# Why does a substance expands upon freezing? What conditions necessitate this?

Why does a substance expands upon freezing? What conditions necessitate this? For example, how does the slope of sublimation or fusion curve in a P-T diagram affect this?

• Very few substances do this, and I have a vague memory suggesting that they all have oddball bond geometry. Can't provide a citation and may be way off base. Nov 4 '14 at 20:00
• All first order phase transitions must have a change of volume. It can either increase or decrease. Water is the everyday example of a substance that increases in volume upon freezing. Amongst the elements, at least Si, Ge, Sb, and Bi do as well. While Sb and Bi are rhombahedral, while Si and Ge are diamond cubic. Nov 4 '14 at 20:14
• Water is one of the few substances to expand upon freezing at $4^\circ C$ by about 9% since the formation of the hexagonal lattice which contains more space than the liquid form. Water is a peculiar substance with a lot of interesting properties due to it's molecular shape, composition, and tendency to form H-bonds.
– user43617
Nov 4 '14 at 20:15