Animated Solution for Chemistry - s and p-Block Elements: Given below are two statements. Statement I Both ${\text{CaCl}}_2\cdot 6{\text{H}}_2\text{O}$ and ${\text{MgCl}}_2\cdot 8{\text{H}}_2\text{O}$ undergo dehydration on heating. Statement II $\text{BeO}$ is amphoteric, whereas the oxides of other elements in the same group are acidic. In the light of the above statements, choose the correct answer from the options given below.

The beauty of inorganic chemistry lies in its exceptions and the underlying physical principles that govern them. This question from JEE Main 2021 perfectly encapsulates how a simple change in ionic size can completely alter a chemical reaction. Let's break down the two statements and uncover the truth behind them.

The Tale of Two Chlorides

The first statement claims that both hydrated calcium chloride (${\text{CaCl}}_2\cdot 6{\text{H}}_2\text{O}$) and hydrated magnesium chloride (${\text{MgCl}}_2\cdot 8{\text{H}}_2\text{O}$) undergo dehydration upon heating. At first glance, this seems logical. When you heat a hydrated salt, the water of crystallization should simply evaporate, leaving behind the anhydrous salt.

For calcium chloride, this is exactly what happens. The calcium ion (${\text{Ca}}^{2+}$) is relatively large. When heated, the thermal energy easily breaks the weak ion-dipole interactions holding the water molecules, resulting in pure dehydration:

However, magnesium is a different beast. The magnesium ion (${\text{Mg}}^{2+}$) is significantly smaller than the calcium ion, giving it a much higher charge density. According to Fajan's rules, this high charge density gives ${\text{Mg}}^{2+}$ a strong polarizing power. It pulls the electron cloud of the oxygen atom in the water molecule so strongly that it weakens the $\text{O-H}$ bonds within the water molecule itself.

When you heat hydrated magnesium chloride, instead of just losing water, the weakened $\text{O-H}$ bonds break. The chloride ions react with the hydrogen ions to form $\text{HCl}$ gas, and the magnesium ion bonds with the remaining oxygen to form magnesium oxide. This process is called hydrolysis:

Because magnesium chloride undergoes hydrolysis rather than simple dehydration, Statement I is false.

The Nature of Alkaline Earth Metal Oxides

The second statement asserts that Beryllium Oxide ($\text{BeO}$) is amphoteric, while the other oxides in Group 2 are acidic. Let's evaluate the two parts of this claim.

First, is $\text{BeO}$ amphoteric? Yes, it is. Beryllium, being the first element of Group 2, exhibits anomalous behavior. Its exceptionally small size and high ionization energy mean that its bonds with oxygen have significant covalent character. Because of this, $\text{BeO}$ can react with both acids (to form beryllium salts) and bases (to form beryllate ions). This dual nature defines it as amphoteric.

But what about the other oxides in the group ($\text{MgO}$, $\text{CaO}$, $\text{SrO}$, $\text{BaO}$)? The statement claims they are acidic. This is a fundamental error. Metal oxides are generally basic in nature. As we move down Group 2, the atomic size increases, and the ionization energy decreases. This makes it easier for the metal to release the oxide ion (${\text{O}}^{2-}$), which readily reacts with water to form hydroxide ions (${\text{OH}}^{-}$), creating a basic solution.

Therefore, the oxides of the other elements in the group are basic, not acidic. This makes Statement II false.

Since both the claim about the dehydration of magnesium chloride and the claim about the acidic nature of heavier Group 2 oxides are factually incorrect, we can confidently conclude that both Statement I and Statement II are false.

Pro Tip for JEE: If heating hydrated ${\text{MgCl}}_2$ leads to hydrolysis, how do chemists ever obtain anhydrous ${\text{MgCl}}_2$? The trick is to heat the hydrated salt in an atmosphere of dry $\text{HCl}$ gas. The presence of excess $\text{HCl}$ pushes the equilibrium backward, suppressing the hydrolysis reaction and allowing pure dehydration to occur!