Bases are the substance which give −OH ions (hydroxyl ions) when dissolved in water. If one molecule of a base gives two hydroxyl ions, it is called diacidic base.
(i) CH2(OH)2 is a covalent compound and C−O bond is more stronger than O−H bond. Hence, it dissociates to give only a less amount of H+ ions instead of OH− ion and acts as acid.
(ii) Ca(OH)2, being an ionic compound readily gives two hydroxyl ions when dissolved in water, hence it is a diacidic base.
diacidic baseCa(OH)2⟶Ca2++2OH−
(iii) CH3CH(OH)2 is also a covalent compound and gives a small amount of H+ ions, hence it is an acid.
Actually when two −OH groups are attached with same carbon atoms, the compound is highly unstable and loses a water molecule, eg,
