Like solid and gas, the capacity of a liquid can easily be determined using a unit of measurement known as a gallon. A gallon measures a unit capacity of liquid in both the US customary units and British imperial system. There are three different sizes of gallons in use today, namely the imperial gallon used in the UK, Canada, and some Caribbean countries, the US gallon used mainly in the US and some Latin American countries, and the US dry gallon which is the least used of the three. The three sizes of measurements are significantly different with an imperial gallon defined as 4.546 liters, US gallon as 3.785 liters, while US dry gallon is defined as 4.405 liters.

## A Gallon of Water

How much does a gallon of water weigh? This is one of the questions that is widely dodged or inaccurately answered by many. Some people have estimated the answer to this question to be between 5 and 15 pounds. Some people trying to be smart have treated it as a tricky question without an exact answer. Other people will go as far as asking the temperature of water. Initially, the weight of water was defined as 1 kg/liter. However, the definition was discarded since the density of water is affected by changes in temperatures and atmospheric pressure. The answer is simple; a gallon of water weighs about 8.3 pounds. The imperial gallon of water is defined as 10.02 pounds at its maximum density while the weight of US dry gallon of water is defined as 9.71 pounds. However, the answer comes with a caveat. The weight per gallon of water fluctuates with temperature.

## Effects of Water Temperature on Weight

Temperature is one of the ways to measure the amount of energy in water. A higher temperature means higher energy and vice versa. Higher water energy causes water molecules to vibrate, pushing their neighbors to the side. Thus, the density of water decreases with the increase in energy of water. Since a gallon is a set amount of space, the number of water molecules in that space will fluctuate with the changing energy levels. So, a gallon of warm water weighs slightly less than a gallon of cold water with the difference depending on the temperatures in question. For instance, at 500 Fahrenheit, a gallon of water would weigh approximately 8.3 pounds while at 2000 Fahrenheit a gallon of the same water would weigh 8.03 pounds. However, the value of water remains the same despite the temperature changes since the temperature does not affect the composition of water.

## Atmospheric Pressure and Gravity

Atmospheric pressure and gravity also affect the weight of a gallon of water or any other liquid. A gallon of water will weigh less on the moon compared to the Earth but will weigh more on Jupiter compared to the Earth. The difference in weight is attributed to the gravitational pull experienced by objects. The earth has a higher gravitational pull compared to the moon, making a gallon of water to weigh more on Earth than in the moon. However, just like temperature, the value of water does not change with a change in gravitational pull.