Appearance: Group 13 elements

    Impure (amorphous) boron is a brown powder, but the pure element forms shiny, silver-grey crystals. Properties including its high melting point and low electrical conductivity make B an important refractory material. Aluminium is a hard, white metal. Thermodynamically, it should react with air and water but it is resistant owing to the formation of an oxide layer, 10^-6 to 10^-4 mm thick. A thicker layer of Al₂O₃ can be obtained by making Al the anode in the electrolysis of H₂SO₄; the result is anodized aluminium which will take up dyes and pigments to produce a strong and decorative finish. Gallium is a silver-coloured metal with a particularly long liquid range (303–2477 K). Indium and thallium are soft metals, and In has the unusual property of emitting a high-pitched ‘cry’ when the metal is bent.