Can all substances be changed for solid to liquid?
If the right conditions apply then yes all substance can be changed from a solid to a liquid. A liquid CO2 can be achieved above 5.2 atmosphere of pressure.
How do waters relatives densities as a solid and a liquid differ from that of most other substances?
All substances do exist in all phases at some point (at least theoretically). However, many substances are unlikely to be seen in a certain phase because of their chemical properties. Carbon dioxide, for example, does have a liquid phase, but it is rarely seen at room temperature because solid CO2 sublimes to a gas without moving through the liquid phase. If the proper circumstances were met (usually extreme changes in temperature or pressure), liquid CO2…