A United States customary gallon of water weighs, on average, 8.34 pounds. Five U.S. gallons, therefore, weighs 41.7 lbs. The British imperial gallon weighs 10.022 pounds. This means that 5 imperial gallons weigh, on average, some 10 pounds more compared to 5 U.S. gallons. Both calculations say "on average" because the weight of water is not constant and can vary depending on the temperature.
The Chemistry of Water
Covering approximately 70 percent of the world's surface, water is the most abundant molecule on the face of the Earth. Just above the surface in the Earth's atmosphere, water also abounds in the form of water vapor. It is only the chemical compound that exists naturally in solid, liquid, and gas states.
The chemical formula of water, H2O, is one of the most well-known chemical formulas. The H stands for hydrogen with the subscript 2 for the number of hydrogen atoms, and O for 1 oxygen atom, All three atoms are joined together via covalent bonding, which means that they share electrons.
Another unique property of water is that it is less dense when it is frozen solid than in liquid form. This is the reason why ice cubes float in beverages. Water is used as a coolant because it is capable of absorbing a good amount of heat before its own temperature rises.
How Temperature Affects the Weight of Water
Due to the chemical composition of water, its density fluctuates along with the changes in temperature. Water is the densest, thus the heaviest when it is cold. As mentioned earlier, it loses some of its density when it freezes, which means that ice is lighter than cold water. Cold water is denser than warm water and when water is just above freezing.
Thermal fluctuations affect the activity of water molecules, causing them to vibrate and push against each other. When weighed in a contained space, such as a gallon container for instance, the density and weight of the water fluctuates along with thermal changes. It should be noted, however, that the variations in weight are minimal.
Other Effects of Temperature on Water
Apart from affecting the density and weight of water, thermal changes also affect it in different ways. Water, particularly groundwater, can dissolve more minerals from underground rocks at higher temperatures. Higher water temperatures, likewise, increases its electrical conductivity.
Thermal fluctuations can also have an impact on aquatic life. Colder water holds more oxygen than warm water. Compounds that are present in water also becomes toxic when water temperature rises too high. Maintaining a constant, ideal temperature is therefore vital in sustaining the balance of aquatic life.
Water, Gravity and Atmospheric Pressure
Gravity and atmospheric pressure affect the weight of water as well. For instance, the Earth has a stronger gravitational pull than the moon, which means that water will weigh much less on the moon. Jupiter, meanwhile, has a much stronger gravity than Earth, which means that water and other liquids will weigh more on Jupiter. Regardless of the changes in the weight of water due to temperature or gravity. However, its volume will remain constant.
Difference Between US Customary and British Imperial Systems
The U.S. customary and British imperial units of measurements are both based on the old English system units, which dates back to 450 A.D. in Anglo-Saxon England. Measurement units such as the inch and the foot came from the Saxons. The English brought this measurement system to North America when it was still a territory of the British Empire.
While most of the units of measurements between the U.S. customary and imperial systems remain identical, the measurements for liquid volume are not the same. This is due to several factors including foreign trade in the colonies, which influenced its liquid volume and weight measurements.
After the American Revolutionary War, the U.S. colonies maintained the English system of measurements for volume along with the influences from foreign traders. The British made changes and standardized its imperial system units of measurement in 1824.
For comparison using the metric system, a U.S. liquid gallon holds 3.875 liters while an Imperial gallon holds a bit more at approximately 4.55 liters. The U.S. gallon may be divided into 4 U.S. quarts, 8 U.S. pints, or 128 U.S. fluid ounces. The imperial gallon can hold 4 imperial quarts, 8 imperial pints, or 160 imperial fluid ounces.