One gallon of pure water weighs 8.345 pounds at standard temperature and pressure. However, the density of water varies based on temperature. Thus, a given volume of water weighs differently at different temperatures.
Water has its greatest density at 39.2 degrees Fahrenheit or 4 degrees Celsius. This means that a gallon of water at that temperature weighs slightly more than a gallon of water at any other temperature. Water is a unique compound as it weighs more in its liquid state than solid. This is why ice floats and rivers and oceans freeze from the top down instead of the bottom up.