# s-Block Elements > s-block chemistry for JEE Chemistry: alkali and alkaline earth trends, flame colours, diagonal relationship, Solvay process, lime, gypsum and plaster of Paris. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/s-block-elements - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "s-Block Elements", PrepElephant, https://prepelephant.com/topics/jee/chemistry/s-block-elements ## Direct answer Lilac for potassium, golden yellow for sodium, brick red for calcium and apple green for barium — the s-block announces itself in flame colours because its loosely held ns1-2 electrons excite and re-emit visible light. Down both groups the pattern is uniform: radii grow, ionisation enthalpies fall, reactivity climbs (Cs explodes spectacularly in water), hydroxide basicity rises, and the alkaline earths add a solubility crossover — sulphates less soluble down the group, hydroxides more soluble. Lithium and beryllium misbehave by diagonal kinship (Li resembles Mg in forming a nitride and a carbonate that decomposes), and the commercial compounds — Solvay's soda, quicklime, gypsum setting into plaster of Paris — carry the industrial weight. ## What you must remember - **Flame colour map:** Li crimson, Na golden yellow, K lilac (viewed through blue cobalt glass to filter sodium), Rb red-violet, Ca brick red, Sr crimson, Ba apple green; Be and Mg give no colour. - **Trend package down a group:** atomic and ionic radii increase, ionisation enthalpy and electronegativity fall, reactivity climbs, electrode potential stays near constant (the hydration enthalpy fall compensates the easier ionisation) — alkali E° values cluster near -2.7 to -3.0 V. - **Diagonal duo:** Li-Mg parallels — both form nitrides (Li3N, Mg3N2), carbonates that decompose on heating, hydroxides that are weakly basic, and normal oxides (Li2O, MgO) rather than peroxides. - **Carbonate stability ladder:** down a group, thermal stability rises — Li2CO3 and Na2CO3 differ (lithium's decomposes), BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3. - **Sulphate solubility falls, hydroxide solubility rises** down group 2: MgSO4 dissolves, BaSO4 is the insoluble X-ray contrast medium; Be(OH)2 is amphoteric while Ba(OH)2 is a strong base. - **Solvay mechanics:** NaCl + NH3 + CO2 + H2O giving NaHCO3 (least soluble, precipitates) plus NH4Cl; limestone supplies CO2, ammonia is recovered — fails for potassium because KHCO3 stays dissolved. - **Construction pair:** gypsum CaSO4·2H2O heated to about 373 K gives plaster of Paris CaSO4·(1/2)H2O, which sets by rehydrating; quicklime CaO from limestone, slaked with water to Ca(OH)2. - **Biological anchors:** the Na+/K+ gradient powers nerve conduction, Mg sits at chlorophyll's centre, and Ca forms bones and teeth (hydroxyapatite). ## The Solvay loop and lithium's exclusion Follow a sodium atom through Solvay: brine is saturated with ammonia, carbon dioxide pushed in, and sodium hydrogen carbonate crystallises out because it is the least soluble species present — that one line of solubility logic is the entire process. Filter, heat the NaHCO3 to washing soda, and recover ammonia from the NH4Cl mother liquor with lime from roasted limestone, so the expensive reagent cycles and only salt and limestone are consumed. Now ask why the same plant cannot make potassium carbonate: potassium hydrogen carbonate is far too soluble to crystallise from such a mixture, so the loop never closes. Pair this with the lattice-versus-hydration reasoning that runs the whole chapter — sulphate solubility falls down group 2 because hydration enthalpy shrinks faster than lattice enthalpy, while hydroxides behave oppositely because the small OH- ion keeps lattice energy nearly constant — and every trend question becomes derivable rather than memorised. ## Where this stands in the current syllabus The s-block was dropped as a standalone chapter from the JEE Main syllabus in the 2023 rationalisation and the revised JEE Advanced list trims it too, so calibrate effort accordingly: revise it for board-style completeness and for the periodic-properties chapter, where its trends remain quotable evidence. What still earns marks indirectly: flame colours, hydration-versus-lattice arguments, and the carbonate stability logic that general chemistry questions recycle. The traps: assuming all alkali carbonates decompose (only lithium's), expecting Be and Mg to colour a flame (they do not), and reading E° of alkali metals as becoming more negative down the group (hydration effects keep it nearly constant — lithium is actually the most negative at -3.05 V). ## Frequently asked questions ### Why do alkali metals show nearly constant electrode potentials? The fall in ionisation enthalpy down the group is offset by the fall in hydration enthalpy, so the net E° barely shifts — lithium's small, heavily hydrated ion even makes it most negative. ### Why does lithium resemble magnesium? Similar charge density (Li+ 76 pm versus Mg2+ 72 pm) and electronegativity produce matching chemistry — nitrides, decomposing carbonates, weakly basic hydroxides. ### Which alkali metal carbonates decompose on heating? Only lithium carbonate, which behaves like the group 2 carbonates; sodium and potassium carbonates resist decomposition at ordinary flame temperatures. ### Why does gypsum set into a hard mass? Plaster of Paris rehydrates, interlocking crystals of CaSO4·2H2O grow and harden — an exothermic setting used in casts and moulds. ### Why does sulphate solubility fall down group 2? Hydration enthalpy decreases faster than lattice enthalpy with growing cation size, so dissolution becomes progressively less favourable from MgSO4 to BaSO4.