Carbon Dioxide (CO2)

Summary: Carbon dioxide (CO₂) is a linear covalent molecule containing two C=O double bonds — carbon reaches its octet through four shared pairs in total; it turns limewater milky, is produced by complete combustion and respiration, and is a major greenhouse gas. Tags: igcse chemistry Created: 2026-07-18

Carbon dioxide (CO₂) is a Compound of carbon and oxygen and the syllabus example of a Molecule containing two double covalent bonds. Carbon has four valence electrons and needs four more for an octet, while each oxygen has six and needs two more; by sharing electrons, carbon forms a double bond with each of the two oxygen atoms, so the carbon atom is surrounded by 4 shared pairs in total and every atom achieves a full outer Electron Shell. The molecule is linear (O=C=O) and, because only weak forces act between molecules, CO₂ is a gas at room temperature. It is produced by the complete combustion of carbon-containing fuels, by respiration, by the thermal decomposition of Carbonates and by the reaction of acids with carbonates, and it is identified because it turns limewater milky (Chemical Tests). Although it makes up only a small fraction of the Composition of Air, carbon dioxide is a major greenhouse gas, and its rising concentration from burning fossil fuels drives global warming (see Climate Change and Greenhouse Gases).

Bonding: Dot-and-Cross Description

In a Dot-and-Cross Diagram of CO₂, carbon’s electrons are drawn as dots and the oxygens’ as crosses:

• Carbon shares 2 of its electrons with each oxygen, and each oxygen shares 2 electrons back, so each C=O bond is a double bond containing 2 shared pairs (two dots and two crosses).
• With two double bonds, carbon is surrounded by 4 shared pairs (8 electrons) — a complete octet.
• Each oxygen counts 8 outer electrons: 4 in its double bond + 2 lone pairs of its own.
• The two double bonds repel each other equally, so the molecule is linear: O=C=O, with a bond angle of 180°.

Displayed formula: O=C=O.

Key Data

Physical Properties

• Simple molecular gas: low melting/boiling point because only weak intermolecular forces are overcome — the strong C=O bonds are not broken on changing state.
• Denser than air — it can be “poured” and collects downwards, which is why it is used in fire extinguishers.
• Slightly soluble in water, forming weakly acidic carbonic acid (rain is naturally slightly acidic — see Acids and Bases).
• Does not conduct electricity — no ions or delocalised electrons.

Test for Carbon Dioxide

Bubble the gas through limewater (calcium hydroxide solution). CO₂ turns it milky/cloudy as insoluble calcium carbonate forms:

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

With excess CO₂ the milkiness slowly clears as soluble calcium hydrogencarbonate forms. See Chemical Tests and Carbonates.

Sources and Roles

• Complete combustion of carbon-containing fuels: CH₄ + 2O₂ → CO₂ + 2H₂O (see Fuels and Petroleum).
• Respiration in living cells releases CO₂ as a waste product.
• Thermal decomposition of carbonates, e.g. CaCO₃ → CaO + CO₂, and acid + carbonate reactions (see Carbonates and Making Salts).
• Greenhouse gas — CO₂ absorbs infrared radiation, trapping heat in the atmosphere; increasing levels cause global warming and climate change (see Climate Change and Greenhouse Gases and Air Pollution).
• Used in fizzy drinks, fire extinguishers and as dry ice for cooling.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions