Water molecules are polar, meaning they have a positive end and a negative end. This polarity allows water molecules to form hydrogen bonds with each other, which are strong intermolecular forces. When water freezes, the water molecules form a crystalline structure in which each water molecule is surrounded by four other water molecules, held together by hydrogen bonds.

Silica, on the other hand, is a covalent network solid. This means that the silicon atoms in silica are covalently bonded to each other, forming a strong, rigid structure. There are no weak intermolecular forces between silica molecules, so silica does not form a crystalline structure when it cools. Instead, it forms a glass, which is a non-crystalline solid.

The difference in the way that water and silica freeze is due to the different types of intermolecular forces that are present in each material. The strong hydrogen bonds between water molecules allow water to form a crystalline structure, while the lack of strong intermolecular forces between silica molecules prevents silica from forming a crystalline structure.