Acids Reacting with Carbonates

Introduction

A drop of dilute hydrochloric acid on a piece of limestone fizzes. Geologists carry small bottles of acid into the field for exactly this reason: fizzing reveals a carbonate rock. Carbonates are bases that neutralise acids like oxides and hydroxides do, but with one extra product — carbon dioxide gas. That gas explains the fizzing and gives us a simple chemical test to confirm what has happened.

Core explanation

The general equation.

metal carbonate + acid → salt + water + carbon dioxide

This reaction has three products, not two. The carbonate neutralises the acid and releases CO₂ as bubbles.

Examples.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g)

Na₂CO₃(aq) + 2HNO₃(aq) → 2NaNO₃(aq) + H₂O(l) + CO₂(g)

The salt again combines the metal ion from the base with the anion from the acid.

What happens to the ions? The carbonate ion accepts two hydrogen ions. This briefly forms carbonic acid, which is unstable and breaks down:

CO₃²⁻ + 2H⁺ → H₂CO₃ → H₂O + CO₂

So the overall ionic equation is:

CO₃²⁻ + 2H⁺(aq) → H₂O(l) + CO₂(g)

For an insoluble carbonate such as calcium carbonate, the solid is included: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g).

Observations. Effervescence (fizzing) as carbon dioxide escapes; an insoluble carbonate disappears; the solution may change colour, such as green copper(II) carbonate giving a blue solution. Once the acid is used up, fizzing stops and any extra carbonate stays as solid — a handy sign that neutralisation is complete.

The limewater test. To show that the gas is carbon dioxide, it is bubbled through limewater , a clear solution of calcium hydroxide. Carbon dioxide reacts to form insoluble calcium carbonate, which appears as a white precipitate , so the limewater turns milky or cloudy:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

This is the standard test for carbon dioxide. A lighted splint is not a reliable test: CO₂ puts out a flame, but so do other gases such as nitrogen.

Carbonates compared with other bases.

Base Products with acid Gas? --- --- --- Metal oxide Salt + water No Metal hydroxide Salt + water No Metal carbonate Salt + water + carbon dioxide Yes, CO₂ Metal (not a base) Salt + hydrogen Yes, H₂

Step-by-step reasoning

To write a carbonate–acid equation:

1. Find the metal ion charge from the carbonate formula (CaCO₃ → Ca²⁺; Na₂CO₃ → Na⁺). 2. Combine the metal ion with the acid's anion to give the salt. 3. Write carbonate + acid → salt + H₂O + CO₂. 4. Balance: one CO₃²⁻ always needs two H⁺ and gives one H₂O and one CO₂. 5. Add state symbols, with CO₂ as (g).

Visual explanation

Imagine a marble chip in acid seen at particle scale. Pairs of H⁺ ions land on each carbonate ion at the surface; the carbonate splits into a water molecule and a CO₂ molecule, which joins other CO₂ molecules as a bubble and rises. The Ca²⁺ ion left behind drifts into the solution.

Real-world analogy

Opening a shaken bottle of fizzy drink releases a rush of carbon dioxide. The acid–carbonate reaction is similar: carbon dioxide is being made in the liquid faster than it can stay dissolved, so it escapes as a stream of bubbles.

Real-world example

Limestone buildings and marble statues slowly wear away in acid rain. The acids react with calcium carbonate, forming soluble salts that wash away and releasing carbon dioxide. Fine carvings lose their detail over decades.

Why?

Why does limewater turn milky? Calcium hydroxide is soluble enough to give a clear solution, but calcium carbonate is insoluble. When CO₂ reacts with the dissolved calcium hydroxide, tiny particles of solid calcium carbonate form and scatter light, making the liquid look cloudy.

Common misconception

"The fizzing is hydrogen, just like with metals." Carbonates contain no free metal to reduce H⁺ ions. The gas comes from the carbonate ion itself breaking down, so it is carbon dioxide, confirmed by limewater, not by the pop test.

Worked example

Question: Write a balanced equation with state symbols for magnesium carbonate reacting with hydrochloric acid.

Reasoning: MgCO₃ contains Mg²⁺; with Cl⁻ the salt is MgCl₂. One carbonate needs two H⁺, so 2HCl, giving one H₂O and one CO₂.

Answer: MgCO₃(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) + CO₂(g).

Quick check

1. What is seen when carbon dioxide is bubbled through limewater? Answer: The limewater turns milky (cloudy) as a white precipitate of calcium carbonate forms.

Exam focus

Learn all three products: salt, water and carbon dioxide — a common error is to leave out water. Describe the limewater test precisely: "limewater turns milky", not "turns white" or "goes cloudy then clear". Be ready to compare this reaction with metal–acid reactions and their different gas tests.

Advanced insight

If carbon dioxide continues to bubble through milky limewater for a long time, the solution clears again. The extra CO₂ converts insoluble calcium carbonate into soluble calcium hydrogencarbonate, Ca(HCO₃)₂. The same reaction, happening slowly underground, dissolves limestone to form caves.

Summary

Metal carbonates react with acids to form a salt, water and carbon dioxide; each CO₃²⁻ ion accepts two H⁺. The reaction fizzes, and insoluble carbonates disappear until the acid is used up. Carbon dioxide is identified by turning limewater milky, because insoluble calcium carbonate is precipitated.

Practice questions

1. Complete the word equation: calcium carbonate + nitric acid → ? Answer: Calcium nitrate + water + carbon dioxide. 2. Write a balanced equation with state symbols for copper(II) carbonate reacting with hydrochloric acid. Answer: CuCO₃(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l) + CO₂(g). 3. Write the ionic equation for a carbonate ion reacting with hydrogen ions. Answer: CO₃²⁻ + 2H⁺(aq) → H₂O(l) + CO₂(g). 4. A student adds a white powder to acid and sees fizzing. Describe how to show whether the gas is carbon dioxide or hydrogen. Answer: Bubble the gas through limewater: carbon dioxide turns it milky. Alternatively, hold a lighted splint at the mouth of the tube: hydrogen gives a squeaky pop.