Question

Madam Curie was studying solution chemistry. She was reading that dissolution happens with ionic crystals in water where ions are separated and the salt gets dissolved.
She had some Magnesium Oxide (MgO) crystals lying around. Being a scientist, she decided to test it out. She tried to mix MgO with water but she observed that MgO did not mix with water. She was bewildered.
But she soon realized the reason.
What could be the reason?
Select from the following options:

• A. Entropy of water was too high.
• B. MgO was not ionic. Madam Curie was mistaken.
• C. The lattice energy of MgO is higher than the hydration energy of water.
• D. It would have dissolved if Madam Curie had given more time.

Answer 1

The correct option is C

The lattice energy of MgO is higher than the hydration energy of water.

When we dissolve NaCl in water, the electrostatic bonds that are holding the NaCl crystals together need to be broken for the ions to separate. The energy for this breakdown of bonds comes from water molecules in the form of another electrostatic force.
Look at the image below.

Here we can see that the water molecules are attracting the NaCl ions and trying to break them apart. If the attraction forces between NaCl ions and water is stronger then the crystal will break apart.
The energy provided by water is called hydration energy and that between the NaCl ions in the crystal form is called lattice energy.
Finally, we have
If hydration energy > lattice energy $\to$ Dissolution occurs
If hydration energy < lattice energy $\to$ No dissolution