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Learn9701Paper 4 · A Level Structured QuestionsGroup 2Group 2: thermal stability and solubility
Group 2 · Subtopic 1 of 1

Group 2: Thermal Stability and Solubility

Group 2 carbonates decompose on heating to the oxide and CO₂; nitrates to the oxide, NO₂ and O₂. Thermal stability increases down the group: small Mg²⁺ has high charge density and polarises (distorts) the large carbonate/nitrate ion, weakening its bonds, so MgCO₃ decomposes most easily. Down the group, cations get larger, polarise less, and higher temperatures are needed. Solubility: hydroxides get more soluble down the group; sulfates get less soluble. Solubility depends on the balance between lattice energy and hydration enthalpies: for sulfates, hydration enthalpy of the cation falls faster than lattice energy as the cation grows, so dissolving becomes less favourable.

Cambridge 9701 Paper 4AQA 7405 Paper 1≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Write equations for thermal decomposition of group 2 nitrates and carbonates.
  • Explain the stability trend using charge density and polarisation.
  • Explain solubility trends using lattice and hydration enthalpies.
Pause and think

Why does magnesium carbonate break down in a Bunsen flame, while barium carbonate needs a furnace?

Section 2 of 14

Core idea

Start hereWhy lime kilns run so hot

Magnesium carbonate breaks down in a Bunsen flame. Calcium carbonate needs about 900 °C. Barium carbonate needs even more. The cause is the size of the cation.

The big idea

Small, highly charged cations polarise large anions, weakening their bonds. Down group 2, cations get bigger, polarise less, and carbonates and nitrates become more stable.

1. Polarisation

Why thermal stability increases down group 2

Lower charge density means less distortion of the anion.

Worked exampleGas from a nitrate

0.0200 mol Ca(NO₃)₂ is heated to constant mass. Total gas volume at r.t.p.?

Goal
Volume in cm³.
Strategy
Balanced equation, then molar volume.
  1. 2Ca(NO3)2→2CaO+4NO2+O2\mathrm{2Ca(NO_3)_2\rightarrow2CaO+4NO_2+O_2}
    2 mol nitrate → 5 mol gas.
  2. n(gas)=0.0200×52=0.0500n(\text{gas})=0.0200\times\frac{5}{2}=0.0500
    Ratio.
  3. V=0.0500×24 000=1200 cm3V=0.0500\times24\,000=1200\ \text{cm}^3
    Molar volume.
Answer1200 cm³.
✓ CheckIncludes both NO₂ and O₂ ✓.
Key takeawaySolubility trends Hydroxides: more soluble down. Sulfates: less soluble down, because the cation's hydration enthalpy falls faster than the lattice energy.

Summary

  • MCO₃ → MO + CO₂; 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
  • Stability ↑ down group.
  • Charge density and polarisation.
Key words
Charge density
Charge of an ion relative to its size.
Polarisation
Distortion of an anion's electron cloud by a nearby cation.
HistoryLime kilns. People have heated limestone (CaCO₃) in kilns for thousands of years to make quicklime for mortar, relying on its decomposition at about 900 °C.
Fun factBrown gas test. Heating a group 2 nitrate in a test tube releases brown NO₂ and oxygen, which relights a glowing splint: a dramatic way to compare stabilities.
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