Carbon forms two main oxides: Carbon (II) oxide (CO) and Carbon (IV) oxide (CO₂). Here, we focus on Carbon (IV) oxide, commonly known as carbon dioxide.

Carbon dioxide is a colorless, odorless gas with the chemical formula CO₂. It is slightly acidic and plays a vital role in respiration, photosynthesis, and various industrial processes.

Carbon dioxide can be prepared in the laboratory by the action of dilute acid on marble chips (Calcium trioxocarbonate (IV). This reaction leads to effervescence (bubbling of gas) with the evolution of a colourless gas which is then collected over water:

1. Step 1: Take a suitable carbonate like calcium carbonate (CaCO₃)
2. Step 2: Add a dilute acid such as hydrochloric acid (HCl) to the solid carbonate:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

3. Step 3: Collect the evolved gas over water or by downward displacement of air.

Note: The gas is dried using conc \(H_2SO_4 \) or fused \(CaCl_2 \). Dilute \(H_2SO_4\) is not used in the preparation of \(CO_2\) because of the formation of insoluble calcium sulphate which prevents the reaction. Industrially, Carbon (IV) oxide is produced during fermentation processes

• Colorless and odorless gas.
• Denser than air (sinks in air).
• Soluble in water to form carbonic acid (H₂CO₃).
• Non-flammable and does not support combustion.
• Exists as a solid (dry ice) below -78°C.
• Gas is slightly acidic in nature, turns damp blue litmus paper red

• Reaction with Water:
  • CO₂ dissolves in water to form carbonic acid which is a weak acid:

  CO₂ + H₂O ⇌ H₂CO₃

• Reaction with Metals:
  • Carbon dioxide can react with very reactive metals like sodium at high temperatures:

  CO₂ + 4Na → 2Na₂O + C

• Reaction with Metal Oxides:
  • CO₂ reacts with metal oxides to form carbonates:

  CO₂ + CaO → CaCO₃

• Reaction with Bases:
  • CO₂ reacts with alkalis to form carbonates and bicarbonates:

  CO₂ + 2NaOH → Na₂CO₃ + H₂O

  CO₂ + NaOH → NaHCO₃ (if limited NaOH)

• Reducing Ability:
  • At very high temperatures, CO₂ can act as a mild oxidizing agent, accepting electrons from some metals and forming CO.

  Fe + CO₂ → FeO + CO

• Reaction with burning magnesium: Magnesium burns in carbon (IV) oxide to yield magnesium oxide ash

  2Mg + CO₂ → 2MgO + C

• Oxidizing Ability:
  • CO₂ can oxidize reactive metals or carbon-containing substances at high temperatures:

  C + CO₂ → 2CO

• Used in carbonated drinks and soft drinks.
• Used as a fire extinguisher for electrical and oil fires.
• Used in the manufacture of dry ice for refrigeration.
• Used in greenhouses to enhance plant growth.
• Used in the preparation of chemicals like urea and sodium carbonate.

Pass a sample of the unknown gas into a solution of lime water, if it turns milky the gas is likely carbon (IV) oxide

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Further Observation: On passing CO₂ for a longer time, the milkiness gradually disappears.

Explanation: Excess CO₂ reacts with the calcium carbonate to form soluble calcium hydrogen carbonate:

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble)

On heating the solution, the milkiness reappears due to the formation of calcium carbonate

Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O

The formation of a milky or cloudy precipitate (which later dissolves in excess gas) confirms the presence of carbon (IV) oxide gas.

• Use freshly prepared lime water; old lime water may already contain carbonates and give a weak test.
• Ensure the gas is pure and free from acidic vapors that can affect the result.
• Do not bubble the gas too vigorously to avoid splashing of the lime water.