AQA A-level Chemistry (7405) · Group 2, the Alkaline Earth Metals
Mini-Lesson
Group 2, the Alkaline Earth Metals
This mini-lesson covers AQA 3.2.2 Group 2, the alkaline earth metals: the trends in atomic radius, first ionisation energy and melting point from Mg to Ba, the reactions with water, the opposite solubility trends of the hydroxides and the sulfates, the test for sulfate ions, and the real uses of these compounds in medicine and agriculture.
Work through each screen, answer the questions as you go (some are wordy, most are calculations) and collect ⭐ stars. Everything here is A-level standard — the maths is done properly, not skipped. Press Start when you're ready.
Group 2 · trends
Trends in radius, ionisation energy and melting point
Group 2 metals all have two electrons in an outer s sub-shell and form 2+ ions by losing them.
Atomic radius increases down the group — each element has an extra occupied electron shell.
First ionisation energy decreases down the group. The outer electron is further from the nucleus and better shielded by more inner shells, so it is attracted less strongly — this outweighs the increase in nuclear charge.
Reactivity therefore increases down the group: the metals react by losing their two outer electrons, and that gets easier.
Melting point generally decreases (Ca → Ba). The metal ions get larger, so the attraction between the positive ions and the delocalised electrons weakens. (Magnesium is anomalous because of its different crystal packing.)
Group 2 · reactions with water
Reactions of Mg to Ba with water
Group 2 metals react with water to give the hydroxide and hydrogen, and the reaction gets more vigorous down the group:
Ca + 2H₂O → Ca(OH)₂ + H₂steady effervescence; the solution turns cloudy (limewater)
Magnesium is the exception: it reacts extremely slowly with cold water. Heated in steam, however, it burns brilliantly to give the oxide:
Mg + H₂O(g) → MgO + H₂the product is the OXIDE, not the hydroxide — different conditions, different product
Magnesium is also used industrially to extract titanium, because it is a strong enough reducing agent to displace titanium from its chloride:
TiCl₄ + 2Mg → Ti + 2MgCl₂done in an argon atmosphere (Ti reacts with O₂ and N₂ when hot). Check: Ti 1, Cl 4, Mg 2 ✓
Calculate
Your turn
10.0500 mol of calcium reacts completely with water: Ca + 2H₂O → Ca(OH)₂ + H₂. Calculate the volume of hydrogen produced at room temperature and pressure, in dm³. (Molar gas volume = 24.0 dm³ mol⁻¹.)
dm³
Hint: The ratio Ca : H₂ is 1 : 1, so n(H₂) = 0.0500 mol. V = n × 24.0.
Quick check
Quick check
?Why does barium react more vigorously with water than calcium?
Group 2 · solubility
The two opposite solubility trends
This is the part of Group 2 that most students get backwards. Learn them as a pair:
Hydroxides: solubility INCREASES down the group.Mg(OH)₂ is sparingly soluble (which makes it a safe antacid); Ba(OH)₂ is reasonably soluble and gives a strongly alkaline solution.
Sulfates: solubility DECREASES down the group. MgSO₄ is soluble (Epsom salts); BaSO₄ is insoluble — and that is exactly why it is used as the test.
The test for sulfate ions: add acidified barium chloride solution. A white precipitate confirms sulfate.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)a white precipitate
Why acidify — and why with HCl?Carbonate and sulfite ions would also give white precipitates with Ba²⁺; adding dilute hydrochloric acid first removes them (as CO₂ and SO₂), so a white precipitate can only be sulfate. You must not acidify with sulfuric acid — that would add sulfate ions and give a false positive every time.
Calculate
Your turn
2A solution containing 0.0100 mol of sulfate ions is treated with excess acidified BaCl₂. Calculate the mass of BaSO₄ precipitated. (Mr(BaSO₄) = 233.4)
g
Hint: The ratio SO₄²⁻ : BaSO₄ is 1 : 1, so n(BaSO₄) = 0.0100 mol. m = n × M_r.
Sort it
Sort each property going DOWN Group 2
Tap a property, then tap what happens to it as you go down the group from Mg to Ba.
🟩 Increases down the group
🟪 Decreases down the group
🟦 Stays the same
Group 2 · uses
Uses of Group 2 compounds
Every one of these uses is a direct consequence of a chemical property — AQA will expect you to explain why, not just recite the use.
Mg(OH)₂ — antacid ("milk of magnesia"). It is a base, so it neutralises excess stomach acid, but it is only sparingly soluble, so it does not make the stomach dangerously alkaline. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O (Mg 1, O 2, H 4, Cl 2 ✓).
Ca(OH)₂ — agriculture. Slaked lime is spread on fields to neutralise acidic soil and raise its pH, which is essential for good crop yields.
BaSO₄ — the "barium meal". Barium ions are toxic, but BaSO₄ is so insoluble that essentially no Ba²⁺ enters the blood. It is opaque to X-rays, so it outlines the gut.
CaO or CaCO₃ — flue-gas desulfurisation. Powdered calcium oxide or carbonate is sprayed into the flue gases of a power station to remove acidic SO₂: CaO + SO₂ → CaSO₃ (Ca 1, S 1, O 3 ✓). This reduces acid rain.
Calculate
Your turn
3Calculate the relative formula mass of Mg(OH)₂. (Mg = 24.3, O = 16.0, H = 1.0)
Hint: M_r = 24.3 + 2(16.0 + 1.0).
Calculate
Your turn
40.583 g of Mg(OH)₂ neutralises stomach acid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Calculate the moles of HCl neutralised.
mol
Hint: n(Mg(OH)₂) = 0.583 ÷ 58.3 = 0.0100 mol. The ratio is 1 : 2.
Match it
Match the compound to its use
Tap a compound on the left, then its use on the right.
Compound
Use
Quick check
Quick check
?Why must barium chloride solution be acidified with hydrochloric acid — and not sulfuric acid — before testing for sulfate ions?
Quick check
Quick check
?Why is BaSO₄ safe to swallow when Ba²⁺ ions are toxic?
Quick check
Quick check
?Magnesium is used to extract titanium from TiCl₄. What is magnesium acting as?
Group 2 · exam traps
The trends people reverse, and the tests they botch
Hydroxides get MORE soluble down the group; sulfates get LESS soluble. They go in opposite directions. Getting this backwards costs several marks in a single question.
Reactivity increases down the group because ionisation energy FALLS — larger radius and more shielding. Do not say "because the atoms get bigger" and stop there.
Acidify BaCl₂ with hydrochloric acid, never sulfuric. Sulfuric acid adds the very ion you are testing for.
Magnesium with steam gives MgO; magnesium with cold water gives Mg(OH)₂ — very slowly.
Why the solubility trends are opposite (worth knowing even though AQA only asks you to state them): as the cation gets bigger, both the lattice enthalpy and the hydration enthalpy fall — but they fall at different rates, and which one falls faster depends on the size of the anion. A small anion (OH⁻) and a large anion (SO₄²⁻) therefore behave oppositely.
Quick check
Quick check
?Which Group 2 sulfate is the least soluble in water?
Calculate
Your turn
5Calculate the moles of Ca(OH)₂ needed to neutralise 0.0400 mol of HCl. Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
mol
Hint: The ratio Ca(OH)₂ : HCl is 1 : 2.
Recap
The big ideas to know
Down the group: atomic radius increases; first ionisation energy decreases; reactivity with water increases
Melting point: generally decreases down the group — the metal ions get larger, so the metallic bonding weakens
With water: Mg is extremely slow in cold water (but burns in steam → MgO); Ca, Sr and Ba react increasingly vigorously → M(OH)₂ + H₂
Hydroxides: solubility increases down the group — Mg(OH)₂ is sparingly soluble, Ba(OH)₂ is quite soluble
Sulfates: solubility decreases down the group — BaSO₄ is insoluble, which is why it is the test
Sulfate test: acidified BaCl₂ → a white precipitate of BaSO₄. Acidify with HCl to remove carbonate and sulfite, which would also give white precipitates
That is the whole of AQA 3.2.2. Press Finish to see your score.
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