In its most basic sense, density is defined as the equivalent weight of a substance for every specific amount of volume. The normal density of water is 1 gram per 1 milliliter at room temperature. However, this measurement can change depending on a lot of factors such as changes in temperature and dissolved substances. Ice has a lesser density than water which is why it floats. Knowing the density of water is important because it can be applied to a lot of other aspects. 

Water Density

The sentence “Density is the mass per unit volume of a substance,” is something that is always mentioned in class. That is the technical definition of the term. Mass and weight are often misused because of the confusion about their respective definitions but since we are only referring to one gravity which is that of Earth’s, mass and weight are pretty much the same. 

Density is a constant factor in all matter that is composed of molecules. In other words, if an object occupies space and has mass, its density can also be measured. It is simply just its weight for every specific volume. Water’s density at normal conditions is 0.9998395 g/ml at 4.0° Celsius (39.2° Fahrenheit) to be specific but is rounded off to 1 gram per mL or cubic centimeter (1g/mL or cc) for uniformity. 

Factors affecting water density

The following are the common factors that affect the density of water.

  • The density of water is approximately 1 gram per cubic centimetre (1 g/cm3) or 1 gram per milliliter (1g/mL).
  • Temperature can also change water density but the correlation is still unclear and inconsistent.
  • When water is cooled at room temperature, it becomes denser over time. This is true for other substances as well. But the limit ends at 4°C where pure water is at its most dense.
  • There are cases where it is usually the other way around where further cooling of a substance decreases its density. This phenomenon is usually referred to as ‘negative thermal expansion’ and applies to those bonded with intermolecular forces along with other chemical composition factors. One example that can be made for this is molten silica.

Temperature-dependent water density

There was a time in my life when I was able to use this concept of water density to get back at my older brother and his friends for always picking on me. It was a hot summer day and they went up this nearby hill to bury their collection of bottle caps. Then they asked me to return home, get a gallon of water and bring it to them to quench their thirst. I was a kid back then and I weighed nothing more than 70 pounds. A gallon of tap water at that temperature was a lot for me to carry. The second time that they asked me, I looked for ways to make my job easier. Thanks to the encyclopedia, I discovered that a gallon of water at boiling point weighed a lot less. So I brought them a gallon of boiled water which was definitely lighter, and I scampered home right away without regard for their reactions. 

Temperature (°F/°C)Density (grams/cm3Weight (pounds/ft3
32°F/0°C0.9998762.416
39.2°F/4.0°C162.424
40°F/4.4°C0.9999962.423
50°F/10°C0.9997562.408
60°F/15.6°C0.9990762.366
70°F/21°C0.9980262.3
80°F/26.7°C0.9966962.217
90°F/32.2°C0.995162.118
100°F/37.8°C0.9931861.998
120°F/48.9°C0.988761.719
140°F/60°C0.9833861.386
160°F/71.1°C0.9772961.006
180°F/82.2°C0.9705660.586
200°F/93.3°C0.9633360.135
212°F/100°C0.9586559.843

Source: U.S. Department of the Interior, Bureau of Reclaimation, 1977, Ground Water Manual, from The Water Encyclopedia, Third Edition, Hydrologic Data and Internet Resources, Edited by Pedro Fierro, Jr. and Evan K. Nyler, 2007

Ice has a lower density than water

Looking at this picture below, a big part of the iceberg is submerged in the water. You can see that the majority of it is below the water surface. This is because of the fact that the density of ice is less than that of the liquid water. An ice’s density is reduced by 9 percent at freezing point. 

photo of iceberg

One way to understand the difference of the densities of water in various phases of matter is to look at the structure of ice. Since ice is basically solid water, the arrangement of molecules that make it up is in a constant position. Whereas for its liquid phase, the molecular arrangement tends to be more random and there is a larger distance between molecules, making it denser than ice. However, there is still air contained within a block of ice that makes up for about 10% of its weight which is why there is a portion of the iceberg that remains above the water surface.

Water is life. The majority of the planet is water and so is the majority of the human body. At higher temperatures, water becomes denser. The denser matter will always sink the less dense. In winter, when the surface of the water is exposed in the cold temperature, the ice that forms sinks to the bottom of the body of water which will cause it to freeze. At the same time, water can also regulate heat so it might take a while for frozen lakes to completely thaw in the summer season.

The scientific explanation of this concept might be harder to follow because of the other factors that could affect it. For example, the content in the water like dissolved solids, organic substances, and microorganisms can increase its weight. A good comparison would be the ocean water and pure water. There is obviously more content in ocean water than there is in fresh water so that makes the former denser.

clear round ball on black textile

The heavy ice cubes sink all the way to the bottom of the glass and the lighter ice cubes float to the water surface. 

Credit: Mike Walker

Does heavy ice float?

We have already established that ice floats because it is less dense than liquid water. However, there are special cases where the ice is of slightly different composition and that is usually referred to as “heavy ice” made from heavy water. This type of water is 10.6% much denser than regular water and has a chemical of D2O instead of the usual H2O since deuterium (an isotope of hydrogen) atoms have replaced the original hydrogen. As a result, the ice formed from that kind of water is denser than ice from regular water.

How to measure density

three clear beakers placed on tabletop

A device intended for measuring the density of liquids is called the hydrometer, usually made of glass and resembles a thermometer. It is quite easy to use. As a matter of fact, you can make a makeshift version of it using just plastic straws. It is a thin glass rod with a round bulb at the tip to serve as its support to float upright. To take the measurement, it needs to be submerged into the liquid until it floats. You can see on the printed lines and numbers where the water level matches. The meter will float lower in liquids with low density and higher in those with high density. The basis for all other densities of liquids is water so when you use it in water, it should hit the 1.000 mark, proving true the specific gravity of water at 4°C. 

Not only is this device used to measure densities of various liquids, it also has other applications in different fields. It can be used to measure water salinity in class experiments, get rough estimates of fat content in milk since density is correlated to how much fat is present in a specific volume of milk. It is also used to determine sugar content in some alcoholic drinks like beer and wine in the brewer industry to know how long it has been going through fermentation.

The reason why water density is 1

The mathematical formula of density (ρ=m/v) is mass divided by volume, which translates to mass for every given volume. Water is also the standard used for the foundation of the metric unit of mass, establishing that 1 cubic centimeter of water weighs 1 gram. This is no coincidence.

It is also a convenience because you won’t have a hard time remembering that number. 1 gram per 1 cc. Like mentioned before, it’s not exactly 1. The exact number is just rounded off to the nearest whole number to avoid errors and maintain uniformity. There is never a constant density for water as well because air pressure and temperature can alter it even at the slightest bit. If you need very accurate numbers, you need to determine these factors and take them into consideration in your calculations. There is a chart in the next part to guide you with these instances.

Keep in mind though that those values only apply to pure water. Other water types like seawater have a different density due to the salt content. It is around 1.02g/cm3 to 1.03g/cm3, a bit higher than pure water.