Acids with Carbonates

Reaction between acids and carbonates

When an acid reacts with a carbonate, a chemical reaction occurs producing three products:

• A salt
• Carbon dioxide gas (CO₂)
• Water (H₂O)

This reaction is easy to spot because it produces effervescence—bubbles of carbon dioxide gas escaping from the mixture. This happens because carbon dioxide is a gas formed during the reaction and escapes as bubbles.

The general word equation for this reaction is:

Acid + Carbonate 12; Salt + Carbon dioxide + Water

For example, when hydrochloric acid reacts with calcium carbonate (found in chalk or limestone), the products are calcium chloride (a salt), carbon dioxide, and water:

Calcium carbonate + Hydrochloric acid 12; Calcium chloride + Carbon dioxide + Water

This reaction is typical of all carbonates reacting with acids, regardless of the acid or carbonate involved. For example, nitric acid reacts with carbonates to form nitrates as the salt.

For instance, if you add dilute hydrochloric acid to calcium carbonate, you will see fizzing as carbon dioxide gas is released:

CaCO₃ (s) + 2HCl (aq) 12; CaCl₂ (aq) + CO₂ (g) + H₂O (l)

Carbon dioxide identification

To confirm that the gas produced in the reaction is carbon dioxide, you can test it using limewater (a solution of calcium hydroxide).

When carbon dioxide gas is bubbled through limewater, it turns the limewater milky (cloudy). This is because carbon dioxide reacts with calcium hydroxide to form calcium carbonate, which is insoluble and appears as a white precipitate:

CO₂ (g) + Ca(OH)₂ (aq) 12; CaCO₃ (s) + H₂O (l)

This test is a simple and reliable way to identify carbon dioxide gas produced in acid-carbonate reactions.

Writing word and symbol equations

The general word equation for the reaction between an acid and a carbonate is:

Acid + Carbonate 12; Salt + Carbon dioxide + Water

To write symbol equations, you need to:

• Write the correct formulae for the acid and carbonate
• Write the formula for the salt formed
• Include carbon dioxide and water as products
• Balance the equation so the number of atoms of each element is the same on both sides
• Use correct state symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water)

For example, the reaction of hydrochloric acid with sodium carbonate is:

2HCl (aq) + Na₂CO₃ (s) 12; 2NaCl (aq) + CO₂ (g) + H₂O (l)

Another example is the reaction between sulfuric acid and magnesium carbonate:

H₂SO₄ (aq) + MgCO₃ (s) 12; MgSO₄ (aq) + CO₂ (g) + H₂O (l)

Common carbonate examples

Several carbonates are commonly used or encountered in acid-carbonate reactions:

• Calcium carbonate (CaCO₃): Found in chalk, limestone, and marble.
• Sodium carbonate (Na₂CO₃): Also known as washing soda, used in cleaning products.
• Magnesium carbonate (MgCO₃): Used as an antacid and in sports chalk.

All these carbonates react with acids in the same way, producing a salt, carbon dioxide, and water.

For example, calcium carbonate reacts with hydrochloric acid to form calcium chloride, carbon dioxide, and water:

CaCO₃ (s) + 2HCl (aq) 12; CaCl₂ (aq) + CO₂ (g) + H₂O (l)

Similarly, sodium carbonate reacts with sulfuric acid to form sodium sulfate, carbon dioxide, and water:

Na₂CO₃ (s) + H₂SO₄ (aq) 12; Na₂SO₄ (aq) + CO₂ (g) + H₂O (l)

Learning example

Example: Write the balanced symbol equation for the reaction between hydrochloric acid and magnesium carbonate.

Step 1: Write the formulae for the reactants and products.

• Hydrochloric acid: HCl
• Magnesium carbonate: MgCO₃
• Salt formed: magnesium chloride, MgCl₂
• Carbon dioxide: CO₂
• Water: H₂O

Step 2: Write the unbalanced equation:

HCl (aq) + MgCO₃ (s) 12; MgCl₂ (aq) + CO₂ (g) + H₂O (l)

Step 3: Balance the equation.

There are 2 chlorine atoms in MgCl₂, so we need 2 HCl molecules:

2HCl (aq) + MgCO₃ (s) 12; MgCl₂ (aq) + CO₂ (g) + H₂O (l)

This is the balanced symbol equation.