Animated Solution for Chemistry - s and p-Block Elements: The amphoteric hydroxide is

A Family Portrait

When we look at the periodic table, Group 2 elements—the alkaline earth metals—stand out as a fascinating family. From Beryllium at the top to Barium at the bottom, these metals form hydroxides with the general formula ${\text{M(OH)}}_2$.

At first glance, you might assume that all these hydroxides behave exactly the same way. After all, they belong to the same group! However, chemistry is rarely that simple. As we journey down the group, a subtle but powerful shift occurs in their chemical personalities.

Size Matters

Let's talk about basicity. A base, in the simplest terms, is a substance that readily releases hydroxide ions (${\text{OH}}^{-}$) when dissolved in water.

As we move down Group 2—from $\text{Be}$ to $\text{Mg}$, $\text{Ca}$, $\text{Sr}$, and finally $\text{Ba}$—the atomic size of the metal increases significantly. Why does this matter? Imagine holding a heavy weight on a very short string versus a very long string. The longer the string, the easier it is to snap.

Similarly, as the metal atom gets larger, the distance between the metal nucleus and the oxygen atom in the hydroxide group increases. This makes the $\text{M-OH}$ bond longer and, consequently, much weaker. Because the bond is weak, it breaks easily in water, releasing ${\text{OH}}^{-}$ ions. Therefore, the basic character increases down the group:

Beryllium

Now, let's focus on the rebel of the family: Beryllium. Sitting at the very top of Group 2, Beryllium is exceptionally small. It has a very high charge density (a $+2$ charge packed into a tiny volume).

Because of this intense concentrated charge, Beryllium holds onto its oxygen atoms with an iron grip. The $\text{Be-O}$ bond is incredibly strong and has a significant covalent character. It refuses to simply let go of its ${\text{OH}}^{-}$ ions like its heavier siblings do.

This unique property makes ${\text{Be(OH)}}_2$ amphoteric. The word "amphoteric" comes from the Greek prefix amphi-, meaning "both." An amphoteric substance is a chemical chameleon; it can act as a base when faced with an acid, and it can act as an acid when faced with a base!

To truly appreciate this chameleon-like behavior, we need to see it in action. Let's put ${\text{Be(OH)}}_2$ to the test.

Test 1: Acting as a Base If we drop ${\text{Be(OH)}}_2$ into a strong acid like Hydrochloric acid ($\text{HCl}$), it behaves exactly like a traditional base. It neutralizes the acid to form a salt and water:

Test 2: Acting as an Acid Here is where the magic happens. If we take that same ${\text{Be(OH)}}_2$ and place it in a strong base like Sodium hydroxide ($\text{NaOH}$), it doesn't just sit there. It reacts! It accepts more hydroxide ions to form a soluble complex salt called sodium beryllate:

This dual reactivity is the absolute hallmark of an amphoteric compound.

Returning to our original question, we were asked to identify the amphoteric hydroxide among ${\text{Be(OH)}}_2$, ${\text{Ca(OH)}}_2$, ${\text{Sr(OH)}}_2$, and ${\text{Mg(OH)}}_2$.

Armed with our understanding of atomic size, bond strength, and the unique high charge density of Beryllium, the answer is crystal clear. While Magnesium, Calcium, and Strontium hydroxides are strictly basic, Beryllium hydroxide stands alone as the amphoteric rebel.