Calcium Oxide

Calcium oxide, or quicklime, is the first product of the limestone cycle: heat calcium carbonate hard enough and it breaks down to CaO. From there a drop of water converts it to slaked lime, so CaO is the link between a rock and an alkali.

Key facts

Property	Detail
Formula	CaO
Relative formula mass (Mr)	56 (Ca 40, O 16)
Bonding / structure	Giant ionic lattice (Ca2+ and O2- ions)
Classification	Basic oxide
Made from	Thermal decomposition of CaCO3
Reaction with water	Vigorous and exothermic, forming Ca(OH)2

Where this compound appears in 0620

• Thermal decomposition (Topic 11 / carbonates): CaO is the classic decomposition product of heating limestone.
• Oxides (Topic 8): a basic oxide that neutralises acids.
• Extraction of metals (Topic 10): in the blast furnace, CaO removes acidic silica impurities as slag.
• Air quality and climate change (Topic 11): neutralising acidic soils and lakes, and scrubbing sulfur dioxide from flue gases.
• Energetics: CaO + water is a standard exothermic reaction.

Reactions you must know

Production (in a lime kiln):

CaCO3(s) -> CaO(s) + CO2(g) (thermal decomposition, strong heating)

With water (slaking):

CaO(s) + H2O(l) -> Ca(OH)2(s)

A vigorous exothermic reaction — the solid crumbles, steams and gets noticeably hot, producing calcium hydroxide (slaked lime).

With acids (basic oxide):

CaO(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l)

In the blast furnace:

CaO(s) + SiO2(s) -> CaSiO3(l) (calcium silicate, the molten slag)

Removing sulfur dioxide from flue gases:

CaO(s) + SO2(g) -> CaSO3(s)

Uses at a glance

Use	How CaO acts
Neutralising acidic soil	Basic oxide raising the soil pH
Flue-gas desulfurisation	Reacts with acidic SO2
Blast-furnace flux	Removes SiO2 as slag
Making cement	Roasted with clay

For treating acidic soil, quicklime acts fastest because it is the most reactive, but it is caustic and awkward to handle, so farmers often use the gentler calcium hydroxide (slaked lime) or powdered calcium carbonate instead. All three neutralise soil acidity — they differ in speed and safety, not in the chemistry.

Worked exam question

Limestone (calcium carbonate) is heated strongly in a lime kiln to make quicklime, which is then used to make slaked lime.

(a) Name quicklime and slaked lime and give the formula of each. (2 marks)

(b) Write the equation for the reaction in the lime kiln and state the type of reaction. (2 marks)

(c) A few drops of water are added to solid quicklime. State one observation and explain it. (2 marks)

Mark scheme

• (a) Quicklime is calcium oxide, CaO [1]; slaked lime is calcium hydroxide, Ca(OH)2 [1]
• (b) CaCO3 -> CaO + CO2 [1]; thermal decomposition [1]
• (c) The solid gets hot / gives off steam / crumbles [1]; because the reaction of calcium oxide with water is exothermic (releases heat) [1]

Examiner note: in (b), “decomposition” alone is insufficient — the mark requires “thermal decomposition”. A frequent error in (a) is swapping the two names; quicklime is the oxide, slaked lime is the hydroxide.

Common exam mistakes

• Swapping quicklime and slaked lime. Quicklime (CaO) is made in the kiln; adding water gives slaked lime, Ca(OH)2.
• Losing the CO2. The decomposition of limestone must show carbon dioxide as a product, and it is not reversible under exam conditions.
• Saying CaO dissolves in water. It reacts with water rather than dissolving, forming slightly soluble calcium hydroxide.
• Calling the slaking reaction endothermic. Adding water to quicklime releases heat strongly — it is exothermic.