9701 Chemistry · Topic 10 · AS Level
Group 2 Cheat Sheet — A Level Chemistry 9701
Group 2 is one of the most predictable sources of marks in 9701 — provided you can state the trends and, more importantly, explain them using ionic radius and charge density. This sheet covers reactivity with water and oxygen, the opposite solubility trends of the hydroxides and sulfates, thermal decomposition of the carbonates and nitrates, and the uses examiners ask about.
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What’s on this cheat sheet
Group 2 — The Alkaline Earth Metals
01 · The group
Be, Mg, Ca, Sr, Ba — all end in ns² and form 2+ ions by losing both outer electrons.
Silvery reactive metals, always found combined in nature. Reactivity increases down the group.
02 · Why reactivity rises down the group
Atomic radius increases and shielding increases, so the outer electrons are held less strongly and the first and second ionisation energies fall.
The metal is therefore a stronger reducing agent further down, and forms its 2+ ion more readily.
03 · Reaction with oxygen
2M + O₂ → 2MO. All burn vigorously to a white ionic oxide.
Flame colours: Mg brilliant white, Ca brick red, Sr crimson, Ba apple green.
04 · Reaction with water
Mg very slow in cold water, but Mg + H₂O(g) → MgO + H₂ with steam
Ca steady fizzing, cloudy suspension
Sr, Ba increasingly vigorous
The resulting solution is alkaline and gets more so down the group as the hydroxides become more soluble.
05 · Reaction with dilute acid
M + 2HCl → MCl₂ + H₂, increasingly vigorous down the group.
With H₂SO₄ the reaction slows or stops for Ca, Sr and Ba: the insoluble sulfate coats the metal and blocks further attack.
06 · Solubility trends
| Compound | Down the group |
|---|---|
| hydroxides | solubility increases |
| sulfates | solubility decreases |
Mg(OH)₂ is nearly insoluble (milk of magnesia); Ba(OH)₂ dissolves freely. BaSO₄ is so insoluble it is safe to swallow as a barium meal, and is the white precipitate in the sulfate test.
07 · Thermal stability of carbonates
Stability increases down the group, so the decomposition temperature rises: MgCO₃ decomposes easily, BaCO₃ needs very strong heating.
Reason: the cation gets larger and less polarising, so it distorts the carbonate ion less. Nitrates follow the same trend: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
08 · Polarising power explained
A small, highly charged cation pulls electron density from the large anion, weakening a C–O bond within CO₃²⁻ and making decomposition easier.
Charge density falls down the group as the ionic radius grows, so polarisation falls and stability rises. The same argument explains why Group 1 carbonates are more stable than Group 2 ones.
09 · The oxides and hydroxides
MO + H₂O → M(OH)₂ — all basic, and increasingly so down the group.
All react with acids: MO + 2HCl → MCl₂ + H₂O and M(OH)₂ + H₂SO₄ → MSO₄ + 2H₂O.
10 · Uses
Ca(OH)₂ / CaO — neutralising acidic soil and acidic industrial gases; CaO in the blast furnace removes silica as slag.
Mg(OH)₂ and CaCO₃ — antacids: they neutralise excess stomach acid without being corrosive.
BaSO₄ — X‑ray contrast medium; Mg — lightweight alloys and sacrificial protection.
11 · Worked example — identify the metal
Cold‑water reaction rules out Mg
Brick‑red flame → calcium
Ca + 2H₂O → Ca(OH)₂ + H₂
Cloudiness is the sparingly soluble Ca(OH)₂.
12 · Worked example — thermal decomposition
n(CaCO₃) = 5.00 ÷ 100.1 = 0.0500 mol
1 : 1 → n(CaO) = 0.0500 mol
m = 0.0500 × 56.1 = 2.80 g
The 2.20 g lost is CO₂, which at r.t.p. occupies 1.20 dm³.
13 · Equations to know
Mg + H₂O(g) → MgO + H₂
Ca + 2H₂O → Ca(OH)₂ + H₂
CaO + H₂O → Ca(OH)₂
MgCO₃ → MgO + CO₂
2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
14 · Tests and observations
| Test | Observation |
|---|---|
| flame test | Ca brick red, Sr crimson, Ba green |
| + BaCl₂ / HCl | white ppt confirms sulfate |
| limewater + CO₂ | milky, then clears in excess |
| carbonate + acid | fizzing, gas turns limewater milky |
Limewater clears because the carbonate converts to soluble hydrogencarbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ — the origin of temporary hardness in water.
Trends in one line
Down Group 2: radius ↑, ionisation energy ↓, reactivity ↑, hydroxide solubility ↑, sulfate solubility ↓, carbonate stability ↑.
Every one of these follows from the growing ionic radius and the falling charge density.
Marks lost here
— Giving the hydroxide and sulfate solubility trends the same way round; they are opposite.
— Writing MgO + H₂ for magnesium with cold water; that is the steam reaction.
— Explaining carbonate stability by “bigger atoms” rather than lower charge density and less polarisation.
— Forgetting that Group 2 nitrates give NO₂ and O₂, not just the oxide.
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Group 2 — Frequently Asked Questions
Why does thermal stability of Group 2 carbonates increase down the group?
The cation gets larger down the group, so its charge density falls. It polarises the carbonate ion less, weakens the C–O bond less, and a higher temperature is needed to decompose it.
Why does hydroxide solubility increase down Group 2 while sulfate solubility decreases?
Both lattice enthalpy and hydration enthalpy fall down the group, but at different rates. For the hydroxides the lattice enthalpy falls faster, so solubility increases; for the sulfates the hydration enthalpy falls faster, so solubility decreases.
What are the Group 2 flame colours?
Calcium burns brick red, strontium crimson and barium apple green. Beryllium and magnesium give no flame colour because their electrons are too tightly held to be excited by a bunsen flame.
