⚗️ Visual Chemistry
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Typical Metal Elements

20min Part 3 / Ch6 / Lesson 2
Prerequisites: 3-6-1

Objectives

  • Compare properties of alkali and alkaline earth metals
  • Know aluminum's amphoteric nature and its extraction
  • Understand precipitation reactions of common metal ions

Alkali Metals (Group 1)

Li, Na, K, Rb, Cs — going down the group:

  • Reactivity increases (ionization energy decreases)
  • Density increases (Li, Na, K float on water)
  • All form monovalent cations M⁺

Common reactions:

  • React vigorously with water: 2Na+2H2O→2NaOH+H2\mathrm{2Na + 2H_2O \rightarrow 2NaOH + H_2}
  • Oxidize in air → stored under oil

Flame colors:

ElementLiNaKCuCaSrBa
ColorRedYellowVioletBlue-greenOrange-redCrimsonYellow-green

Alkaline Earth Metals (Group 2)

Ca, Sr, Ba — form divalent cations M²⁺.

CompoundFormulaUses/Properties
QuicklimeCaODesiccant, heat packs
Slaked limeCa(OH)₂Neutralization, plaster
LimewaterCa(OH)₂ solutionCO₂ detection (turns milky)
GypsumCaSO₄·2H₂OPlaster casts, building

Alkaline earth metal compounds

Ca(OH)2+CO2→CaCO3↓+H2O\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O}

Passing excess CO₂:

CaCO3+CO2+H2O→Ca(HCO3)2\mathrm{CaCO_3 + CO_2 + H_2O \rightarrow Ca(HCO_3)_2} (soluble → milkiness disappears)

Aluminum (Amphoteric Metal)

Aluminum Al is amphoteric — dissolves in both acids and strong bases:

With acid: 2Al+6HCl→2AlCl3+3H2\mathrm{2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2}

With base: 2Al+2NaOH+6H2O→2Na[Al(OH)4]+3H2\mathrm{2Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2}

Aluminum oxide Al₂O₃ (alumina) is also amphoteric.

Thermite reaction: 2Al+Fe2O3→Al2O3+2Fe\mathrm{2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe} (reduces iron at high temperature)

Hall–Héroult process (aluminum extraction):

  • Bauxite → purified alumina Al₂O₃
  • Dissolved in cryolite Na₃AlF₆ as a flux → molten-salt electrolysis

Separating Metal Ions

ReagentIon precipitatedPrecipitate color
NaOH solutionFe³⁺ → Fe(OH)₃Red-brown
NaOH solutionFe²⁺ → Fe(OH)₂Green-white
NaOH solutionCu²⁺ → Cu(OH)₂Blue-white
NaOH (excess)Al³⁺ → precipitates then redissolves—
NH₃ (excess)Cu²⁺ → [Cu(NH₃)₄]²⁺Deep blue
H₂SPb²⁺, Cu²⁺Black (PbS, CuS)

Common precipitation reactions

Check Your Understanding

Q1 What color is sodium's flame test?

Q2 Why is aluminum called amphoteric?

Q3 What happens when CO₂ is bubbled into limewater?

Exercises

Q1. Excess NaOH is added to a mixed solution of Al³⁺ and Fe³⁺. Describe what happens.

Solution

Both initially precipitate as hydroxides:

Al3++3OH−→Al(OH)3↓\mathrm{Al^{3+} + 3OH^- \rightarrow Al(OH)_3 \downarrow} (white) Fe3++3OH−→Fe(OH)3↓\mathrm{Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3 \downarrow} (red-brown)

With excess NaOH:

  • Al(OH)3\mathrm{Al(OH)_3} is amphoteric and redissolves: Al(OH)3+OH−→[Al(OH)4]−\mathrm{Al(OH)_3 + OH^- \rightarrow [Al(OH)_4]^-}
  • Fe(OH)3\mathrm{Fe(OH)_3} does not dissolve (basic oxide only)

→ Only the red-brown Fe(OH)₃ precipitate remains. This separates Al³⁺ from Fe³⁺.