Does mass decrease when temperature decreases?

The inertia mass of an object does not change with temperature -- an intrinsic properties. You need to elaborate more on the specific situation.

Not neccessarily, density is measured by M/V mass divided by volume this means that the density will decrease if the volume is raised and the mass stays the same or if the mass is lowered and the...

thermal contraction

This behavior relates to Charles Law of Gases (Gay-Lussac's Law also applies in terms of pressure changes in a rigid container). You may have noticed that car tires and balloons shrink in cold...

Water is most dense at +4 deg centigrade. When it freezes it becomes lighter.