Why does a mass thermodynamically schrink when cooled down?

Answer 1

There is no simple answer.. some materials do the
opposite when cooled down.. it dependes

Answer 2

An ideal gas is predicted to have zero volume at a temperature of 0 K (absolute zero) according to the ideal gas law.

PV = nRT
When T = 0 K, V = 0

As the temperature of a substance is lowered, the molecules making up that substance begin to slow down in their movements. The slow the particles are moving, the less space they take up and the closer they can fit together (the substance shrinks). At 0 K, all motion stops for the particles making up the system, and with no motion, the ideal gas is said to have no volume.
But this is just or ideal gasses…and in reality, there is no such thing as an ideal gas. Ideal gasses are treated as point masses; they have no volume of their own. This assumption is obviously contradictory to reality, since all real materials are made of atoms (in turn made up of smaller, sub atomic, particles) which do have a real volume.

Answer 3

I depends how the molecules line up. Water crystals must line up with 6 molecules making a ring shape. This causes a space in the center that isn't there when water is liquid, so it expands. The 6 sides is also why snowflakes all have 6 sides. The crystals grow on each other.