydroxide to make calcium carbonate, which is insoluble. The tiny suspended particles are the cloudiness you see.
On the delivery tube
You do not need a bent glass delivery tube for this. A 30 cm length of 6 mm PVC hose pushed through a one-hole rubber stopper does the job, costs about a tenth as much, and cannot shatter.
Glass is only necessary when something is heated. This reaction is cold, so flexible tubing is correct rather than merely acceptable.
When the limewater does not go milky
| Problem | Cause | Fix |
|---|---|---|
| Nothing happens at all | The stopper is not sealing. The commonest failure by a long way | Press it firmly and check before you add the acid |
| Gas escapes before you connect | Too slow | Have the stopper in your hand. Acid in, stopper home, tube under, one movement |
| The limewater was cloudy to begin with | It has already absorbed carbon dioxide from the air | Filter it, or use fresh. This is why the instruction says clear limewater |
| It goes milky, then clears again | Too much carbon dioxide. The calcium carbonate reacts further into calcium hydrogen carbonate, which is soluble | That is a real result and a good one. Stop bubbling sooner |
Reaction 4: an acid and a metal
Mg + 2HCl → MgCl2 + H2
- Cut about 3 cm of magnesium ribbon. If it is dull, clean it with sandpaper
- Ribbon into a test tube, acid to about halfway. It fizzes hard
- Hold a second test tube upside down over the first to trap the hydrogen
- Take it away and hold a lit splint at the mouth
- A squeaky pop means hydrogen
Hydrogen is the least dense gas there is, which is why an upside-down tube holds it and why it collects at the top. Tilt the tube slightly before testing so some air can mix in, or it will not pop.
Clean the ribbon first. Magnesium reacts with air to form a dull coating of magnesium oxide, and that coating slows the reaction with the acid or stops it entirely. Fresh metal fizzes immediately.
Ribbon, never powder. Powdered magnesium reacts far too vigorously for a school bench.
The reaction that does not happen
This is the most useful thing on the page.
A current Grade 9 textbook prints the following, twice, once as a worked example and once in a practice exam:
copper + nitric acid → copper nitrate + hydrogen
Cu + 2HNO3 → Cu(NO3)2 + H2
That reaction does not occur.
Copper is below hydrogen in the reactivity series. A metal can only displace hydrogen from an acid if it sits above hydrogen. Copper does not, so it cannot, and no hydrogen is produced.
Nitric acid does attack copper, because nitric acid is an oxidising acid and works by a different route. The gas is a nitrogen oxide:
| Dilute nitric acid | 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O |
|---|---|
| Concentrated nitric acid | Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O |
Do not do this reaction with a class. Nitrogen monoxide oxidises in air to nitrogen dioxide, which is toxic. It is a paper example only.
Why this is worth teaching rather than just correcting
The rule on this page says metal + acid → salt + hydrogen. Copper is the metal for which that rule is false.
A rule is only useful when you know where it stops. The reactivity series is the list of where it stops.
Put three metals in the same acid and the point makes itself:
| Metal | In dilute hydrochloric acid |
|---|---|
| Magnesium | Fizzes hard. Gone in a minute |
| Zinc | Slower, steady fizzing |
| Copper | Nothing at all |
Telling the two gases apart
| Gas | Comes from | Test | Result |
|---|---|---|---|
| Carbon dioxide | Acid + carbonate | Bubble through clear limewater | Goes milky |
| Hydrogen | Acid + metal | Lit splint at the mouth of the tube | Squeaky pop |
Both reactions fizz and they look identical. The test is the only way to tell which gas you have, and that is exactly why both tests exist.
One thing worth being clear about. The limewater test detects carbon dioxide, not the reaction that made it. You can turn limewater milky by blowing through a straw, because you breathe out carbon dioxide. The test tells you the gas is present, not where it came from.
Safety
- Goggles for all four practicals. Textbooks often list them for three and omit the carbonate activity, which is the one where acid goes drop by drop into an open fizzing beaker. That is an oversight, not a permission
- Never dilute a laboratory acid yourself. Acid into water, slowly, and only by someone trained
- The sodium hydroxide is the hazard, not the acid. At these dilutions the acid is an irritant. Concentrated sodium hydroxide is corrosive and does not hurt at first, which is what makes it dangerous
- Nobody tastes the recovered salt
- Hydrogen in test tube quantities only, in an open tube, never a sealed vessel
- Keep the lit splint away from the acid tube. It goes at the mouth of the collecting tube only
- Magnesium ribbon, never powder
Disposal: neutralise the leftovers until universal indicator reads green, then down the sink with plenty of water. Everything here ends as a benign salt.
How the 62 marks are made up
| Section | Marks |
|---|---|
| The pattern | 8 |
| Neutralising a hydroxide | 11 |
| A carbonate and an acid | 10 |
| Testing the gas | 9 |
| A metal and an acid | 10 |
| The metal that does not react | 9 |
| Everyday chemistry | 5 |
No mark allocation is prescribed for these activities. The worksheet and this split are ours.
The most valuable question on the sheet asks learners to use the reactivity series to explain why the printed copper equation cannot be right. A class that can find a mistake in its own textbook and say how it knows has done something better than the practical.
If you have time
Blow through a straw into limewater. It goes milky, with no chemicals at all, because you breathe out carbon dioxide.
Then add vinegar to the milky limewater and watch it clear. That is a nineteenth-century travelling-salesman trick: an audience member blew into limewater, the cloudiness was announced as proof of disease, and the vinegar that cleared it was sold as the cure.
Compare magnesium, zinc and copper in the same acid. Three metals, one acid, and the reactivity series in about ninety seconds.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 62 marks, with all four reactions, both gas tests, the equations to balance and a full section on the copper equation that does not work
- Marking memorandum, with worked equations, what to do when the limewater clears again, and the five answers that look right and score nothing
Related practicals
- The pH scale and indicators, Grade 9. The chapter before this one, and where the universal indicator used here is explained
- Types of reaction in aqueous solution, Grade 10. Where these reactions go next year
- Physical and chemical change. Collecting a gas over water, with the same stopper and tubing
- Conservation of matter. Why the masses have to balance
What you need to run it
We are putting together a Reactions of Acids Kit for Grade 9 with the test tubes, rack, beakers, stopper and delivery tubing, watch glasses, pH paper and goggles in one box, plus a printed teacher guide and the marking memo. Coming shortly.
You can run most of this on supermarket chemicals. Vinegar is the acid, bicarbonate of soda is the carbonate, and old blackboard chalk is the calcium carbonate. The fizzing, the limewater test and the neutralisation all work without a single laboratory reagent. Only the sodium hydroxide and the magnesium need the real thing.
The one part worth buying properly is the gas-collecting setup. A one-hole rubber stopper and a length of 6 mm PVC hose is about R2 a set from a ten-metre roll, and it is what makes the carbon dioxide test possible.
The rest is ordinary laboratory glassware, and the watch glasses are what you evaporate the salt on.