Carbon Across IGCSE Chemistry

Carbon is arguably the most connected element in the 0620 syllabus. This page traces its appearances across every major topic to help you see links that examiners love to test.

1. Bonding and structure — allotropes

Carbon has three allotropes at IGCSE:

Allotrope	Structure	Key properties
Diamond	Giant covalent, each C bonded to 4 others	Very hard, very high MP, does not conduct
Graphite	Giant covalent layers, each C bonded to 3 others	Soft, very high MP, conducts electricity
Fullerenes (C60)	Molecular (hollow cages)	Lower MP, used in nanotechnology

The difference between diamond and graphite is a classic exam comparison. See covalent vs ionic bonding and giant covalent structure.

2. Organic chemistry — the backbone

Carbon forms the backbone of all organic compounds because it:

• Makes 4 covalent bonds
• Bonds to itself to form chains, branches, and rings
• Bonds to H, O, N, and halogens

Homologous series

Series	General formula	Functional group
Alkanes	CnH(2n+2)	C-C single bonds only
Alkenes	CnH(2n)	C=C double bond
Alcohols	CnH(2n+1)OH	-OH
Carboxylic acids	CnH(2n+1)COOH	-COOH

See organic naming rules and organic chemistry equations.

3. Metal extraction — reducing agent

Carbon (as coke) and carbon monoxide reduce metal oxides in the blast furnace:

Fe2O3 + 3CO -> 2Fe + 3CO2

Carbon can only reduce metals below it in the reactivity series. Metals above carbon (Al, Mg, Na, K, Ca) require electrolysis.

See extraction of metals and metals equations.

4. Environmental chemistry — CO2 and CO

Carbon dioxide (CO2)

• Product of complete combustion of any carbon-containing fuel
• A greenhouse gas that contributes to climate change
• Causes the greenhouse effect: absorbs infrared radiation re-emitted by Earth
• Tested with limewater (turns milky): Ca(OH)2 + CO2 -> CaCO3 + H2O
• See carbon dioxide

Carbon monoxide (CO)

• Product of incomplete combustion
• Colourless, odourless, toxic
• Binds to haemoglobin, preventing oxygen transport
• Removed from car exhausts by catalytic converters: 2CO + 2NO -> 2CO2 + N2

The carbon cycle

CO2 is removed from the atmosphere by photosynthesis and dissolving in oceans, and returned by respiration, combustion, and decomposition. Human burning of fossil fuels has increased atmospheric CO2, accelerating climate change.

5. Electrochemistry — graphite electrodes

Graphite is used as electrodes in electrolysis because:

• It conducts electricity (delocalised electrons)
• It has a very high melting point
• It is relatively inert (unreactive with most electrolytes)

Exception: in aluminium extraction, the carbon anodes burn in the oxygen produced at high temperature (C + O2 -> CO2) and must be replaced.

6. Acids and carbonates

All metal carbonates react with acids:

metal carbonate + acid -> salt + water + CO2

This is a key pattern in acid-base equations. The thermal decomposition of carbonates also produces CO2.

7. Carbon in stoichiometry

Carbon’s Ar (12) is used as the basis for the relative atomic mass scale. Many calculation questions involve carbon compounds (e.g. Mr of CaCO3 = 100).

Exam connections to watch for

• “Describe the environmental impact of burning fossil fuels” — links combustion, CO2, greenhouse effect, acid rain
• “Compare diamond and graphite” — links bonding, structure, properties
• “Explain why iron is extracted by carbon but aluminium is not” — links reactivity series, extraction methods
• “Distinguish complete from incomplete combustion” — links products, safety, pollution

Carbon appears in metal extraction, combustion, and bonding. Give one role of carbon in each context. [3 marks]

• Metal extraction: carbon acts as a reducing agent to remove oxygen from metal oxides (e.g. Fe2O3 + 3CO -> 2Fe + 3CO2) [1]
• Combustion: carbon in fuels burns to produce CO2 (complete) or CO/C (incomplete) [1]
• Bonding: carbon exists as allotropes (diamond, graphite) with giant covalent structures; graphite conducts electricity due to delocalised electrons [1]

Explain why increased burning of fossil fuels contributes to climate change. [3 marks]

• Burning fossil fuels produces carbon dioxide: e.g. CH4 + 2O2 -> CO2 + 2H2O [1]
• CO2 is a greenhouse gas that absorbs/traps infrared radiation re-emitted by the Earth [1]
• Increased CO2 concentration enhances the greenhouse effect, raising global temperatures [1]