Hydroxide formation: reaction of group 13 elements with water
Can anyone tell me why this is the case?


Though the elements reside in same group, they react with water and form different products. This is mainly due to inert pair effect.


Boron


$$\ce{2B + 3H2O -&gt;[700-800 C] B2O3 + 3H2}$$


  Boron react with water to produce boron trioxide(III) and hydrogen.
  This reaction takes place at a temperature of 700-800°C.(source)



Aluminium


$$\ce{2Al + 6H2O → 2Al(OH)3 + 3H2}$$


Gallium


$$\ce{2Ga + 6H2O → 2Ga(OH)3 + 3H2 ~~~~~~(1)}$$

$$\ce{2Ga + 4H2O -&gt;[350 C] 2GaO(OH) + 3H2O~~~~~(2)}$$


  Gallium react with water to produce gallium oxohydroxide and water.
  This reaction takes place at a temperature near 350°C. (source)



Thallium


$$\ce{2Tl + H2O -&gt;[atm. O2 ~~~50-70 C] 2TlOH}$$


  Thallium react with oxygen and water to produce thallium(I) hydroxide.
  This reaction takes place at a temperature of 50-70°C. (source)