Reactions of Acids and Making Salts

Acids react with some metals to produce a salt and hydrogen gas. For AQA GCSE Chemistry, you only need the reactions of magnesium, zinc and iron with hydrochloric acid and sulfuric acid. Copper is below hydrogen in the reactivity series and does not react.

$\text{metal}+\text{acid}\to \text{salt}+\text{hydrogen}$

Hydrochloric acid makes chloride salts and sulfuric acid makes sulfate salts. Two worked equations:

$Mg+2HCl\to MgC{l}_2+H_2$

$Zn+H_{2}S{O}_4\to ZnS{O}_4+H_2$

The hydrogen produced can be tested with a lit splint: it burns with a squeaky pop. The more reactive the metal, the faster the fizzing, so magnesium reacts faster than zinc, which reacts faster than iron.

Acid + metal → salt + hydrogen. Keep this limited to Mg, Zn and Fe with hydrochloric and sulfuric acid, exactly as the specification requires.

(Higher Tier only) Explaining the reaction of acids with metals as a redox reaction in terms of electrons is assessed at Higher Tier.

When a metal reacts with an acid, electrons are transferred, so the reaction is a redox reaction. The metal atoms lose electrons (are oxidised) and the hydrogen ions from the acid gain electrons (are reduced).

Take magnesium reacting with hydrochloric acid. The ionic equation, with chloride as a spectator ion removed, is:

$Mg+2H^{+}\to M{g}^{2+}+H_2$

Splitting this into half equations shows the electron transfer:

$Mg\to M{g}^{2+}+2e^{-}\text{(oxidation: loses electrons)}$

$2H^{+}+2e^{-}\to H_2\text{(reduction: gains electrons)}$

So the metal is oxidised and the hydrogen ions are reduced. This is the same electron-transfer idea (OIL RIG) used for displacement reactions.

An acid is neutralised by a base. Bases include alkalis (soluble metal hydroxides), insoluble metal hydroxides and metal oxides, and metal carbonates. The general patterns are:

Reactant with acid	Products
Alkali (soluble metal hydroxide)	salt + water
Metal oxide or insoluble hydroxide (base)	salt + water
Metal carbonate	salt + water + carbon dioxide

The salt produced depends on two things:

• The acid sets the second part of the name: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
• The positive ion of the base, alkali or carbonate sets the first part of the name.

Worked examples:

$\text{sodium hydroxide}+\text{hydrochloric acid}\to \text{sodium chloride}+\text{water}$ $NaOH+HCl\to NaCl+H_{2}O$

$\text{copper oxide}+\text{sulfuric acid}\to \text{copper sulfate}+\text{water}$ $CuO+H_{2}S{O}_4\to CuS{O}_4+H_{2}O$

$\text{calcium carbonate}+\text{hydrochloric acid}\to \text{calcium chloride}+\text{water}+\text{carbon dioxide}$ $CaC{O}_3+2HCl\to CaC{l}_2+H_{2}O+C{O}_2$

Only the carbonate reaction releases carbon dioxide. Oxides and hydroxides give salt and water with no gas.

To write a salt's formula, balance the charges of its ions so the overall charge is zero. You need the common ion charges: the metal ion (from its group or given), and the acid ion (chloride $C{l}^{-}$, nitrate $N{O}_3^{-}$, sulfate $S{O}_4^{2-}$).

Worked example — magnesium chloride. Magnesium forms $M{g}^{2+}$; chloride is $C{l}^{-}$. To balance a $2+$ charge you need two $C{l}^{-}$ ions, giving $MgC{l}_2$.

Worked example — aluminium sulfate. Aluminium forms $A{l}^{3+}$; sulfate is $S{O}_4^{2-}$. The lowest common multiple of $3$ and $2$ is $6$, so you need two $A{l}^{3+}$ (total $+6$) and three $S{O}_4^{2-}$ (total $-6$), giving $A{l}_2(S{O}_4{)}_3$.

Ion	Formula	Charge
Chloride	$C{l}^{-}$	$1-$
Nitrate	$N{O}_3^{-}$	$1-$
Sulfate	$S{O}_4^{2-}$	$2-$
Magnesium	$M{g}^{2+}$	$2+$
Aluminium	$A{l}^{3+}$	$3+$

The total positive charge must equal the total negative charge. Add subscripts to balance the charges, then wrap a polyatomic ion in brackets before adding a subscript, as in $A{l}_2(S{O}_4{)}_3$.

Required Practical 1 is to prepare a pure, dry sample of a soluble salt from an insoluble base — a metal oxide or carbonate. The method makes copper sulfate from copper(II) oxide and sulfuric acid, but the same steps work for any insoluble base and acid pair.

Method:

1. Warm the dilute sulfuric acid gently with a Bunsen burner (this speeds up the reaction). Do not boil the acid dry.
2. Add the copper oxide a little at a time, stirring, until some solid remains unreacted even after stirring. This shows an excess of the solid has been added.
3. Filter the mixture. The unreacted excess copper oxide stays on the filter paper; the blue copper sulfate solution passes through.
4. Pour the filtered solution into an evaporating basin and heat gently, using a water bath or electric heater, to evaporate about half the water. Stop heating once crystals begin to form and leave the rest to crystallise slowly.
5. Remove the crystals, pat them dry between filter paper, and you have a pure, dry sample of copper sulfate.

The reaction is:

$CuO+H_{2}S{O}_4\to CuS{O}_4+H_{2}O$

Understanding why each step is done matters more than the result:

Step	Why it is done
Add solid in excess	Ensures all the acid reacts, so no acid contaminates the salt
The base is insoluble	Any excess can be filtered off; a soluble base could not be removed
Filter	Removes the unreacted excess solid, leaving pure salt solution
Evaporate gently / water bath	Removes water slowly so large, well-formed crystals grow and the salt is not decomposed by overheating

Excess insoluble base is the key idea: it guarantees the acid is used up, and being insoluble it can be filtered away. This is why the method starts from an oxide or carbonate, not from a soluble alkali.

1. Wrong second part of the salt name

The acid decides the ending: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates. Do not write "sulfur" for the sulfate from sulfuric acid.

2. Forgetting the carbon dioxide from carbonates

Acid + carbonate gives salt + water and carbon dioxide. Only the carbonate reaction fizzes with a gas; oxides and hydroxides give just salt and water.

3. Not explaining why excess solid is added

The mark is for the reasoning: excess solid ensures all the acid reacts, and because the solid is insoluble the excess can be filtered off. State both parts.

4. Boiling the salt solution dry

Evaporate gently and let the solution crystallise; do not boil it to dryness. Slow crystallisation gives larger, purer crystals, and strong heating can decompose some salts.

5. Saying copper reacts with acid to give hydrogen

Copper is below hydrogen in the reactivity series, so it does not react with dilute acid. To make a copper salt, react copper oxide (a base) with the acid, not copper metal.

6. Confusing acid + metal with acid + base

Acid + metal gives salt + hydrogen; acid + base (oxide or hydroxide) gives salt + water only. Check whether the reactant is the metal or its oxide before choosing the products.