Acid reactions and neutralisation
Predict products, connect gas observations to reactions, and explain neutralisation with ions and balanced equations.
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The core idea
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Learning objectives
- describe the characteristic properties of acids as in reactions with metals, bases and carbonates to form salts (description of the preparation of pure salts is not required)
- describe the reaction between hydrogen ions and hydroxide ions to produce water, H+ + OH– → H2O, as neutralisation
- describe the characteristic properties of bases in reactions with acids and with ammonium salts
An acid can make a metal bubble, dissolve a basic oxide, or release gas from a carbonate. The products depend on both reactants. This lesson shows how to choose the reaction pattern, name the salt, and connect observations to balanced equations.
Use the aqueous-ion definitions from the previous lesson. You also need to read formulae and state symbols and balance an equation by changing coefficients, not the formulae of substances.
Identify the other reactant first
For the dilute-acid reactions below, the acid determines the salt ending: hydrochloric acid gives a chloride, sulfuric acid a sulfate, and nitric acid a nitrate. The metal in the other reactant supplies the first part of the salt name. This naming rule does not tell you whether hydrogen, carbon dioxide or water also forms: that depends on the other reactant’s chemical class.
| Other reactant | Products with an acid | Evidence to look for |
|---|---|---|
| Suitable metal, such as magnesium with dilute hydrochloric acid | Salt + hydrogen | Metal gets smaller and bubbles form; the collected hydrogen burns with a squeaky pop with a lighted splint |
| Base, such as copper(II) oxide | Salt + water | The solid can dissolve as it reacts; gas is not a required product |
| Alkali, such as aqueous sodium hydroxide | Salt + water | No gas is required; an indicator or pH measurement can follow the changing acidity |
| Carbonate, such as calcium carbonate | Salt + water + carbon dioxide | Bubbles form; the gas turns limewater milky |
“Suitable metal” matters. Copper does not release hydrogen with dilute hydrochloric acid; do not apply the pattern to every metal and every acid. Use dilute hydrochloric acid or dilute sulfuric acid for the familiar metal–hydrogen pattern. Nitric acid does not follow that simple generalisation with metals.
Bubbles are an observation, not a gas identity. Both the metal and carbonate reactions can effervesce. A gas test distinguishes their products. In a supervised practical, test a small collected sample as instructed; never identify gases by directly smelling them.
Modelled example 1
From magnesium and acid to an equation
Problem
Magnesium ribbon reacts with dilute sulfuric acid. Predict the products, describe the gas test, and write the equation.
Study the worked solution
Choose the pattern
Method
Magnesium is a suitable metal, so the other product alongside the salt is hydrogen.
Reason
Water and carbon dioxide belong to a different reactant pattern; magnesium metal is not a carbonate.
Working
magnesium + dilute sulfuric acid → salt + hydrogen
Name and represent the salt
Method
The salt is magnesium sulfate, MgSO₄.
Reason
Sulfuric acid supplies sulfate; Mg²⁺ and SO₄²⁻ combine in a 1:1 ratio.
Working
Ion ratio Mg²⁺ : SO₄²⁻ = 1 : 1; salt formula MgSO₄.
Connect observation and equation
Method
The metal gets smaller and bubbles form. The collected gas gives a squeaky pop with a lighted splint.
Reason
The test supports identification of hydrogen; the equation conserves every atom.
Working
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
Carbonates: account for all three products
When calcium carbonate reacts with dilute hydrochloric acid, the salt is calcium chloride. The carbonate also gives carbon dioxide and water:
The coefficient 2 supplies two chlorine atoms for CaCl₂ and two hydrogen atoms for water. The three oxygen atoms from the carbonate appear as one in water and two in carbon dioxide. Omitting water breaks atom conservation as well as the reaction pattern.
Guided practice 2
Change the acid, then rebuild the products
Problem
Dilute nitric acid is added to calcium carbonate. Name all products and state the test and positive result for the gas.
Predict before revealing the worked answer
Hints
Hint 1: Salt ending
Which salt ending comes from nitric acid?
Hint 2: Conserve the carbonate oxygen
There are three products, not two.
View solution step by step
Name the salt
Method
Calcium nitrate.Reason
The acid changed from hydrochloric to nitric acid, so the salt is a nitrate rather than a chloride.
Working
calcium + nitrate → calcium nitrate, Ca(NO₃)₂
Complete the reaction
Method
The other products are water and carbon dioxide. Bubble the gas through limewater; it turns milky.
Reason
Changing the acid does not turn the carbonate reaction into the metal–hydrogen pattern.
Working
CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)
Neutralisation: what actually changes?
Copper(II) oxide is a base. When it reacts with dilute hydrochloric acid, the products are copper(II) chloride and water:
The black solid is used up as it reacts. There is no carbonate to produce carbon dioxide and no metal to produce hydrogen. Calling the oxide a metal would choose the wrong reaction pattern: a metal oxide is a compound, not the metal element.
With an aqueous alkali, the hydrogen ions from the acid react with hydroxide ions to form water. For hydrochloric acid and sodium hydroxide:
Sodium and chloride ions remain in the solution. These unchanged spectator ions are omitted when writing just the particles that react:
This is the ionic equation for aqueous acid–alkali neutralisation. There are two hydrogen atoms and one oxygen atom on each side. The total charge is zero on each side: + 1 + (-1) = 0.
Do not replace OH⁻ with O²⁻: an aqueous alkali supplies hydroxide ions. Do not cancel a species merely because its element appears on both sides; a spectator is the same ion, unchanged.
Neutralisation occurring does not establish that the final mixture has pH 7. With excess acid, hydrogen ions remain in excess and the mixture is acidic. With excess alkali, hydroxide ions remain in excess and it is alkaline. Without the reacting amounts or a final measurement, you cannot choose among those outcomes.
Bases also react with ammonium salts
Warm an ammonium salt with an alkali and ammonia is released. For example:
The pattern is ammonium salt + alkali → salt + water + ammonia. It differs from acid–alkali neutralisation: the ammonium salt supplies nitrogen, and a gas forms. In a supervised test, warm gently and hold damp red litmus in the escaping gas; ammonia turns it blue. The moisture allows the gas to dissolve and produce an alkaline solution. Ammonia manufacture is a separate topic, not needed to explain this reaction.
Repair a reaction, then try a new one
A learner writes ZnCO₃ + 2HCl → ZnCl₂ + H₂. The first error is treating a carbonate as a metal. Identify the reactant class before adjusting coefficients: the missing products are water and carbon dioxide. Balancing the wrong product list cannot repair the chemical prediction.
Try these before opening the answers:
- Zinc carbonate reacts with dilute hydrochloric acid. Write the corrected equation with state symbols, then name the gas test and positive result.
- Dilute nitric acid reacts with aqueous potassium hydroxide. Write the full equation and the neutralisation ionic equation. Explain which ions you omit and why.
- An acid–alkali mixture has a final meter reading of pH 4. Explain why this does not show that neutralisation failed to occur.
- A warmed ammonium chloride solution releases a gas after sodium hydroxide is added. State the gas and the observation with damp red litmus.
Compare your reactions and reasoning
1.
ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l) + CO₂(g). Carbon dioxide turns limewater milky.
- HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)
. The ionic equation is H + (aq) + OH-(aq) → H₂O(l). Potassium and nitrate ions remain unchanged in solution, so they are spectators.
- Some acid may have reacted, while excess acid remains. The final pH describes the remaining mixture, not whether any reaction took place.
- Ammonia; damp red litmus turns blue. Do not substitute the limewater test for carbon dioxide.
Next, use reaction evidence to choose a soil treatment and distinguish acidic, basic, amphoteric and neutral oxides.