Group 2

— Chemistry · AS Level (7405) · Alkaline Earth Metals —

★ Exam Tip: Group 2 is a small chapter and mostly memorisation (the teacher's words: "majority of it is rattā") — but examiners keep asking for observations, state symbols, correct formulae (Mg(OH)₂, Sr(NO₃)₂ — not SrNO₃!) and the trends. Learn the equation and the observation follows from it.

1. Group 2 & the Reactivity Trend

We study only Mg, Ca, Sr, Ba. But beryllium (above Mg) and radium (below Ba) can still be asked — extend the trends up or down.

Q. Why does the reactivity of Group 2 metals increase down the group?

Answer: down the group, the nuclear charge increases but this is counteracted by increased shielding by inner-shell electrons. The atomic radius increases as the number of shells increases, so it becomes easier to lose the valence (outer) electrons.

Down the group the metals become stronger reducing agents (they lose electrons: Mg⁰ + Cl₂⁰ → Mg²⁺Cl₂⁻).

All Group 2: outer electron configuration ends in s²; all form 2+ ions and ionic compounds.

Ions: M²⁺ have formula patterns X(OH)₂, XCO₃, X(NO₃)₂, XSO₄ (X is 2+, so hydroxide/nitrate need a ×2).

Q10(a)Structured

(i) Why do both lines rise steadily? Nuclear charge and shielding by inner electrons both increase down the group and counteract each other; the radius increases due to the increased number of shells. [2]

(ii) Which line is which? Lower line = ionic radii, upper line = atomic radii — in ionic form the atom has lost its outermost shell by donating its electrons to another species (M → M²⁺ + 2e⁻). [2]

both radii rise steadily (more shells; extra nuclear charge cancelled by shielding). Ions are smaller — atom lost its outer shell

2. Reactions with Oxygen

All Group 2 metals react with oxygen to give oxides, which are white solids.

2X(s) + O₂(g) → 2XO(s) e.g. 2Ca(s) + O₂(g) → 2CaO(s)

Observations (examiners ask these repeatedly): coloured flame and white solid.

burning colours — fireworks get their colours from compounds of these elements

Compare: Group 1 metals mostly burn with a white flame. Group 2: Mg white, Ca and Sr red, Ba green.

3. Reactions with Water and Steam

Apart from magnesium (which does react, but very slowly), all Group 2 metals react readily with liquid water to form the hydroxide + hydrogen.

X(s) + 2H₂O(l) → X(OH)₂(aq) + H₂(g) e.g. Sr(s) + 2H₂O(l) → Sr(OH)₂ + H₂(g)

Magnesium with steam reacts readily to form the oxide + hydrogen (not the hydroxide):

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Observations: Ca → Ba: solid disappears; Mg: white solid forms. Effervescence of hydrogen, and a pop sound with a burning splint.
Mg + cold water: slow bubbling, white solid forms. Mg + steam: silver-grey metal turns white.

Careful: hydrogen is colourless — no "white smoke" or "fumes" (fumes = coloured smoke, e.g. NH₄Cl or brown NO₂). The pop test is a test done after you see effervescence; effervescence/bubbling is the observation.

Why Mg is slow with cold water: the Mg(OH)₂ (and MgO) formed is insoluble and stays on the metal; steam makes MgO because the hydroxide is hard to form/keep at high temperature.

4. Reactions with Dilute Acids

X + acid → salt + hydrogen

Examples

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Ca(s) + H₂SO₄(aq) → CaSO₄(aq) + H₂(g) (CaSO₄ is sparingly soluble — the board writes aq)
Sr(s) + 2HNO₃(aq) → Sr(NO₃)₂(aq) + H₂(g)

Observations: solid disappears, effervescence of hydrogen, pop sound with burning splint.

Formulae trap: nitrate salts of Group 2 are Sr(NO₃)₂, Ca(NO₃)₂ — not SrNO₃ (that is a Group 1 pattern). Sulphates: CaSO₄, MgSO₄ — not Mg₂SO₄. Work the formula from the charges: X²⁺ with NO₃⁻ → 1 : 2.

Can't remember the observation? Look at the equation and its state symbols — the observation follows (solid → aq = disappears; gas formed = effervescence).

5. Group 2 Oxides with Water and Dilute Acids

MgO is insoluble in water and does not react with it (observation: solid does not disappear).
All other oxides (CaO, SrO, BaO) are soluble and form hydroxides: XO(s) + H₂O(l) → X(OH)₂(aq) e.g. CaO(s) + H₂O(l) → Ca(OH)₂(aq). Observation: solid disappears.

Alkali vs base: a base is a big category (proton acceptor: oxides, hydroxides, carbonates…). If the base dissolves in water it is an alkali (soluble base). Oxides/hydroxides that dissolve give strongly alkaline solutions.

With dilute acids: all oxides give salt + water: XO + acid → salt + H₂O. Observation: solid disappears.

Examples

MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
CaO(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l)
SrO(s) + H₂SO₄(aq) → SrSO₄(s) + H₂O(l) (SrSO₄ is insoluble; board writes aq)

Note: a solution + solution neutralisation (e.g. hydroxide + acid) has no clear visible observation — you would have to monitor pH. Oxides are ionic ⇒ high melting point.

6. Group 2 Hydroxides

Solubility of hydroxides increases down the group:

Mg(OH)₂	Ca(OH)₂	Sr(OH)₂	Ba(OH)₂
insoluble	soluble	more soluble	very soluble
pH ≈ 9–10	↑	↑	pH ≈ 13–14

More soluble ⇒ more OH⁻ in solution ⇒ higher pH (Ba(OH)₂ is the most alkaline). Oxides and hydroxides that dissolve give strongly alkaline solutions.

pH answers: give one value (e.g. 13 or 14; 9 or 10), not a range. The reason for the solubility trend (lattice energy and hydration enthalpy) is A2 — not needed now.

With dilute acids: X(OH)₂ + acid → salt + water. e.g. Sr(OH)₂ + 2HNO₃ → Sr(NO₃)₂ + 2H₂O; Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.

Ca(OH)₂ (slaked lime) neutralises acidic soils. Ionic equation: H⁺ + OH⁻ → H₂O.

7. Group 2 Carbonates

None of the Group 2 carbonates is soluble in water (carbonates are mostly insoluble; only Na⁺, K⁺, NH₄⁺ carbonates dissolve).
With dilute acids: XCO₃ + acid → salt + CO₂ + H₂O e.g. CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Observations: solid disappears, effervescence. (Limewater turns milky is a test for CO₂, not an observation of the reaction itself.)

Thermal decomposition (endothermic)XCO₃(s) → XO(s) + CO₂(g) e.g. MgCO₃(s) → MgO(s) + CO₂(g)
Thermal stability increases down the group — more heat needed to decompose (explanation is A2).

Observations: solid changes colour to white (e.g. MgCO₃ → MgO). No effervescence — the gas is colourless and there is no liquid; you cannot see it. Test the gas: limewater turns milky.

heat the carbonate strongly (boiling tube); pass the gas into limewater

Heating the carbonate produces CO₂; use a boiling tube and heat strongly. Barium carbonate needs much more heat than MgCO₃ — it is the most thermally stable.

8. Thermal Decomposition of Nitrates

X(NO₃)₂(s) → XO(s) + 2NO₂(g) + ½O₂(g) e.g. Ca(NO₃)₂ → CaO + 2NO₂ + ½O₂

Observations: solid changes colour, brown gas evolved (NO₂), a burning splint burns more brightly (O₂).

All Group 2 nitrates decompose on strong heating; they also become more thermally stable down the group.
NO₂ = nitrogen dioxide. Careful: "dinitrogen" would mean N₂ — the examiner may try to confuse: N₂O is dinitrogen monoxide.
Hydrated Mg(NO₃)₂·7H₂O gives NO₂, O₂ and water vapour when heated.

9. Solubility Trends and Solubility Rules

Mg(OH)₂ → Ba(OH)₂	MgSO₄ → BaSO₄
↓ down the group: become more soluble	↓ down the group: become less soluble

Trick: hydroxide ↑, sulfate ↓ (opposite). BaSO₄ is very insoluble — used later to test for sulfate; radium sulfate is "extremely insoluble".

Basic solubility rules (O-level revision)All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Chlorides mostly soluble. Carbonates mostly insoluble.

Trick (choose a reagent): if the anion is specified, use its Na⁺/K⁺/NH₄⁺ salt (always soluble). If the cation is specified, use its nitrate (always soluble) — e.g. need Hg⁺ ions? use Hg(NO₃)₂.

Precipitation: mixing solutions of a soluble Ba salt and Na₂CO₃/Na₂SO₄ gives an insoluble white BaCO₃/BaSO₄ — a way to make insoluble salts.

10. Past-Paper MCQs

Q4MCQ

Which oxide (MgO, CaO, SrO, BaO) forms the most alkaline solution in water?

Solubility increases down the group ⇒ BaO dissolves most, highest [OH⁻], pH ≈ 14. MgO is insoluble.

Answer: D (BaO)

Q6MCQ

Steam over heated Mg gives X + H₂. What is not a property of X (MgO)?

MgO: ionic ⇒ high m.p.; basic oxide; white solid; but insoluble in water.

A, B, C ✗ all true properties

Answer: D (very soluble in water)

Q9MCQ · stoichiometry

Mass of solid residue from thermal decomposition of 4.10 g anhydrous calcium nitrate?

Ca(NO₃)₂ → CaO + 2NO₂ + ½O₂M_r Ca(NO₃)₂ = 40.1 + 28 + 96 = 164.1n = 4.10 / 164.1 = 0.025 moln(CaO) = 0.025 mol; M_r = 56.1mass = 0.025 × 56.1 = 1.40 g

Answer: C

Q12MCQ

Mg(NO₃)₂·7H₂O is heated — which three gases are given off?

Nitrate ⇒ NO₂ + O₂; hydrated ⇒ water vapour. Not N₂O ("dinitrogen"), not H₂.

Answer: D (NO₂, O₂, water vapour)

Q21MCQ · mineral

Mineral heated in a Bunsen flame to constant mass: colourless gas turning limewater milky. Residue cooled + HCl(aq): vigorous effervescence. Which mineral?

Gas = CO₂ ⇒ a carbonate. Residue still fizzes with acid ⇒ some carbonate did not decompose — BaCO₃ is so thermally stable that a Bunsen flame cannot decompose it. Aragonite (CaCO₃), artinite (Mg) and dolomite (Ca, Mg) leave oxides only.

Note: the teacher said he would double-check this question; the board only shows each mineral converting to its oxide. The answer marked is C.

Answer: C (barytocalcite, BaCO₃·CaCO₃)

Q24MCQ · CO₂ from limestone

1000 million t limestone for cement + 200 million t for lime, roasted. Total CO₂ output?

CaCO₃ → CaO + CO₂ (1 : 1)total = 1200 million t; M_r CaCO₃ = 100.1n = 1200/100.1 = 11.988mass CO₂ = 11.988 × (12 + 32)= 527 million t

Answer: B

Q25MCQ

Volume of O₂ (room conditions) from thermal decomposition of 8.2 g Ca(NO₃)₂ (M_r = 164)?

n(Ca(NO₃)₂) = 8.2 / 164 = 0.05 molCa(NO₃)₂ → CaO + 2NO₂ + ½O₂ ⇒ n(O₂) = 0.025 molV = 0.025 × 24 000 cm³ = 600 cm³

(not marked on the board — left as an exercise; worked here)

Answer: C (600 cm³)

Q37MCQ · rat poison

Must be insoluble in rainwater but soluble at low pH of stomach acid. Which barium compound?

BaCl₂ and Ba(OH)₂ are soluble (would wash away) ✗. BaSO₄ is insoluble even in acid ✗. BaCO₃ is insoluble in water but reacts with HCl (gives soluble BaCl₂ + CO₂ + H₂O).

Answer: A (barium carbonate)

Q61MCQ · fire retardants

Three minerals decompose in the heat giving CO₂: barytocite BaCa(CO₃)₂ (most stable), dolomite CaMg(CO₃)₂ (2nd), huntite Mg₃Ca(CO₃)₄ (least stable — most Mg). Best retardant?

Best = decomposes most easily to give CO₂ = least stable: huntite (more Mg) → dolomite → barytocite.

Answer: D (huntite, dolomite, barytocite)

Q72MCQ · property that decreases

Which property decreases down Group II?

Cation radius ↑, reactivity with water ↑, shielding ↑, but carbonates become harder to decompose (ease of thermal decomposition ↓).

Answer: D

Q79MCQ · precipitates

Mg(NO₃)₂ and Ba(NO₃)₂ each: Na₂CO₃(aq) → excess HCl → excess NaOH. How many of M, N, P, Q, R, S are white precipitates?

	Na₂CO₃	excess HCl	excess NaOH
Mg	M = MgCO₃ (insoluble ✔ ppt)	N = MgCl₂ (soluble)	P = Mg(OH)₂ (insoluble ✔ ppt)
Ba	Q = BaCO₃ (insoluble ✔ ppt)	R = BaCl₂ (soluble)	S = Ba(OH)₂ (soluble)

Precipitates: M, P, Q = 3 (carbonates insoluble; chlorides soluble; Mg(OH)₂ insoluble but Ba(OH)₂ soluble).

Note: the circled option on the board looks like C (4), but the working ("insoluble, insoluble, insoluble") gives 3 ⇒ B.

Answer: B (3)

11. Past-Paper Structured Questions

Q2 — Mg reaction schemeStructured

(a) Identify A–F: A MgSO₄ (Mg + dil H₂SO₄) B MgCl₂ (Mg + dil HCl) C MgCO₃ (A + Na₂CO₃) D MgO (Mg heated in air) E Mg(OH)₂ (B + NaOH) F Mg(NO₃)₂ (E + dil HNO₃).

(b)(i) equationsMg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)MgCO₃(s) → MgO(s) + CO₂(g) (heat)Mg(NO₃)₂(s) → MgO(s) + 2NO₂(g) + ½O₂(g) (heat)

(ii) Heat on E: Mg(OH)₂(s) → MgO(s) + H₂O(g) [4]. Give the state symbols if you are sure of them; if you are not, leave them out.

Q3 — RadiumStructured

(a)(i) All Group II electronic configurations end in s². (ii) Another very insoluble Ra salt: RaCO₃ — carbonates are insoluble, so it is the safest choice (sulfates become less soluble down the group).

(c)(i) Mg + cold water: slow bubbling, white solid forms; Mg + steam: silver-grey metal turns white. (ii) Mg(s) + H₂O(g) → MgO(s) + H₂(g).

(d)(i) Ra(s) + 2H₂O(l) → Ra(OH)₂(aq) + H₂(g); (ii) observations: vigorous effervescence and solid disappears; (iii) pH ≈ 12 on the board (strongly alkaline — 13/14 would also be expected; give a single value); (iv) more vigorous than barium — radium has a larger radius so it is easier to lose its electrons. [6]

Q4 — Calcium schemeStructured

(a) U CaCl₂, V CaO, W Ca(NO₃)₂, X Ca(OH)₂, Y CaCO₃ (Z = CaSO₄).

(b) Y → V: strongly heat in a boiling tube (thermal decomposition).

(c)(i)Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)CaO(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l)CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq)(ii) Ca(NO₃)₂(s) → CaO(s) + 2NO₂(g) + ½O₂(g)

(d) Reagent (not an acid) to make CaSO₄ from Ca(NO₃)₂: Na₂SO₄. (e) X from Ca(s): effervescence, solid disappears; X from V (CaO): solid disappears.

Q5 — Barium schemeStructured

(a) R BaO, S BaCl₂, T Ba(OH)₂, U BaSO₄, V BaCO₃, W Ba(NO₃)₂.

(b)(i) Ba(OH)₂(aq) + 2HNO₃(aq) → Ba(NO₃)₂(aq) + 2H₂O(l); roasting V: BaCO₃(s) → BaO(s) + CO₂(g). (ii) T → V: gaseous reagent CO₂: Ba(OH)₂ + CO₂ → BaCO₃ + H₂O. (c) Reaction 1 (W → U) needs an aqueous non-acid reagent: Na₂SO₄.

(d) Oxide X (heated Ba in oxygen) is 18.9 % O by mass:

n(Ba) = 81.1 / 137.3 = 0.5907n(O) = 18.9 / 16 = 1.181ratio = 0.5907 : 1.181 = 1 : 2empirical formula = BaO₂

(ii) Y = BaSO₄. (iii) BaO₂ + H₂SO₄ → BaSO₄ + H₂O₂ [4]

Q10(b)–(d)Structured

(b) L is a Group 2 salt: heated strongly → brown gas + white solid; the solid dissolves to give a colourless solution of M(OH)₂; dilute H₂SO₄ gives a dense white precipitate.

(i) anion = NO₃⁻ (brown gas NO₂). (ii) M = Ba (BaSO₄ ppt): Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). (iii) L = Ba(NO₃)₂: Ba(NO₃)₂(s) → BaO(s) + 2NO₂(g) + ½O₂(g).

(c)(i) Neutralisation of acid — Ca(OH)₂: H⁺ + OH⁻ → H₂O; CaCO₃: CO₃²⁻ + 2H⁺ → H₂O + CO₂. (ii) In areas of high rainfall CaCO₃ is better because it is insoluble and won't get washed away into water bodies (Ca(OH)₂ is soluble; excess CaCO₃ does not make soil too alkaline).

(d) Mg + cold water → Mg(OH)₂; Mg + steam → MgO.

12. Quick Recap — Full Chapter Cheat-Sheet

Property (down Mg → Ba)	Trend
Atomic radius, reactivity, reducing power	increase
Solubility of hydroxides (and pH of solutions)	increases
Solubility of sulfates	decreases
Thermal stability of carbonates and nitrates	increases (harder to decompose)

✓ Reactions & observations

+ O₂: 2X + O₂ → 2XO; coloured flame (Mg white, Ca/Sr red, Ba green), white solid.
+ H₂O: X + 2H₂O → X(OH)₂ + H₂ (Mg very slow; Mg + steam → MgO + H₂); solid disappears, effervescence, pop.
+ acid: X + acid → salt + H₂; oxide → salt + H₂O; hydroxide → salt + H₂O; carbonate → salt + CO₂ + H₂O.
Oxides: MgO insoluble, others soluble → alkaline hydroxides. Carbonates all insoluble.
Heating: XCO₃ → XO + CO₂ (white solid; limewater milky). X(NO₃)₂ → XO + 2NO₂ + ½O₂ (brown gas, relights splint).
Formulae: X(OH)₂, X(NO₃)₂, XSO₄, XCO₃. Group 1/NH₄⁺ salts and all nitrates are soluble.