HookThe toxic metal radiologists feed you on purpose
Barium is genuinely poisonous — soluble barium salts disrupt nerve and muscle function, and barium carbonate was sold for decades as rat poison. Yet every day, hospital radiology departments hand patients a thick white suspension of barium sulfate and instruct them to drink the lot. The 'barium meal' works because barium's heavy, electron-rich atoms absorb X-rays, coating the gut in a layer that shows up in crisp outline on the scan. And it is safe for exactly one reason: barium sulfate's solubility is roughly 2 mg per litre of water — so close to nothing that barely any Ba²⁺ ever reaches the bloodstream. The compound passes straight through you, toxic metal and all. The safety case is not toxicology; it is solubility chemistry, sitting at the bottom rung of a trend that runs the full height of Group 2.
That is the flavour of this section: trends with consequences. 3.2.2 asks four things. Explain the trends in atomic radius, first ionisation energy and reactivity down the group — the vertical version of the machine you built in 3.2.1. Know the reactions with water, from magnesium's near-refusal to barium's enthusiasm. Learn two solubility trends that run in opposite directions — hydroxides more soluble down the group, sulfates less. And connect each fact to a job: slaked lime on acidic fields, milk of magnesia in stomachs, barium meals in radiology, magnesium in the titanium works, and limestone scrubbing sulfur dioxide out of power-station flues.
ModelDown the group — one extra shell per row, and everything follows
Every Group 2 element has the same outer configuration, ns²: magnesium is [Ne]3s², calcium [Ar]4s², and so on down. Each step down the group adds one complete electron shell, and that single structural fact drives all three trends.
Atomic radius increases — magnesium 0.160 nm, calcium 0.197 nm, strontium 0.215 nm — because each new shell sits further out than the last. First ionisation energy decreases: Be 900, Mg 738, Ca 590, Sr 550, Ba 503 kJ mol⁻¹. The nuclear charge does rise down the group, but it is beaten twice over: the outer electrons are further from the nucleus, and every added inner shell shields them more. Distance and shielding outrun charge — the exact opposite of the across-a-period story, where shielding was frozen and charge won.
Reactivity increases down the group, and the logic is bookkeeping: Group 2 chemistry is almost entirely the formation of M²⁺ ions, so the easier the two outer electrons are to remove, the faster and more vigorously the element reacts. The second ionisation energy follows the same downward trend as the first, so the total electron bill falls from magnesium to barium. When AQA asks 'explain why barium reacts faster with water than calcium', it is asking for this chain: larger radius, more shielding, weaker attraction on the outer electrons, lower ionisation energies, M²⁺ forms more readily.
MechanismWater — from geological patience to a fizzing beaker
Magnesium and cold water is a reaction in name only: it proceeds so slowly that collecting a test tube of hydrogen takes days, forming a little magnesium hydroxide as it crawls. Give magnesium steam, though, and it burns with a brilliant white light: \(\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}\). Notice the product — the oxide, not the hydroxide, because at steam temperatures the hydroxide is not stable. The state symbol on the water is doing real work in that equation, and AQA marks it.
Calcium reacts with cold water at a civilised fizz: \(\text{Ca(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}\) — steady effervescence, the metal skating on bubbles, and the liquid turning cloudy. That cloudiness is a solubility observation smuggled into a reactivity experiment: calcium hydroxide is only sparingly soluble, so it saturates the solution and precipitates as a white suspension. Strontium reacts faster. Barium goes fastest of all — and its beaker stays colourless and clear, because barium hydroxide is soluble enough to remain dissolved. Two metals, same reaction type, opposite appearance, and the difference is the hydroxide solubility trend of the next block.
The observations are the marks here: effervescence, the metal disappearing, the solution clouding (calcium) or not (barium), and — for magnesium and steam — the white solid and bright white light.
DataTwo solubility ladders, running in opposite directions
Group 2 compounds obey two clean trends. Hydroxides become more soluble down the group: magnesium hydroxide is effectively insoluble (a white precipitate), calcium hydroxide is sparingly soluble (its saturated solution is limewater), and barium hydroxide dissolves readily. Sulfates become less soluble down the group: magnesium sulfate is Epsom salts, dissolved by the spoonful in bathwater, while barium sulfate at ~2 mg per litre is the most insoluble common compound on the A-level course. Same family, mirrored ladders — and the pairing is unique enough that the two trends together fingerprint any Group 2 metal.
The insolubility of barium sulfate powers the standard test for sulfate ions: acidify the unknown with dilute hydrochloric acid, add barium chloride solution, and a white precipitate means sulfate: \(\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}\). The acid step is where the marks hide. Carbonate and sulfite ions also throw white precipitates with barium, so an unacidified test proves nothing; the hydrochloric acid destroys them first (you may see the carbon dioxide fizz off), leaving only sulfate to answer. And the acid must never be sulfuric — acidifying with H₂SO₄ adds the very ion you are testing for and guarantees a false positive.
An exam classic: a student adds barium chloride solution to an unknown and sees a white precipitate. Explain why this does not prove sulfate is present, and how to make the test valid. Model answer: carbonate ions would also give a white precipitate with barium ions, as barium carbonate (1). Repeat the test after adding dilute hydrochloric acid: the acid reacts with and removes any carbonate — seen as effervescence — so a white precipitate that still forms must be barium sulfate (1). Name the acid. 'Add acid' unqualified is not creditworthy, because sulfuric acid would itself produce the precipitate; hydrochloric (or nitric) is the answer the scheme wants.
CaseJobs for the family — every use is one property in overalls
AQA's use questions are property-to-job links, and each Group 2 application hangs off exactly one fact. Agriculture: calcium hydroxide — slaked lime — is spread on fields to neutralise acidic soil, a straightforward base–acid reaction: \(\text{Ca(OH)}_2 + 2\text{H}^+ \rightarrow \text{Ca}^{2+} + 2\text{H}_2\text{O}\). Medicine: magnesium hydroxide is 'milk of magnesia', the antacid — basic enough to neutralise stomach acid, \(\text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}\), yet so weakly soluble that it is gentle enough to swallow. Barium sulfate's radiology job you already know — and note the pattern: both medical uses are insolubility wearing a stethoscope.
Industry: titanium cannot be extracted by reducing TiO₂ with cheap carbon — that yields titanium carbide, which embrittles the metal. So the ore is converted to titanium(IV) chloride and reduced with magnesium at around 1,000 °C under argon: \(\text{TiCl}_4 + 2\text{Mg} \rightarrow \text{Ti} + 2\text{MgCl}_2\). Magnesium is the sacrificial electron donor; the argon blanket stops both hot metals reacting with oxygen or nitrogen in air. Environment: coal- and oil-fired power stations scrub acidic sulfur dioxide from flue gases with cheap calcium bases — \(\text{CaO(s)} + \text{SO}_2\text{(g)} \rightarrow \text{CaSO}_3\text{(s)}\), or the same job done by powdered limestone, CaCO₃. The calcium sulfite is oxidised to calcium sulfate — gypsum — and sold on for plasterboard, which is why flue-gas desulfurisation plants are quietly profitable.
The Kroll process as a reacting-mass calculation — what mass of magnesium does 1.00 tonne of titanium cost? Moles of titanium: \(n = 1.00 \times 10^6\ \text{g} \div 47.9\ \text{g mol}^{-1} = 2.09 \times 10^4\ \text{mol}\). The equation demands 2 mol of Mg per mol of Ti: \(n(\text{Mg}) = 4.18 \times 10^4\ \text{mol}\). Mass: \(4.18 \times 10^4 \times 24.3 = 1.01 \times 10^6\ \text{g} \approx 1.0\ \text{tonne}\). Titanium consumes its own weight in magnesium before anyone pays for the 1,000 °C, the argon or the batch downtime — a one-line calculation that explains why titanium is priced like a precious metal despite its ore being abundant.
CaseOne unknown, two test tubes — the trends as a fingerprint
Here is the section working as a whole. You are handed a white solid that is one of MgCl₂, CaCl₂ or BaCl₂. Dissolve it, split the solution, and interrogate it with the two ladders. Tube one: add sodium hydroxide solution. A thick white precipitate says magnesium — Mg(OH)₂ is the insoluble end of the hydroxide ladder; a faint precipitate says calcium; little or none says barium. Tube two: add dilute sulfuric acid or any soluble sulfate. Nothing says magnesium — MgSO₄ is happily soluble; a slight cloudiness says calcium; a dense white precipitate says barium.
Because the two trends run in opposite directions, every metal returns a different pair of results — magnesium (precipitate, clear), barium (clear, precipitate), calcium (faint, faint). No flame test, no spectrometer: the periodicity itself is the analytical instrument. This is precisely the logic Required Practical 4 drills, where these cation tests join the anion sequence — carbonate, sulfate, halide — into one method; the full RP4 workflow lives with the halide tests in 3.2.3.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Group 2 is one of AQA's heaviest AO1 sections: equations and observations are recalled, not derived, so drill them until they are reflexes. Equations need state symbols where the chemistry depends on them — H₂O(g) in the steam reaction, BaSO₄(s) in the sulfate test — and the steam product is MgO, not the hydroxide. Observation marks are earned in observation language: 'effervescence', 'white precipitate', 'solution remains colourless', not 'hydrogen is produced' (an inference, not an observation).
Trend explanations run the vertical machine in full: more shells, greater distance and shielding, weaker attraction on the outer electrons, lower ionisation energies, so M²⁺ forms more easily. Examiners' reports repeatedly note that candidates who mention rising nuclear charge without stating that shielding and distance OUTWEIGH it lose the explanation mark — the trend is a tug-of-war, and you must say who wins.
Use questions are one-line property→job links and the property is usually solubility: milk of magnesia works because Mg(OH)₂ is a base AND barely soluble; the barium meal is safe because BaSO₄ is insoluble; the sulfate test needs hydrochloric, never sulfuric, acid. For titanium, be ready for the 'why' pair: why not carbon (titanium carbide forms, embrittling the metal) and why argon (to exclude air, which would react with both hot metals). And if a calculation appears, it is reacting-mass stoichiometry off TiCl₄ + 2Mg → Ti + 2MgCl₂ — show moles, ratio, mass, units, three significant figures.