Cambridge AS Level Chemistry 9701
Group 2
Down Group 2, from magnesium to barium, the atoms get larger, the first ionisation energy falls and the metals get more reactive. The hydroxides become more soluble, the sulfates less soluble, and the carbonates and nitrates need more heat to decompose. Almost every Group 2 question is one of those five trends, or the reason behind it.
It is the single sub-topic Cambridge AS Chemistry 9701 Paper 1 returns to most.
Updated 28 September 2026
How much it is worth
Group 2: 53 questions across 23 real Cambridge 9701 papers — 5.8% of the total, and it came up on every one of them. How that ranks against every other 9701 topic →
The trends at a glance
| Down the group (Mg → Ba) | Trend |
|---|---|
| Atomic and ionic radius | Increases — one more electron shell each period |
| First ionisation energy | Decreases — outer electrons further out and more shielded |
| Reactivity of the metal | Increases — the two outer electrons are lost more easily |
| Solubility of the hydroxides | Increases — Mg(OH)₂ sparingly soluble, Ba(OH)₂ much more soluble |
| pH of the hydroxide solution | Increases — more OH⁻ in solution |
| Solubility of the sulfates | Decreases — MgSO₄ soluble, BaSO₄ insoluble |
| Thermal stability of carbonates and nitrates | Increases — they need a higher temperature to decompose |
Melting point is the one to be careful with: it does not follow a clean trend down the group, so a question offering “melting point decreases steadily” is usually a distractor.
Why reactivity increases down the group
Each Group 2 metal reacts by losing its two outer electrons to form a 2+ ion. Going down, the outer electrons are in a shell further from the nucleus and shielded by more inner shells, so less energy is needed to remove them — the first and second ionisation energies fall. The easier the electrons are lost, the more reactive the metal. The atomic structure guide covers the ionisation energy argument in full.
Reactions of the metals
With oxygen — each metal burns to form its oxide:
With water — magnesium reacts only very slowly with cold water but readily with steam; calcium, strontium and barium react with cold water, increasingly vigorously:
With dilute hydrochloric acid — a salt and hydrogen:
With dilute sulfuric acid — the same pattern, but for calcium, strontium and barium the sulfate formed is sparingly soluble or insoluble, coats the metal and slows or stops the reaction.
Oxides, hydroxides and sulfates
The oxides are basic. They react with water to form hydroxides, and with acids to form a salt and water:
The hydroxides get more soluble down the group, so their saturated solutions get more alkaline. The sulfates go the other way — barium sulfate is insoluble, which is why adding barium chloride solution to a sulfate gives a white precipitate, the standard test for sulfate ions.
Thermal decomposition of carbonates and nitrates
Both become more thermally stable down the group. The explanation is polarisation: every Group 2 ion is 2+, but a smaller ion packs that charge into a smaller volume — a higher charge density. Mg²⁺ distorts (polarises) the large carbonate or nitrate ion more than Ba²⁺ does, weakening its bonds so it breaks up at a lower temperature.
Nitrates give off brown nitrogen dioxide and oxygen, which is how a question tells you the decomposition is of a nitrate, not a carbonate.
Uses in agriculture
Calcium hydroxide (slaked lime) and calcium carbonate (limestone) are added to acidic soil to neutralise it. Both are bases; the carbonate acts more slowly and gives off carbon dioxide as it reacts.
Worked example
Worked example
Which statement about the Group 2 elements magnesium to barium is correct? A: the first ionisation energy increases down the group. B: the solubility of the sulfates increases down the group. C: the thermal stability of the carbonates increases down the group. D: magnesium reacts more vigorously with cold water than barium.
C. Down the group the cation’s charge density falls, so it polarises the carbonate ion less and the carbonate needs more heat to decompose.
A is backwards — ionisation energy falls. B is backwards — sulfate solubility falls, down to insoluble barium sulfate. D is backwards — barium is the more reactive metal.
Common mistakes
1.Mixing up the hydroxide and sulfate trends.
They run in opposite directions. Hydroxides: more soluble down the group. Sulfates: less soluble. Anchor it on BaSO₄ being the insoluble one.
2.Explaining thermal stability with “stronger metallic bonding”.
It is about the cation polarising the anion. Smaller cation, higher charge density, more polarisation, less stable.
3.Writing Mg + H₂O → MgO + H₂ for cold water.
With steam, magnesium gives the oxide. With cold water the slow reaction gives the hydroxide: Mg + 2H₂O → Mg(OH)₂ + H₂.
4.Forgetting oxygen in the nitrate equation.
Group 2 nitrates give the oxide, nitrogen dioxide and oxygen. Balance with 2M(NO₃)₂ on the left.
Common questions
Why does reactivity increase down Group 2?
The outer electrons are further from the nucleus and more shielded, so the ionisation energies are lower and the two electrons are lost more easily.
Why are Group 2 carbonates more stable down the group?
The cations get larger with the same 2+ charge, so their charge density falls. They polarise the carbonate ion less, so more heat is needed to break it into the oxide and carbon dioxide.
How often does Group 2 come up on 9701 Paper 1?
On 23 of the 23 sittings Quanta has mapped — 53 questions, 5.8% of the total, more than any other single sub-topic. The full ranking →
Group 2 on real papers
The papers that leaned on it hardest, as a share of the paper. Each one has its own page on Quanta with the full topic breakdown:
Practise group 2 against real mark schemes
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