Some people say that ice belongs to a covalent crystal. Why is this statement incorrect?

1. Different structural particles in covalent crystals (such as diamond, silicon dioxide, etc.):

The entire crystal is formed by atoms connected through covalent bonds into a "super-molecule".

Ice: The basic unit that constitutes ice is an independent $H_2O$ molecule. Although the internal part of the water molecule (between H and O) is a covalent bond, the molecules remain independent from each other.

2. Nature of the Force (Core Difference)

This is the most crucial criterion for determining the type of crystal.

Covalent Crystal: Held together by covalent bonds. Covalent bonds are extremely strong and require an enormous amount of energy to break them.

Ice: Held together by intermolecular forces (mainly hydrogen bonds and van der Waals forces). Although hydrogen bonds are stronger than ordinary van der Waals forces, they are much weaker than covalent bonds (with an intensity of approximately 1/10 to 1/20 of a covalent bond).

3. Differences in physical properties

If ice is a covalent crystal, its physical properties will be completely overturned: Properties of ice (molecular crystal) analogy: Diamond/Silicon dioxide (covalent crystal)

Summary of misunderstandings:

Many people make mistakes because they see that water molecules contain covalent bonds and casually call it a "covalent crystal".

Underlying logic:

• Examine the interior: $H_2O$ is a covalent compound (because it contains covalent bonds).
• Examine the whole: Ice is a molecular crystal (because molecules are stacked together by hydrogen bonds).

This is similar to a box of Lego bricks. The individual bricks (molecules) are glued together by glue, but the bricks are simply connected to each other through simple assembly (hydrogen bonds). To disassemble these Lego bricks, we don't need to melt the plastic blocks; we just need to separate them.