Water contracts on melting and has a negative slope on the fusion curve.

Water is a unique substance when it comes to its phase changes. Normally, most substances expand when they melt. This is because when a solid melts and transitions into a liquid, the intermolecular forces between the particles weaken, allowing them to move more freely and occupy a larger volume, resulting in expansion. However, water behaves differently.

Water contracts on melting:
Water is one of the few exceptions to the general rule. When water melts, it actually undergoes contraction instead of expansion. This means that the volume of the liquid water is smaller than the volume of the solid ice. This contraction is due to the unique arrangement of water molecules in the solid state.

The structure of ice:
In the solid state, water molecules arrange themselves in a regular pattern held together by hydrogen bonds. Each water molecule is bonded to four neighboring water molecules through these hydrogen bonds, forming a tetrahedral structure. This arrangement causes the water molecules to be more closely packed compared to the liquid phase.

Melting of ice:
When ice is heated, the energy provided breaks the hydrogen bonds holding the water molecules in the solid state. As the hydrogen bonds break, the water molecules gain kinetic energy and start moving more freely. However, even with the increased molecular motion, the hydrogen bonding network still influences the arrangement of the molecules.

Negative slope on the fusion curve:
The fusion curve represents the relationship between temperature and pressure at which a substance coexists in both the solid and liquid states. For water, the fusion curve has a negative slope. This means that as pressure increases, the melting point of water decreases. This unusual behavior is also related to the strength of the hydrogen bonding in water.

The negative slope of the fusion curve is a result of the contraction of water on melting. As pressure increases, the water molecules are forced closer together, which enhances the strength of the hydrogen bonding. This leads to a decrease in the melting point. In other words, under higher pressures, the water molecules are more likely to remain in the solid state, resulting in a lower temperature required for melting.

In conclusion, water is a unique substance that contracts on melting and has a negative slope on the fusion curve. This behavior is a result of the unique structure and hydrogen bonding in water, which causes the water molecules to be more closely packed in the solid state and influences the melting point under different pressures.