Reactions and Stoichiometry of Alkaline Earth and Alkali Metal Compounds

Ionic nitrides hydrolyse in water to give the metal hydroxide plus ammonia: Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃, so ONE mole of magnesium nitride (which contains 2 nitrogen atoms per formula unit, as N₃⁻) releases TWO moles of ammonia on complete hydrolysis with excess water, not one. Different metal carbides hydrolyse to different hydrocarbons depending on the specific carbon-containing anion present: methanides (C⁴⁻, as in Be₂C or Al₄C₃) give methane; acetylides/dicarbides (C2²-, as in CaC₂) give ethyne (acetylene); and the less common sesquicarbide/allylide ion (C3⁴-, as in Mg₂C₃) gives propyne (methylacetylene) specifically — recognise Mg₂C₃ as the one classic exam example that breaks from the more familiar methane/acetylene pattern.

Bleaching powder (calcium oxychloride, roughly CaOCl₂) is made by passing chlorine gas over slaked lime, Ca(OH)₂ (not over quicklime, CaO, directly)

Separately, a classic cycle links three calcium compounds: heating limestone (CaCO₃, compound X) releases CO₂ gas and leaves quicklime (CaO) as a residue; mixing that residue with water gives slaked lime, Ca(OH)₂ (compound Y); passing EXCESS CO₂ through a suspension/solution of Y converts it to the water-soluble calcium bicarbonate, Ca(HCO₃)₂ (compound Z, giving a clear solution, since Ca(HCO₃)₂ is soluble while Ca(OH)₂ and CaCO₃ are not); and boiling that clear solution of Z drives off CO₂ and reforms solid CaCO₃ (compound X), completing the cycle. This exact cycle (limestone to lime to bicarbonate solution to limestone again on boiling) is the chemistry behind temporary hard water and behind stalactite/stalagmite formation in caves.

When a mixture of two Group 1 metals reacts completely with water, each mole of metal (regardless of which one) releases exactly one mole of hydroxide ion, since each is a +1 cation. Neutralising the resulting mixed hydroxide solution with a known volume and molarity of a diprotic acid like H₂SO₄ (which neutralises 2 moles of OH⁻ per mole of acid) lets you calculate the TOTAL moles of metal present from the acid used; combining that total-moles equation with the given total MASS of the mixture (via each metal atomic mass) gives two simultaneous equations in two unknowns, solvable for the mass of each individual metal — the calculation works the same way regardless of which two alkali metals are mixed, since basicity (charge) is identical across the group.

Magnesium hydroxide, Mg(OH)₂, is only very sparingly soluble in water. Milk of magnesia (the familiar antacid) is prepared as a SUSPENSION of finely divided, undissolved Mg(OH)₂ particles dispersed in water, not a true solution (which would require the compound to actually dissolve at the molecular/ionic level) and not a colloid either (a suspension has larger, visible/settling particles, distinct from the much smaller, stably dispersed particles of a true colloid) — the deliberately low aqueous solubility of Mg(OH)₂ is exactly what makes it useful as a slow-acting, sustained-release antacid rather than a fast-dissolving one.