In general, if ice and water have the same volume, then the ice weighs less. This is because ice is less dense than water and occupies more space. Therefore, a gallon of ice weighs less than a gallon of water.
When water freezes into ice, the individual water molecules expand to occupy more space, increasing in volume by about 9 percent. As the liquid turns into a crystalline solid, the water molecules stay expanded and mesh together to form a solid. Due to this phenomenon, the same volume of ice and water have different masses and different densities. For this reason, ice floats in water.