The alkaline earth metal nitrate that does not crystallise with water molecules, is
(a) Ca(NO3)2
(b)

Q: Why do some salts have water molecules in their crystals while others don't?

It depends on the balance of energies. If the energy released by the cation binding to water (hydration enthalpy) is high enough to stabilize the crystal structure, hydrates form. If not, the salt crystallizes without water.

Q: What exactly is charge density?

Charge density is the ratio of an ion's charge to its volume. A smaller ion with the same charge has a higher charge density, making its electric field more concentrated and its pull on polar water molecules stronger.

Q: Does Barium nitrate dissolve in water if it doesn't form hydrates?

Yes, Barium nitrate is soluble in water! Dissolving in water and forming solid hydrated crystals are two different phenomena. It dissolves because its lattice energy is also relatively low, but when you evaporate the water, it prefers to pack into a crystal without water.

Q: What are the formulas for the hydrated nitrates of the other Group 2 metals?

Magnesium and Calcium nitrates typically crystallize as hexahydrates (e.g., $\text{Mg(NO}_3\text{)}_2 \cdot 6\text{H}_2\text{O}$), while Strontium nitrate can form a tetrahydrate. Barium nitrate is the exception, forming an anhydrous salt.

Q: Is Barium the only element that forms anhydrous salts?

Not at all! Many large cations with low charge densities, like Potassium ($K^+$) and Rubidium ($Rb^+$), also tend to form anhydrous salts for the exact same reason—their hydration enthalpies are too low.

Question

The alkaline earth metal nitrate that does not crystallise with water molecules, is

Options:

(A) $	ext{Ca(NO}_3	ext{)}_2$

(B) $	ext{Sr(NO}_3	ext{)}_2$

(C) $	ext{Ba(NO}_3	ext{)}_2$

(D) $	ext{Mg(NO}_3	ext{)}_2$

Sigma Percentile · Accepted Answer

### The Mystery of the Anhydrous Nitrate

Have you ever noticed how some salts, like copper sulfate, form beautiful blue crystals that contain water, while others, like table salt, are completely dry? This trapped water is known as the **water of crystallization**.

In this problem, we are on a mission to find out which alkaline earth metal nitrate refuses to trap water molecules and instead forms an **anhydrous** crystal.

### The Tug of War: Hydration vs. Lattice

To understand why a salt forms a hydrate, we need to look at the forces at play. When a salt crystallizes from an aqueous solution, there is a competition.

The metal cation wants to surround itself with polar water molecules due to ion-dipole interactions. The energy released in this process is called the **hydration enthalpy** ($\Delta H_{	ext{hyd}}$).

On the other hand, the cation also needs to pack tightly with the nitrate anions to form a stable solid lattice. If the hydration enthalpy is highly negative (meaning a lot of energy is released), it provides enough stability to keep the water molecules locked inside the crystal lattice.

### The Role of Charge Density

So, what determines the strength of this hydration enthalpy? It all comes down to **charge density**, which is the ratio of the ion's charge to its size.

$$ 	ext{Charge Density} \propto \frac{	ext{Charge}}{	ext{Size}} $$

For the Group 2 alkaline earth metals, the charge is constant at $+2$. Therefore, the hydration enthalpy depends entirely on the ionic size. A smaller ion will have a higher charge density, creating a more concentrated electric field that strongly attracts water molecules.

### Walking Down Group 2

Let's observe what happens as we move down Group 2 in the periodic table:

$$ 	ext{Be}^{2+} < 	ext{Mg}^{2+} < 	ext{Ca}^{2+} < 	ext{Sr}^{2+} < 	ext{Ba}^{2+} $$

Beryllium ($	ext{Be}^{2+}$) is tiny. Its $+2$ charge is packed into a very small volume, giving it a massive charge density. It strongly attracts water and easily forms hydrated crystals.

As we move down to Barium ($	ext{Ba}^{2+}$), the number of electron shells increases significantly. Barium is the largest ion in this group. Its $+2$ charge is spread out over a much larger volume, resulting in the lowest charge density.

### The Final Verdict

Because Barium has the lowest charge density, its pull on water molecules is the weakest.

$$ \Delta H_{	ext{hyd}} (	ext{Ba}^{2+}) 	ext{ is the least negative.} $$

The energy released by hydrating the Barium ion is simply not enough to compensate for the disruption it would cause in the solid crystal lattice. As a result, when Barium nitrate crystallizes, it prefers to pack together without any water molecules.

Thus, $	ext{Ba(NO}_3	ext{)}_2$ is the only nitrate in this list that crystallizes as an **anhydrous** salt!

Animated Solution for Chemistry - s and p-Block Elements: The alkaline earth metal nitrate that does not crystallise with water molecules, is

Select Answer:

Visualized Solution

$Question Analysis$

$Condition for Hydration$

$Ionic Size Trend$

$Hydration Enthalpy Trend$

$Conclusion$

$The Way Forward$

The Sigma Insight: Alkaline Metals

The Mystery of the Anhydrous Nitrate

The Tug of War

The Role of Charge Density

Walking Down Group 2

The Final Verdict

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