Carbonates are salts formed when all or part of the hydrogen in carbonic acid (H₂CO₃) is replaced by a metal or an ammonium ion. Their general formula is MCO₃ or (NH₄)₂CO₃, where M is a metal.

• Most carbonates are solids at room temperature.
• All carbonates decompose on heating to give the metal oxide and carbon dioxide, except Na₂CO₃ and K₂CO₃.
• Carbonates of metals like Na, K, and NH₄ are soluble in water; others are insoluble.
• Carbonates react with acids to produce carbon dioxide, water, and a salt.
• Many carbonates (e.g., CaCO₃) occur naturally as minerals such as limestone, marble, and chalk.

1. Preparation of Soluble Carbonates

By reacting an alkali with carbon dioxide gas:

$$ 2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O $$

Example: Sodium carbonate can be prepared by passing carbon dioxide through a solution of sodium hydroxide.

2. Preparation of Insoluble Carbonates

By precipitation — reacting a soluble carbonate with a soluble salt of the required metal:

$$ \small{Na_2CO_3 + CaCl_2 \rightarrow CaCO_3 \downarrow + 2NaCl} $$

Example: Calcium carbonate is obtained as a white precipitate when sodium carbonate solution is added to calcium chloride solution.

Carbonates can be identified by their reaction with acids to produce carbon dioxide gas, which turns limewater milky.

1. Procedure: Add dilute hydrochloric acid to a small amount of the carbonate.
2. Observation: Effervescence (bubbling) occurs — gas is evolved.
3. Confirmatory Test: Pass the gas through limewater (Ca(OH)₂ solution).
4. Result: Limewater turns milky, confirming the presence of a carbonate.

Reactions:
$$ \small{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2} $$
$$ Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O $$

• Calcium carbonate (CaCO₃): Used in building materials (limestone, marble, chalk), cement, and glass manufacture.
• Sodium carbonate (Na₂CO₃): Used in detergents, glass, and water softening.
• Potassium carbonate (K₂CO₃): Used in glass and soap making.
• Ammonium carbonate ((NH₄)₂CO₃): Used as a smelling salt and leavening agent in baking.
• Magnesium carbonate (MgCO₃): Used in medicine and cosmetics as an antacid or drying agent.
• Copper(II) carbonate (CuCO₃): Used as a pigment and in fungicides.
• Lead(II) carbonate (PbCO₃): Formerly used in paints (now avoided due to toxicity).