Reactions of Acids with Metals and Carbonates

General Equation

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

What Happens

When a reactive metal is added to an acid:

• Bubbles of hydrogen gas are produced
• The metal dissolves (disappears)
• The solution may get warm (exothermic)
• A salt is formed in solution

Examples

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$ Magnesium + hydrochloric acid → magnesium chloride + hydrogen

$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$ Zinc + sulfuric acid → zinc sulfate + hydrogen

$2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2$ Aluminium + hydrochloric acid → aluminium chloride + hydrogen

Testing for Hydrogen

Hold a burning splint near the mouth of the test tube. Hydrogen burns with a squeaky pop.

Which Metals React?

Only metals above hydrogen in the reactivity series react with dilute acids:

Copper, silver, and gold are below hydrogen in the reactivity series, so they don't react with dilute acids.

Note: Potassium and sodium react too violently with acids to be safe in the lab. Typically, magnesium, zinc, and iron are used.

General Equation

$\text{acid} + \text{metal oxide} \rightarrow \text{salt} + \text{water}$

Examples

$2\text{HCl} + \text{CuO} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O}$ Hydrochloric acid + copper oxide → copper chloride + water

$\text{H}_2\text{SO}_4 + \text{MgO} \rightarrow \text{MgSO}_4 + \text{H}_2\text{O}$ Sulfuric acid + magnesium oxide → magnesium sulfate + water

What Happens

• The metal oxide (often a coloured powder) dissolves
• The solution changes colour (e.g. CuO gives blue CuCl₂ solution)
• No gas is produced
• This is a neutralisation reaction

General Equation

$\text{acid} + \text{metal hydroxide} \rightarrow \text{salt} + \text{water}$

Examples

$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

$\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$

$2\text{HNO}_3 + \text{Cu(OH)}_2 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2\text{H}_2\text{O}$

This is also a neutralisation reaction.

General Equation

$\text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$

What Happens

• Fizzing/effervescence (bubbles of CO₂ gas)
• The carbonate dissolves
• A salt solution is formed

Examples

$2\text{HCl} + \text{CaCO}_3 \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$

$\text{H}_2\text{SO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2$

$2\text{HNO}_3 + \text{MgCO}_3 \rightarrow \text{Mg(NO}_3\text{)}_2 + \text{H}_2\text{O} + \text{CO}_2$

Testing for Carbon Dioxide

Bubble the gas through limewater ($\text{Ca(OH)}_2$ solution). If CO₂ is present, the limewater turns milky/cloudy:

$\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$

The cloudiness is caused by insoluble calcium carbonate particles.

Method 1: Acid + Excess Insoluble Reactant

Used when the base is insoluble (e.g. CuO, MgCO₃, Zn).

1. Warm the acid gently
2. Add excess insoluble reactant (metal, metal oxide, or carbonate) until no more dissolves
3. Filter to remove the excess solid
4. Evaporate/crystallise the filtrate to obtain salt crystals

Adding excess solid ensures all the acid has reacted.

Method 2: Titration

Used when both acid and base are soluble (e.g. HCl + NaOH).

1. Use a titration to find the exact volumes needed
2. Mix the exact volumes (without indicator)
3. Evaporate/crystallise to obtain the salt

• Learn the four general equations — they're used in most GCSE chemistry exams
• Know the gas tests: squeaky pop (H₂) and limewater turns milky (CO₂)
• When writing equations, check: is the acid HCl, H₂SO₄, or HNO₃? This tells you the salt ending
• For "describe how to make a salt" questions, include: add excess, filter, evaporate, crystallise
• Don't forget state symbols if asked: (s), (l), (g), (aq)

• Acid + metal → salt + hydrogen (only if metal is above H in reactivity series)
• Acid + metal oxide → salt + water
• Acid + metal hydroxide → salt + water
• Acid + metal carbonate → salt + water + CO₂
• Gas tests: H₂ = squeaky pop; CO₂ = limewater turns milky
• To make a salt: add excess insoluble reactant → filter → evaporate/crystallise