Calcium

Calcium (Ca) is a Group II metal that ties together metals, acids-bases, and environmental chemistry. Its compounds — limestone, quicklime, and slaked lime — are among the most examined at IGCSE.

Where it appears in 0620

Few elements reach into as many topics as calcium:

• Reactions of metals: calcium with water is the “in-between” case — steadier than sodium, brisker than magnesium.
• Oxides and neutralisation: calcium oxide and calcium hydroxide are bases used to neutralise acidic soils and lakes.
• Air quality and climate change: limewater is the test for carbon dioxide, and powdered calcium carbonate treats acid rain damage.
• Reacting mass calculations: the thermal decomposition of calcium carbonate is a standard mole-calculation setup.

Position in the periodic table

• Symbol: Ca
• Atomic number: 20
• Group 2 (II), Period 4
• Electron configuration: 2, 8, 8, 2
• Forms Ca2+ ions by losing 2 electrons

Physical properties

Property	Detail
Appearance	Silver-white, tarnishes in air
Melting point	842 C
Density	1.55 g/cm3
Conductivity	Good conductor
Hardness	Relatively soft for a metal

Reactivity

Calcium is fairly high in the reactivity series, above hydrogen and above carbon.

With water

Ca(s) + 2H2O(l) -> Ca(OH)2(aq) + H2(g)

Observations:

• Calcium sinks at first, then rises as hydrogen bubbles coat it
• Steady effervescence
• The mixture turns slightly milky (Ca(OH)2 is only slightly soluble)
• The solution is alkaline (universal indicator turns blue/purple)

With dilute acids

Ca(s) + 2HCl(aq) -> CaCl2(aq) + H2(g)

Ca(s) + H2SO4(aq) -> CaSO4(s) + H2(g) — the reaction slows as insoluble CaSO4 coats the metal.

Flame test

Calcium gives an orange-red (brick-red) flame. See flame tests.

The limestone cycle

One of the most important reaction sequences at IGCSE, connecting three calcium compounds.

Step 1: Thermal decomposition of calcium carbonate

CaCO3(s) -> CaO(s) + CO2(g)

Calcium carbonate (limestone) is heated strongly. CaO is calcium oxide (quicklime).

Step 2: Calcium oxide + water

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

An exothermic reaction. Calcium hydroxide, Ca(OH)2, is slaked lime.

Step 3: Limewater test

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

Limewater (calcium hydroxide solution) turns milky as carbon dioxide is bubbled through, because a white precipitate of calcium carbonate forms.

Key calcium compounds

Compound	Formula	Common name	Key fact
Calcium carbonate	CaCO3	Limestone/marble	Decomposes on heating; reacts with acids
Calcium oxide	CaO	Quicklime	Basic oxide; exothermic with water
Calcium hydroxide	Ca(OH)2	Slaked lime	Alkali; limewater test for CO2
Calcium sulfate	CaSO4	Gypsum	Insoluble; coats calcium in H2SO4
Calcium chloride	CaCl2	—	Soluble; used as a drying agent

Test for Ca2+ ions

Adding NaOH(aq) gives no precipitate at IGCSE level, because Ca(OH)2 is slightly soluble. The flame test (orange-red) is the standard identification.

Key facts at a glance

Fact	Detail
Symbol	Ca
Proton number	20
Group / Period	2 (II) / 4
Ion formed	Ca2+
Flame colour	Orange-red (brick-red)
Signature reagent	Ca(OH)2 solution = limewater (tests for CO2)

Common exam mistakes

• Expecting a precipitate in the Ca2+ test. At IGCSE, calcium gives no precipitate with sodium hydroxide — you identify it by the orange-red flame instead.
• Mislabelling the limewater precipitate. The white solid that turns limewater milky is calcium carbonate (CaCO3), not calcium hydroxide.
• Not balancing Ca + water. Calcium is 2+ and needs two hydroxides: Ca + 2H2O -> Ca(OH)2 + H2, never “CaOH”.
• Mixing up the three limes. Limestone is CaCO3, quicklime is CaO, slaked lime is Ca(OH)2 — get these straight before the limestone cycle question.

Exam-style questions

Describe the limestone cycle. Write a balanced equation for each of the three steps. (6 marks)

Mark scheme

• calcium carbonate is heated (thermal decomposition): CaCO3 -> CaO + CO2 [2]
• water is added to calcium oxide to make calcium hydroxide (exothermic): CaO + H2O -> Ca(OH)2 [2]
• carbon dioxide is bubbled through calcium hydroxide solution, turning it milky: Ca(OH)2 + CO2 -> CaCO3 + H2O [2]

Examiner note: each step scores one mark for the description and one for a correct balanced equation — give both to secure full marks.

Calcium reacts with dilute sulfuric acid but the reaction soon stops even when acid remains. Explain why. (2 marks)

Mark scheme

• calcium sulfate (CaSO4) is insoluble [1]
• it coats the surface of the calcium, keeping the acid away from the metal so the reaction stops [1]

Examiner note: contrast this with hydrochloric acid, where soluble CaCl2 forms and the reaction runs to completion.