Question

The increase in the solubility of Sodium halides, in water at 25°C is:

• A. NaCl > NaBr > NaI
• B. NaBr > NaI > NaCl
• C. NaI > NaBr > NaCl
• D. NaCl = NaBr > NaI

Hint:

Larger ions like iodide (I-) tend to form weaker ionic bonds with sodium, making NaI more soluble in water than NaBr and NaCl.

Answer 1

The Correct Option is C

The increase in the solubility of Sodium halides, in water at 25°C is: 

• Option 1: NaCl > NaBr > NaI - This is incorrect. As the size of the halide ion increases, the solubility tends to decrease.
• Option 2: NaBr > NaI > NaCl - This is also incorrect. The solubility does not follow this trend at 25°C.
• Option 3: NaI > NaBr > NaCl - This is the correct answer. The solubility of sodium halides in water increases as the size of the halide ion increases. NaI has the highest solubility, followed by NaBr, and NaCl has the lowest solubility.
• Option 4: NaCl = NaBr > NaI - This is incorrect because NaI has a higher solubility than both NaCl and NaBr at 25°C.

Explanation:

The solubility of sodium halides in water at 25°C increases in the order of NaI > NaBr > NaCl. This is because as the ionic radius of the halide ion increases, the lattice energy of the salt decreases, allowing it to dissolve more easily in water.

Answer 2

The solubility of an ionic solid in water depends on the balance between its lattice energy (the energy holding the ions together in the solid) and its hydration energy (the energy released when the ions get surrounded by water molecules). For a series of sodium halides, the cation stays fixed as Na+ while the halide ion gets bigger going from Cl- to Br- to I-. Let's check each option against this balance.

1. NaCl > NaBr > NaI: This would mean solubility falls as the halide ion grows, but a bigger halide ion has a lower charge density, so it lowers the lattice energy more than it lowers the hydration energy. Solubility should rise, not fall, so this order is wrong.
2. NaBr > NaI > NaCl: This puts NaCl at the bottom, which fits the general rise with ion size, but it places NaBr above NaI, breaking the expected steady increase from Cl to Br to I. This order does not hold up.
3. NaI > NaBr > NaCl: As the halide ion size grows from Cl- to Br- to I-, the lattice energy of the salt drops faster than its hydration energy drops. That makes the salt easier to pull apart in water, so solubility climbs steadily from NaCl to NaBr to NaI. This order matches the expected trend.
4. NaCl = NaBr > NaI: This claims NaCl and NaBr dissolve equally well and both beat NaI, but NaI is actually the most soluble of the three because it has the lowest lattice energy. This order is incorrect.

Since lattice energy falls faster than hydration energy as the halide ion gets bigger, solubility keeps rising from NaCl to NaBr to NaI.

So the correct answer is NaI > NaBr > NaCl.