Carbon Dioxide, Carbon Monoxide & Carbonates

Carbon Dioxide (CO₂)

Definition

Carbon dioxide is a colourless, odourless gas with formula CO₂. It is formed when carbon or carbon compounds burn in excess oxygen, and is produced in respiration and many industrial processes. In the atmosphere its concentration is about 0.04% (approx. 400 ppm; varies with location and time).

Preparation / Sources

1. Action of acids on carbonates (common laboratory method):

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

   (e.g., reaction of limestone or marble with dilute acid)
2. Thermal decomposition of metal carbonates:

   CuCO₃(s) → CuO(s) + CO₂(g)

   (also: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ on heating)
3. By-product of fermentation — alcoholic fermentation produces CO₂ (used in bread and brewing).
4. Complete combustion of hydrocarbons:

   CH₄ + 2O₂ → CO₂ + 2H₂O

Tests for Carbon Dioxide

• Limewater test: Bubble gas through limewater (Ca(OH)₂). A white precipitate of CaCO₃ forms and the solution turns milky.

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

• If excess CO₂ is passed the white precipitate dissolves to give a clear solution of calcium hydrogen carbonate:

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

Physical Properties

• Colourless, odourless gas.
• Denser than air (so it collects in low places).
• Does not support combustion (it extinguishes flames).
• Sublimes at about −78 °C to form solid "dry ice".
• Slightly soluble in water; dissolved CO₂ forms carbonic acid (H₂CO₃).

Chemical Properties

• CO₂ is slightly acidic in water:

  CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

  (carbonic acid is weak and partly dissociates)
• Reacts with alkalis to form carbonates or hydrogen carbonates:

  CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)

  CO₂(g) + NaOH(aq) → NaHCO₃(aq)  (if limited)

• Redox with very reactive metals (e.g., magnesium at high temperature) can reduce CO₂:

  2Mg(s) + CO₂(g) → 2MgO(s) + C(s)

  (This is a high-temperature reaction — not for school practicals.)

Uses

• Used in fizzy drinks to give sparkle (carbonation).
• Used in fire extinguishers because it does not support combustion.
• Used in refrigeration as dry ice for cooling and transport.
• Used in the manufacture of carbonates and as a raw material (e.g., in soda ash production).

Carbon Monoxide (CO)

Definition

Carbon monoxide is a colourless, odourless and highly poisonous gas with formula CO. It is produced by incomplete combustion of carbon-containing fuels (when oxygen supply is limited).

Formation (common sources)

• Incomplete combustion of fuels: e.g.,

  2CH₄ + 3O₂ → 2CO + 4H₂O

  (insufficient oxygen)
• Car exhausts, faulty heaters, and poorly ventilated combustion appliances.

Tests & Identification

There is no simple safe home test for CO gas. In laboratory settings, the gas can be burnt in air to form CO₂ (blue flame) but handling CO is dangerous. The most important real-world safety measure is prevention and detection (carbon monoxide alarms).

Properties

• Colourless and odourless — hard to detect without an alarm.
• Very poisonous — it combines with haemoglobin to form carboxyhaemoglobin, preventing oxygen transport and causing suffocation.
• Burns to form carbon dioxide:

  2CO + O₂ → 2CO₂

Uses (industrial)

• Used as a reducing agent in metallurgy and in the production of synthesis gas (CO + H₂) for chemical manufacture.

Carbonates (CO₃²⁻)

Definition

Carbonates are salts that contain the carbonate ion CO₃²⁻. Common examples include calcium carbonate (CaCO₃) and sodium carbonate (Na₂CO₃).

Formation & Reactions

• Carbonates + acid → salt + water + CO₂:

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

• Thermal decomposition of carbonates (some):

  CuCO₃(s) → CuO(s) + CO₂(g)

• Thermal decomposition of bicarbonates:

  2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)

Tests for Carbonate Ion

1. Add dilute acid (e.g., HCl) to the suspected solid or solution. Effervescence (bubbles) shows gas evolution.
2. Collect the gas and test with limewater — it turns milky (CO₂ test).

Common Carbonates & Uses