Group 2 Elements: Alkaline Earth Metals

Q: Why do the solubilities of Group 2 hydroxides and sulphates change in opposite directions?

Solubility depends on lattice enthalpy against hydration enthalpy. For hydroxides the small anion makes lattice enthalpy fall faster than hydration enthalpy down the group, so solubility increases. For sulphates the large anion keeps lattice enthalpy nearly constant while hydration enthalpy falls, so solubility decreases and barium sulphate is insoluble.

Q: Why does thermal stability of Group 2 carbonates increase down the group?

A smaller cation polarises the large carbonate ion more strongly, weakening its bonds, and forms an oxide with a higher lattice enthalpy. Both effects favour decomposition, so beryllium carbonate is unstable and magnesium carbonate decomposes easily, while barium carbonate needs about 1633 K. Larger cations give more stable carbonates.

Q: What is the structure of beryllium chloride?

Solid beryllium chloride is a polymeric chain in which each beryllium atom is bonded tetrahedrally to four bridging chlorine atoms. In the vapour it forms a chlorine bridged dimer, which breaks into linear monomer molecules at about 1200 K. The electron deficient beryllium atom accepts lone pairs from chlorine.

Q: What is plaster of Paris and how does it set?

Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum to 393 K. When mixed with water it takes up one and a half water molecules per calcium sulphate and changes back into gypsum, forming a hard mass within 5 to 15 minutes. Heating above 393 K gives dead burnt plaster, which does not set.

Q: How is bleaching powder made?

Bleaching powder is made by passing chlorine into milk of lime, a suspension of slaked lime in water. Two moles of calcium hydroxide react with two moles of chlorine to give calcium chloride, calcium hypochlorite and water. The hypochlorite is the active bleaching and disinfecting part.

Q: What is the composition of Portland cement?

Portland cement contains about 50 to 60 percent lime, 20 to 25 percent silica, 5 to 10 percent alumina and small amounts of magnesia, iron oxide and sulphur trioxide. Its clinker is mainly tricalcium silicate, dicalcium silicate and tricalcium aluminate, and 2 to 3 percent gypsum is added to slow down setting.

Q: Which Group 2 compound trends are most asked in JEE Advanced?

JEE Advanced most often asks the opposite solubility trends of hydroxides and sulphates, the thermal stability order of carbonates, the three structures of beryllium chloride with hybridisation of beryllium, and the products of heating nitrates, usually as multiple correct statements or matching lists.

Q: Which calcium reactions should I know for JEE Advanced?

For JEE Advanced learn the decomposition of limestone, slaking of lime, lime water turning milky and then clear, bleaching powder from milk of lime and chlorine, quicklime with silica and phosphorus pentoxide, and the conversion of gypsum into plaster of Paris at 393 K.

ChemistryThe s-Block ElementsFor JEE aspirants

The compounds of alkaline earth metals contain the $M^{2 +}$ ion and are predominantly ionic, though less so than alkali metal compounds; beryllium and magnesium compounds are the most covalent. This page covers their oxides, hydroxides, halides (including the structures of $BeC l_{2}$ ) and salts of oxoacids, with the solubility and thermal-stability trends, and then the important calcium compounds: quicklime, slaked lime, limestone, plaster of Paris and Portland cement. JEE Advanced tests the trends with their lattice-versus-hydration reasons and the reactions of the calcium compounds.

On this page1Oxides2Hydroxides3Halides and BeCl₂4Carbonates, sulphates, nitrates5Solubility logic6Quicklime7Slaked lime8Limestone9Plaster of Paris10Cement11Solved examples

Key Formulas - Quick Reference

★ Must learnOxides MO (rock salt) are basic: $MO + H_{2} O \to M (OH)_{2}$ ; BeO is covalent and amphoteric.
★ Must learnHydroxides: solubility, thermal stability and basic strength increase $Mg (OH)_{2} < Ca (OH)_{2} < Sr (OH)_{2} < Ba (OH)_{2}$ ; $Be (OH)_{2}$ amphoteric.
★ Must learnCarbonates and sulphates: solubility decreases down the group; thermal stability of carbonates increases ( $BeC O_{3} < MgC O_{3} < CaC O_{3} < SrC O_{3} < BaC O_{3}$ ).
Nitrates: $2M (N O_{3})_{2} \to 2MO + 4N O_{2} + O_{2}$ (all Group 2); hydrate tendency falls ( $Mg (N O_{3})_{2} \cdot 6 H_{2} O$ , $Ba (N O_{3})_{2}$ anhydrous).
$BeC l_{2}$ : chain (solid, Be $s p^{3}$ ), Cl-bridged dimer (vapour), linear monomer (about 1200 K).
★ Must learnLime: $CaC O_{3} \to CaO + C O_{2}$ (1070-1270 K); $Ca (OH)_{2} + C O_{2} \to CaC O_{3} + H_{2} O$ ; $CaC O_{3} + C O_{2} + H_{2} O \to Ca (HC O_{3})_{2}$ .
★ Must learnBleaching powder: $2Ca (OH)_{2} + 2C l_{2} \to CaC l_{2} + Ca (OCl)_{2} + 2 H_{2} O$ ; plaster of Paris $CaS O_{4} \cdot \frac{1}{2} H_{2} O$ from gypsum at 393 K.
Cement: CaO 50-60%, $Si O_{2}$ 20-25%; $Si O_{2} /A l_{2} O_{3}$ = 2.5-4; $CaO/ (Si O_{2} + A l_{2} O_{3} + F e_{2} O_{3})$ ≈ 2; 2-3% gypsum slows setting.

1. General Character

The dipositive oxidation state ( $M^{2 +}$ ) is the predominant valence of Group 2 elements. Their compounds are predominantly ionic, but less ionic than the corresponding compounds of the alkali metals, because of the increased nuclear charge and smaller size. The oxides and other compounds of beryllium and magnesium are more covalent than those of the heavier, larger members (Ca, Sr, Ba).

1.1 Oxides and hydroxides

The alkaline earth metals burn in oxygen to form the monoxide MO, which, except for BeO, has the rock-salt structure. BeO is essentially covalent. The enthalpies of formation of these oxides are quite high, so they are very stable to heat. BeO is amphoteric; the other oxides are ionic and basic, and react with water to form sparingly soluble hydroxides:

MO + H_{2} O M (OH)_{2}

The solubility, thermal stability and basic character of the hydroxides increase from $Mg (OH)_{2}$ to $Ba (OH)_{2}$ . They are, however, less basic and less stable than the alkali metal hydroxides. Beryllium hydroxide is amphoteric, as it reacts with both alkali and acid:

Be (OH)_{2} + 2O H^{-} beryllate ion [Be (OH)_{4}]^{2 -}

Be (OH)_{2} + 2HCl + 2 H_{2} O [Be (H_{2} O)_{4}] C l_{2}

1.2 Halides

Except for the beryllium halides, the halides of the alkaline earth metals are ionic. Beryllium halides are essentially covalent and soluble in organic solvents. Beryllium chloride has a chain structure in the solid state. In the vapour phase it tends to form a chloro-bridged dimer, which dissociates into the linear monomer at high temperatures, of the order of 1200 K.

Figure 1:

BeC l_{2}

in three states (NCERT 10.19). Be is electron-deficient with only four valence electrons around it in the monomer, so it accepts lone pairs from Cl of neighbours: sp (monomer),

s p^{2}

(dimer),

s p^{3}

(chain). The chain is drawn flat; each

BeC l_{4}

unit is tetrahedral.

The tendency to form halide hydrates gradually decreases down the group ( $MgC l_{2} \cdot 6 H_{2} O$ , $CaC l_{2} \cdot 6 H_{2} O$ , $SrC l_{2} \cdot 6 H_{2} O$ , $BaC l_{2} \cdot 2 H_{2} O$ ). The hydrated chlorides, bromides and iodides of Ca, Sr and Ba can be dehydrated by heating; the corresponding hydrated halides of Be and Mg suffer hydrolysis on heating. The fluorides are relatively less soluble than the chlorides owing to their high lattice energies.

Figure 2: The tendency to form hydrates falls down the group (hydration enthalpy falls). The hydrated halides of Be and Mg hydrolyse on heating instead of losing water.

JEE Advanced

Reading $BeC l_{2}$ like an exam question. In the monomer, Be has only four valence electrons (two Be-Cl bonds), so it is electron-deficient and a strong Lewis acid. It makes up the deficiency by accepting a lone pair from a Cl of a neighbouring molecule:

Form	Where	Be hybridisation	Be coordination	Shape at Be
Monomer	vapour, above about 1200 K	$s p$	2	linear (180°)
Dimer $B e_{2} C l_{4}$	vapour, lower temperature	$s p^{2}$	3	trigonal planar
Chain $(BeC l_{2})_{n}$	solid	$s p^{3}$	4	roughly tetrahedral

The same electron deficiency makes $BeC l_{2}$ (like $AlC l_{3}$ ) a Friedel-Crafts catalyst. In the chain, each Be-Cl-Be bridge is a normal two-electron donor bond from Cl to Be, not a three-centre bond (contrast $Be H_{2}$ chains, which are three-centre two-electron bonds, since H has no lone pair).

Key idea

Oxides and hydroxides become more basic and more soluble down the group; beryllium's are amphoteric.

BeC l_{2}

is covalent and electron-deficient, so it bridges: chain, dimer, monomer.

2. Salts of Oxoacids

Carbonates are insoluble in water and can be precipitated by adding sodium or ammonium carbonate to a solution of a soluble salt of the metal. Their solubility decreases as the atomic number of the metal increases. All of them decompose on heating to give $C O_{2}$ and the oxide. Beryllium carbonate is unstable and can be kept only in an atmosphere of $C O_{2}$ . Thermal stability increases with increasing cationic size.
Sulphates are white solids and stable to heat. $BeS O_{4}$ and $MgS O_{4}$ are readily soluble in water; the solubility decreases from $CaS O_{4}$ to $BaS O_{4}$ . The greater hydration enthalpies of $B e^{2 +}$ and $M g^{2 +}$ overcome the lattice enthalpy, so their sulphates are soluble.
Nitrates are made by dissolving the carbonates in dilute nitric acid. Magnesium nitrate crystallises with six molecules of water, whereas barium nitrate crystallises as the anhydrous salt, again a decreasing tendency to form hydrates with increasing size. All of them decompose on heating to give the oxide, like lithium nitrate.

2M (N O_{3})_{2} heat M = Be, Mg, Ca, Sr, Ba 2MO + 4N O_{2} + O_{2}

Figure 3: Thermal stability rises with cation size,

BeC O_{3} < MgC O_{3} < CaC O_{3} < SrC O_{3} < BaC O_{3}

(NCERT uses about 1200 K for

CaC O_{3}

), so

BaC O_{3}

is the most stable (NCERT 10.32).

2.1 Why the solubility trends run in opposite directions

Solubility depends on the balance between lattice enthalpy (cost) and hydration enthalpy (return). Hydration enthalpy always falls as the cation grows. What differs is the lattice enthalpy. With a small anion ( $O H^{-}$ , $F^{-}$ ), the lattice enthalpy depends strongly on the cation radius and falls faster than the hydration enthalpy, so solubility increases down the group. With a large anion ( $C O_{3}^{2 -}$ , $S O_{4}^{2 -}$ ), the lattice enthalpy is set mainly by the big anion and stays almost constant, while hydration enthalpy falls, so solubility decreases.

Figure 4: The anion size decides the direction (NCERT Problems 10.4, 10.5). Small anion: lattice enthalpy falls faster, hydroxides and fluorides get more soluble. Large anion: lattice enthalpy barely changes, carbonates and sulphates get less soluble (bars schematic).

Figure 5: Real numbers for the two opposite trends. Hydroxides become more soluble down the group (Mg to Ca: a factor of about

1 0^{6}

K_{s p}

); sulphates become less soluble (BaSO

_{4}

is the insoluble one used in barium meals and gravimetric analysis).

Exam Trick 'Small anion, soluble down; big anion, insoluble down.' Hydroxides and fluorides: solubility ↑ from Mg to Ba. Carbonates, sulphates (and chromates): solubility ↓. Thermal stability of any oxo-salt: always ↑ with cation size.

Hydroxides, fluorides (small anion)Solubility increases down the group:

Ba (OH)_{2}

most soluble and the strongest base;

Mg (OH)_{2}

(

K_{s p}

5.6 × 10

^{- 12}

) nearly insoluble.

Carbonates, sulphates (large anion)Solubility decreases down the group:

BeS O_{4}

MgS O_{4}

soluble;

BaS O_{4}

(

K_{s p}

1.1 × 10

^{- 10}

) insoluble.

Figure 6: Ask one question, the size of the anion, and the solubility trend follows. Thermal stability always rises with cation size.

Key idea

Solubility of Group 2 salts depends on anion size; thermal stability of carbonates and nitrates depends on cation size (larger is more stable).

Quick Recall: tap to check

Why does the solubility of alkaline earth metal hydroxides increase down the group?

With the common small

O H^{-}

anion, lattice enthalpy decreases much more than hydration enthalpy as the cation grows.

Why are

BeS O_{4}

and

MgS O_{4}

soluble while

BaS O_{4}

is not?

The very large hydration enthalpies of

B e^{2 +}

and

M g^{2 +}

overcome the (almost constant) lattice enthalpy;

B a^{2 +}

is hydrated too weakly.

What is the hybridisation of Be in solid

BeC l_{2}

s p^{3}

: each Be is bonded tetrahedrally to four bridging Cl atoms.

3. Important Compounds of Calcium

3.1 Calcium oxide (quicklime), CaO

Quicklime is prepared on a commercial scale by heating limestone in a rotary kiln at 1070-1270 K. The $C O_{2}$ is removed as soon as it forms, so that the reaction goes to completion.

CaC O_{3} heat CaO + C O_{2}

Calcium oxide is a white amorphous solid with a melting point of 2870 K. On exposure to air it absorbs moisture and $C O_{2}$ . Adding a limited amount of water breaks the lumps of lime (slaking); quicklime slaked with soda gives solid soda lime. Being a basic oxide, it combines with acidic oxides at high temperature:

CaO + H_{2} O Ca (OH)_{2}

CaO + Si O_{2} CaSi O_{3}

6CaO + P_{4} O_{10} 2C a_{3} (P O_{4})_{2}

Figure 7: CaO is a basic oxide, so it grabs acidic oxides. Milk of lime with

C l_{2}

gives hypochlorite, the active part of bleaching powder (NCERT 10.17 ii, iii).

Uses: primary material for cement and the cheapest form of alkali; manufacture of sodium hydroxide from sodium carbonate (causticising: $N a_{2} C O_{3} + Ca (OH)_{2} \to 2NaOH + CaC O_{3}$ ); purification of sugar and manufacture of dyestuffs.

3.2 Calcium hydroxide (slaked lime), Ca(OH)₂

Calcium hydroxide is prepared by adding water to quicklime. It is a white amorphous powder, sparingly soluble in water. Its aqueous solution is lime water, and a suspension of slaked lime in water is milk of lime. When $C O_{2}$ is passed through lime water it turns milky because calcium carbonate forms; with excess $C O_{2}$ the precipitate dissolves as calcium hydrogencarbonate. Milk of lime reacts with chlorine to form hypochlorite, a constituent of bleaching powder.

Ca (OH)_{2} + C O_{2} CaC O_{3} + H_{2} O

CaC O_{3} + C O_{2} + H_{2} O Ca (HC O_{3})_{2}

2Ca (OH)_{2} + 2C l_{2} bleaching powder CaC l_{2} + Ca (OCl)_{2} + 2 H_{2} O

Figure 8: Lime water turns milky with

C O_{2}

and clears with excess. The clearing step is why passing too much

C O_{2}

is avoided when preparing

CaC O_{3}

Uses: mortar (a building material); white wash, because it is a disinfectant; glass making, tanning, preparation of bleaching powder, and purification of sugar.

3.3 Calcium carbonate, CaCO₃

Calcium carbonate occurs in nature as limestone, chalk, marble and other forms. It is prepared by passing $C O_{2}$ through slaked lime, or by adding sodium carbonate to calcium chloride. Excess $C O_{2}$ must be avoided, since it forms water-soluble calcium hydrogencarbonate. It is a white fluffy powder, almost insoluble in water; at 1200 K it decomposes to CaO and $C O_{2}$ , and it reacts with dilute acids to liberate $C O_{2}$ .

CaC l_{2} + N a_{2} C O_{3} CaC O_{3} + 2NaCl

CaC O_{3} + 2HCl CaC l_{2} + H_{2} O + C O_{2}

CaC O_{3} + H_{2} S O_{4} CaS O_{4} + H_{2} O + C O_{2}

Figure 9: Every NCERT calcium reaction sits on this loop. The same chemistry sets mortar (

Ca (OH)_{2}

C O_{2}

) and builds stalactites (

Ca (HC O_{3})_{2}

losing

C O_{2}

Uses: building material (marble) and manufacture of quicklime; with $MgC O_{3}$ as a flux in the extraction of metals such as iron; specially precipitated $CaC O_{3}$ in high-quality paper; antacid, mild abrasive in toothpaste, a constituent of chewing gum, and a filler in cosmetics.

3.4 Calcium sulphate (plaster of Paris), CaSO₄·½H₂O

Plaster of Paris is a hemihydrate of calcium sulphate. It is obtained when gypsum, $CaS O_{4} \cdot 2 H_{2} O$ , is heated to 393 K. Above 393 K no water of crystallisation is left and anhydrous $CaS O_{4}$ , dead burnt plaster, is formed. Plaster of Paris has the remarkable property of setting with water: mixed with an adequate quantity of water it forms a plastic mass that becomes a hard solid in 5 to 15 minutes.

2 (CaS O_{4} \cdot 2 H_{2} O) 393 K 2CaS O_{4} \cdot H_{2} O + 3 H_{2} O

Figure 10:

2 (CaS O_{4} \cdot 2 H_{2} O) \to 2CaS O_{4} \cdot H_{2} O + 3 H_{2} O

at 393 K. Plaster of Paris is a hemihydrate; overheating gives dead burnt plaster, which has lost the ability to set.

Exam Trick Count the waters: 2, then ½, then 0. Gypsum (

CaS O_{4} \cdot 2 H_{2} O

) → plaster of Paris (

CaS O_{4} \cdot \frac{1}{2} H_{2} O

, 393 K) → dead burnt (

CaS O_{4}

, above 393 K). Only the middle one sets. Likewise for lime: quicklime has no water, slaked lime has 'drunk' one.

Uses: the building industry and plasters; immobilising a fractured bone or sprain; dentistry, ornamental work, and casts of statues and busts.

3.5 Portland cement

Cement was first introduced in England in 1824 by Joseph Aspdin. It is called Portland cement because it resembles the natural limestone quarried in the Isle of Portland. It is made by combining a material rich in lime, CaO, with clay, which contains silica ( $Si O_{2}$ ) along with oxides of aluminium, iron and magnesium. When clay and lime are strongly heated together they fuse and react to form cement clinker, which is mixed with 2-3% by weight of gypsum to form cement. On mixing with water, the constituents hydrate and rearrange to give a hard mass (setting); gypsum is added only to slow down setting so that the cement hardens properly.

Figure 11: Portland cement is mostly lime and silica; the clinker holds

C a_{3} Si O_{5}

(51%),

C a_{2} Si O_{4}

(26%) and

C a_{3} A l_{2} O_{6}

(11%). Gypsum is added only to slow down setting.

Uses: concrete and reinforced concrete, plastering, and the construction of bridges, dams and buildings; next to iron and steel, it is a commodity of national necessity.

Key idea

The calcium compounds form one loop: limestone → quicklime → slaked lime → limestone, with bleaching powder and plaster of Paris as side branches.

Figure 12: Group 2 compounds on one page. Solubility depends on anion size; thermal stability rises with cation size; beryllium is covalent throughout.

Quick Recall: tap to check

What happens when quicklime is heated with silica?

CaO + Si O_{2} \to CaSi O_{3}

(a basic oxide combines with an acidic oxide; calcium silicate slag).

What is dead burnt plaster?

Anhydrous

CaS O_{4}

, formed when gypsum is heated above 393 K; it does not set with water.

Why is gypsum added to cement clinker?

Only to slow down the setting, so the cement hardens properly.

4. Solved Examples

Solved Example 1

Why does the solubility of alkaline earth metal hydroxides in water increase down the group?

Solution:

The anion ( $O H^{-}$ ) is common and small, so the cation radius controls the lattice enthalpy. As the cation grows, lattice enthalpy decreases much more than hydration enthalpy, so the balance shifts towards dissolving and solubility increases down the group.

Solved Example 2

Why does the solubility of alkaline earth metal carbonates and sulphates in water decrease down the group?

Solution:

The anions are much larger than the cations, so the lattice enthalpy stays almost constant within the group. Hydration enthalpy decreases down the group, so solubility decreases.

Solved Example 3

Which is the correct order of thermal stability?
(A)

BaC O_{3} < SrC O_{3} < CaC O_{3} < MgC O_{3}

(B)

MgC O_{3} < CaC O_{3} < SrC O_{3} < BaC O_{3}

(C)

CaC O_{3} < MgC O_{3} < BaC O_{3} < SrC O_{3}

(D) all decompose at the same temperature

Solution:

Answer: (B). The smaller the cation, the more it polarises $C O_{3}^{2 -}$ and the more stable (higher lattice enthalpy) the oxide product, so smaller cations decompose their carbonates more easily (about 813, 1173, 1563, 1633 K).

Solved Example 4

Which statements about beryllium chloride are correct? (One or more options.)
(A) In the solid it is a polymeric chain with

s p^{3}

Be.
(B) In the vapour at about 1200 K it is linear.
(C) It is an ionic solid like

CaC l_{2}

.
(D) It acts as a Lewis acid.

Solution:

Answer: (A), (B), (D). $BeC l_{2}$ is covalent and soluble in organic solvents, so (C) is wrong. Electron-deficient Be accepts lone pairs, which is why it bridges and acts as a Lewis acid (Friedel-Crafts catalyst).

Solved Example 5

How much plaster of Paris can be obtained from 1.00 kg of gypsum? (Ca = 40.1, S = 32.1, O = 16.0, H = 1.0)

Solution:

$M (CaS O_{4} \cdot 2 H_{2} O) = 172.2$ ; $M (CaS O_{4} \cdot \frac{1}{2} H_{2} O) = 145.2$ g mol $^{- 1}$ . One mole gives one mole (per CaSO $_{4}$ ).

Mass $= 1000 \times \frac{145.2}{172.2} =$ 843 g (the rest, 15.7%, is water lost).

Solved Example 6

C O_{2}

is bubbled through lime water. The solution turns milky (P) and then clear (Q); on boiling it turns milky again. Identify P and Q with equations.

Solution:

P = $CaC O_{3}$ : $Ca (OH)_{2} + C O_{2} \to CaC O_{3} + H_{2} O$ .

Q = $Ca (HC O_{3})_{2}$ (soluble): $CaC O_{3} + C O_{2} + H_{2} O \to Ca (HC O_{3})_{2}$ . Boiling reverses the second step and precipitates $CaC O_{3}$ again.

Solved Example 7

How many water molecules are lost per formula unit of

CaS O_{4}

when gypsum is converted into plaster of Paris? (Answer as a decimal.)

Solution:

$CaS O_{4} \cdot 2 H_{2} O \to CaS O_{4} \cdot \frac{1}{2} H_{2} O$ : 1.5 water molecules per $CaS O_{4}$ (3 per two formula units, as in the NCERT equation).

Practice Questions

Compare the alkali metals and alkaline earth metals with respect to (ii) basicity of oxides and (iii) solubility of hydroxides. (NCERT 10.6)Answer: Alkali metal oxides and hydroxides are more basic and more soluble; Group 2 oxides are less basic (BeO amphoteric) and the hydroxides are sparingly soluble, with solubility increasing down the group.
Compare the solubility and thermal stability of the nitrates, carbonates and sulphates of the alkali metals with those of the alkaline earth metals. (NCERT 10.15)Answer: Alkali metal salts are soluble and stable (except Li: $L i_{2} C O_{3}$ , $LiN O_{3}$ decompose); Group 2 carbonates and sulphates are much less soluble, carbonates decompose to MO, and all Group 2 nitrates give MO + $N O_{2}$ + $O_{2}$ .
What happens when (ii) quicklime is heated with silica, (iii) chlorine reacts with slaked lime, (iv) calcium nitrate is heated? (NCERT 10.17)Answer: (ii) $CaO + Si O_{2} \to CaSi O_{3}$ ; (iii) $2Ca (OH)_{2} + 2C l_{2} \to CaC l_{2} + Ca (OCl)_{2} + 2 H_{2} O$ ; (iv) $2Ca (N O_{3})_{2} \to 2CaO + 4N O_{2} + O_{2}$ .
Draw the structure of (i) $BeC l_{2}$ (vapour), (ii) $BeC l_{2}$ (solid). (NCERT 10.19)Answer: Vapour: linear Cl-Be-Cl monomer at high temperature, Cl-bridged dimer at lower temperature; solid: polymeric chain with each Be tetrahedrally bonded to four bridging Cl.
Describe the importance of (i) limestone (ii) cement (iii) plaster of Paris, and two uses of quicklime. (NCERT 10.21, 10.18 iii)Answer: Limestone: quicklime, cement, flux, marble. Cement: concrete, bridges, dams. Plaster of Paris: plasters, fracture casts, dentistry, statues. Quicklime: cement, cheapest alkali, sugar purification, causticising Na2CO3.
Explain: (i) BeO is almost insoluble but $BeS O_{4}$ is soluble in water; (ii) BaO is soluble but $BaS O_{4}$ is insoluble. (NCERT 10.29)Answer: (i) BeO has a very high lattice enthalpy (small ions, covalent); in $BeS O_{4}$ the lattice enthalpy is low (large anion) and the huge hydration enthalpy of $B e^{2 +}$ wins. (ii) BaO reacts with water to give soluble $Ba (OH)_{2}$ (lattice enthalpy with small $O^{2 -}$ is low for large $B a^{2 +}$ ); for $BaS O_{4}$ the hydration enthalpy of $B a^{2 +}$ is too small to overcome the lattice enthalpy.
Which alkaline earth metal carbonate is thermally the most stable? (a) $MgC O_{3}$ (b) $CaC O_{3}$ (c) $SrC O_{3}$ (d) $BaC O_{3}$ (NCERT 10.32)Answer: (d) $BaC O_{3}$ .

Common Mistakes to Avoid

Watch out

Saying all Group 2 salts become less soluble down the group. Hydroxides and fluorides become more soluble; carbonates and sulphates less.
Calling BeO basic. It is covalent and amphoteric; MgO to BaO are basic.
Drawing solid $BeC l_{2}$ as linear molecules. The solid is a chain of tetrahedral Be; the linear monomer exists only in the vapour at high temperature.
Writing that hydrated $MgC l_{2}$ gives anhydrous $MgC l_{2}$ on heating. It hydrolyses to MgO and HCl.
Giving Group 2 nitrates the nitrite on heating (as for NaNO $_{3}$ ). All Group 2 nitrates give the oxide, $N O_{2}$ and $O_{2}$ .
Writing plaster of Paris as $CaS O_{4} \cdot H_{2} O$ or confusing it with dead burnt plaster. It is the hemihydrate $CaS O_{4} \cdot \frac{1}{2} H_{2} O$ ; dead burnt plaster is anhydrous and does not set.
Thinking gypsum is added to cement to make it set. It slows setting down.
Writing lime water clearing with excess $C O_{2}$ as a reaction of $Ca (OH)_{2}$ . It is $CaC O_{3}$ dissolving as $Ca (HC O_{3})_{2}$ .

Frequently Asked Questions

Why do the solubilities of Group 2 hydroxides and sulphates change in opposite directions?

Solubility depends on lattice enthalpy against hydration enthalpy. For hydroxides the small anion makes lattice enthalpy fall faster than hydration enthalpy down the group, so solubility increases. For sulphates the large anion keeps lattice enthalpy nearly constant while hydration enthalpy falls, so solubility decreases and barium sulphate is insoluble.

Why does thermal stability of Group 2 carbonates increase down the group?

A smaller cation polarises the large carbonate ion more strongly, weakening its bonds, and forms an oxide with a higher lattice enthalpy. Both effects favour decomposition, so beryllium carbonate is unstable and magnesium carbonate decomposes easily, while barium carbonate needs about 1633 K. Larger cations give more stable carbonates.

What is the structure of beryllium chloride?

Solid beryllium chloride is a polymeric chain in which each beryllium atom is bonded tetrahedrally to four bridging chlorine atoms. In the vapour it forms a chlorine bridged dimer, which breaks into linear monomer molecules at about 1200 K. The electron deficient beryllium atom accepts lone pairs from chlorine.

What is plaster of Paris and how does it set?

Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum to 393 K. When mixed with water it takes up one and a half water molecules per calcium sulphate and changes back into gypsum, forming a hard mass within 5 to 15 minutes. Heating above 393 K gives dead burnt plaster, which does not set.

How is bleaching powder made?

Bleaching powder is made by passing chlorine into milk of lime, a suspension of slaked lime in water. Two moles of calcium hydroxide react with two moles of chlorine to give calcium chloride, calcium hypochlorite and water. The hypochlorite is the active bleaching and disinfecting part.

What is the composition of Portland cement?

Portland cement contains about 50 to 60 percent lime, 20 to 25 percent silica, 5 to 10 percent alumina and small amounts of magnesia, iron oxide and sulphur trioxide. Its clinker is mainly tricalcium silicate, dicalcium silicate and tricalcium aluminate, and 2 to 3 percent gypsum is added to slow down setting.

Which Group 2 compound trends are most asked in JEE Advanced?

JEE Advanced most often asks the opposite solubility trends of hydroxides and sulphates, the thermal stability order of carbonates, the three structures of beryllium chloride with hybridisation of beryllium, and the products of heating nitrates, usually as multiple correct statements or matching lists.

Which calcium reactions should I know for JEE Advanced?

For JEE Advanced learn the decomposition of limestone, slaking of lime, lime water turning milky and then clear, bleaching powder from milk of lime and chlorine, quicklime with silica and phosphorus pentoxide, and the conversion of gypsum into plaster of Paris at 393 K.

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1. General Character

1.1 Oxides and hydroxides

1.2 Halides

2. Salts of Oxoacids

2.1 Why the solubility trends run in opposite directions

3. Important Compounds of Calcium

3.1 Calcium oxide (quicklime), CaO

3.2 Calcium hydroxide (slaked lime), Ca(OH)2

3.3 Calcium carbonate, CaCO3

3.4 Calcium sulphate (plaster of Paris), CaSO4·½H2O

3.5 Portland cement

4. Solved Examples

Common Mistakes to Avoid

Frequently Asked Questions

Why do the solubilities of Group 2 hydroxides and sulphates change in opposite directions?

Why does thermal stability of Group 2 carbonates increase down the group?

What is the structure of beryllium chloride?

What is plaster of Paris and how does it set?

How is bleaching powder made?

What is the composition of Portland cement?

Which Group 2 compound trends are most asked in JEE Advanced?

Which calcium reactions should I know for JEE Advanced?

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3.2 Calcium hydroxide (slaked lime), Ca(OH)₂

3.3 Calcium carbonate, CaCO₃

3.4 Calcium sulphate (plaster of Paris), CaSO₄·½H₂O