Reactive metals, calcium chemistry and hard water
Group position predicts reactivity, while dissolved calcium and magnesium ions explain a familiar property of water: hardness.
Learning goals. Compare Groups 1 and 2; write key reactions; connect limestone, quicklime and slaked lime; distinguish temporary and permanent hardness; and select a softening method.
1. Groups 1 and 2
Alkali metals have one valence electron and usually form M+ ions. Alkaline earth metals have two and usually form M2+ ions. Both are reactive reducing agents, but Group 1 metals generally react more vigorously. Down either group, the outer electron is farther from the nucleus and more shielded, so it is removed more easily and metallic reactivity increases.
| Feature | Group 1 | Group 2 |
|---|---|---|
| Typical ion | M+ | M2+ |
| Reaction with water | Often vigorous: hydroxide + H2 | Varies; Ca reacts, Mg slowly with cold water |
| Oxides/hydroxides | Usually strongly basic and soluble | Basic; solubility varies |
Reactive metals are handled only with appropriate laboratory controls. Sodium and potassium must never be added to water outside a supervised demonstration.
2. Representative reactions
Sodium reacts with water: 2Na + 2H2O → 2NaOH + H2. Magnesium burns in oxygen: 2Mg + O2 → 2MgO. Calcium reacts with water less violently than sodium: Ca + 2H2O → Ca(OH)2 + H2. The hydroxides make the solution alkaline because they provide OH− ions.