What phase is water most dense?
For most substances, the solid phase is more dense than the liquid and gas phases. The liquid phase is less dense than the solid phase but more dense than the gas phase, and the gas phase is less dense than either the solid or liquid phases. Water is an exception. Its solid phase (ice) is less dense than the liquid phase.
Under what we here on Earth would consider "normal" conditions, ice is less dense than water, which is why ice floats on water. The full phase diagram for water is complicated, and the extremely high pressure solid phases (Ice III, for example) are more dense than the liquid phase, but unless you're working at pressures of thousands of atmospheres, you don't need to worry about those.
Above 4° Celsius water expands slightly as its temperature increases, this is due to the faster moving molecules taking up more space when they hit each other. Water is most dense at 4° Celsius (density of 1,000) at 100°C the density is 0,958 kg/liter Ice floats on water because it is less dense than the liquid phase.
No. Mass does not change when the phase changes. But the gallon of liquid water will expand and be more than a gallon when it is ice. That is why ice floats. The solid phase of water (ice) is less dense than the liquid phase. A Gallon of ice weighs less than a gallon of water (one gallon is a precise volume).