Question

Which of the following is only acidic in nature?
A. $Be{(OH)_2}$
B. $Mg{(OH)_2}$
C. $B{(OH)_3}$
D. $Al{(OH)_3}$

Answer 1

Lewis acids and bases are basically described by the Lewis theory of acids and bases. Lewis acids are chemical species which have empty orbitals and are able to accept electron pairs from Lewis bases. Moreover, they are electron pair acceptors and electron pair donors respectively.

Complete step by step answer:
An acidic oxide is basically an oxide which when combined with water gives off an acid. Non-metals react with oxygen to form acidic compounds of oxides which are held together by covalent bonds. These compounds are also known as acid anhydrides.
Now, according to the Lewis acid concept, a compound which can easily accept lone pairs of electrons is known as a Lewis acid. So, among the given options $B{(OH)_3}$ is acidic in nature. This is due to the fact that B has empty p-orbital which can easily accept an extra pair of electrons to act as an acid. The equation is as shown:
$B{(OH)_3} + 2{H_2}O \to B{(OH)_4}^ - + H{}_3{O^ + }$
Moreover, the hydroxide ion bonds to the boron atom and forms a tetrahydroborate ion, leaving the poor acidic hydrogen ion alone. However, the hydrogen ions in boric acid which are responsible for its acidic character actually come from water since the hydroxide are absorbed by the $B{(OH)_3}$ molecule.
Now, talking about the other options, $Mg{(OH)_2}$ is a strong base whereas $Al{(OH)_3}$ and $Be{(OH)_2}$ are amphoteric in nature i.e. they have both basic and acidic properties.

Hence, option C is correct.

Note:
There are some compounds that react with oxygen to form oxides which do not further exhibit acidic or basic characteristics. Such compounds are known as neutral compounds of oxygen. Some examples of these types of oxides are CO, NO etc.