Question

How and why does the addition of a hydrophilic molecule like NaCl to water change (H2O) the specific heat of the solution? Would the addition of a hydrophobic molecule to this solution change the number of hydrogen bonds present?

Answer 1

1. Addition of hydrophilic molecules like NaCl to water changes the specific heat of the solution. This is due to the fact that NaCl when dissolved in water is dissociated into sodium and chloride ions. These charged ions interfere with the intermolecular forces between water molecules. The positively charged sodium ions align with the negative oxygen side while the negatively charged chloride ions align with the positive hydrogen side. This ion-dipole interaction is stronger than the hydrogen bonding between the water molecules so more energy is needed to move water away from the ions and into the vapour phase.

2. Hydrophobic molecules when added to water tend to aggregate minimizing their surface contact and associated surface energy with water. However smaller hydrophobic molecules dissolve to small extent as water molecule can arrange themselves around small hydrophobic molecules without breaking hydrogen bonds or losing much energy.