Question

In solid NaCl, there is one kind of force holding the crystal together. In solid H2O, there are two kinds of forces within the crystal. Identify the forces and explain.
I know that Hydrogen bonds occur in between water molecules in ice, but what other intermolecular force exists?

Answer 1

Solid NaCl:

• Ionic bonds.

Solid H₂O:

• Hydrogen bonds, and
• Dipole-dipole interactions.

Explanation

NaCl is an ionic compound. It contains myriads of Na⁺ ions and Cl⁻. The two types of ions carry opposite charges. They attract each other via electrostatic forces. This type of electrostatic force is known as ionic bonds. Those bonds hold the ions in a gigantic ionic lattice.

H₂O is a covalent compound. Two H atoms are bonded to one O atom in each molecule. The O-H single bond is highly polar. Also, there are two lone pairs of electrons on the central O atom in each molecule. The O-H bond is so polar that the H atom carries a very strong partial positive charge. The H atom would be attracted to lone pairs on the O atom in neighboring H₂O molecules. Hydrogen bonds between the H₂O molecules hold them in place in their solid state.

There are two O-H single bonds in each H₂O molecule. H₂O molecules are V-shaped due to the presence of the two lone pairs on the central oxygen. Dipoles due to each O-H bonds do not line up within the molecule. As a result, H₂O molecules carry non-zero net dipole. They would attract each other by dipole-dipole interactions. That intermolecular force coexists with hydrogen bonds. It adds to the strength of the attractions between H₂O molecules.