How Dense is Water? The Answer May Surprise You!

How Dense is Water? The Answer May Surprise You!

You’ve probably heard the expression, Dense like water, which is meant to describe someone who is slow-witted or mentally sluggish. But how dense is water, really? In order to understand that, you need to understand what density actually means! Read on to learn more about water density and its effects on oceans, lakes, pools, and our bodies. You might be surprised at what you learn!

Density Explained

Water density varies based on temperature and can be calculated by the following equation:

where ρ is density, γ is the specific heat capacity of water, k is the thermal conductivity of water, and t represents temperature. For instance, if we were to calculate the density of seawater at a particular temperature (say, for example, 32 degrees Fahrenheit), we would do so using this equation:. 

For reference, when our sea-faring friends are at zero degrees Celsius (or 273 Kelvin), the density is 1 g/cm3 which translates to an approximate volume of 9 liters per cubic foot. At 10 degrees Celsius (or 283 Kelvin) it becomes denser at 1.001 g/cm3 or about 8 liters per cubic foot. At 20 degrees Celsius (or 293 Kelvin) it becomes even denser with a density of 0.998 g/cm3 or about 7 liters per cubic foot. By the time you get up to 100 degrees Celsius (or 573 Kelvin), water density has reached 0.97 g/cm3, or about 6 liters per cubic foot. 

The three most common ways that humans interact with seawater are through recreational activities like swimming, fishing, and boating; commercial uses such as harvesting shellfish; and industrial purposes such as desalination plants and power plants that extract salt from ocean water for energy production. It's safe to say that there's never been a better time than now to explore the wide world of water density but remember: being aware means being prepared.

Why Is Density Important?

Density (mass per unit volume) has a big effect on the properties of water, like how it interacts with other materials. For example, if two cups of water are different densities, the lighter cup will be able to float in the heavier one. 

If you have ice cubes and liquid water that are at different densities, the ice cubes will sink to the bottom of your glass. When seawater freezes into sea ice, the salt is left behind in brine form because its density is lower than that of fresh water; this causes problems for coastal dwellers who rely on freshwater sources but live close enough to sea ice to have freezing temperatures all year round. A cubic meter of pure water weighs about 1000 kilograms or 1 tonne. It also varies by temperature, which is why ice floats in colder climates. 

The average density of surface ocean water ranges from 1,000 kg/m3 near the poles to 1030 kg/m3 near the equator. The variation in density is due to changes in temperature and salinity. As the water gets warmer, the molecules break apart faster, making them less dense and causing them to rise. 

Colder water molecules do not break apart as easily so they stay together more tightly, making them denser and causing them to sink. Salinity can affect density too - salty water is less dense than fresh water because there are ions in it that take up space without adding any mass. Oceanographers measure the density of seawater using either an echosounder or an underway probe that measures pressure instead of depth. 

One problem with these tools is that they cannot measure accurately below 1500 meters deep, meaning we don't know what happens below those depths when calculating things like climate change models.

What Affects the Density of Water?

Surprisingly, water's density doesn't change no matter where it's from. Other than the number of impurities and the temperature, the only way to change water's density is to add or remove hydrogen ions. When adding hydrogen ions, the water becomes denser because there are more molecules in a fixed space; when removing them, the water becomes less dense. 

It sounds like this would happen as you travel deeper underwater, but that's not the case. Since pressure increases with depth, water expands and therefore becomes less dense with depth. Thus, if you're scuba diving at 100 feet below sea level, the same volume of water will have a greater mass due to being compressed. That means the buoyancy force on your body will be greater and make for a more challenging dive. 

That's why it can be difficult to swim up from deep depths; you need a lot of energy to overcome gravity so your body can stay buoyant. One thing that is important to note is that hot water has a lower density than cold water. For example, hot water contains many more bubbles of air dissolved into it, which decreases its density. 

To test this out, all you need to do is take two cups of water - one at room temperature and one heated to about 120 degrees Fahrenheit. Fill both cups until they overflow and then measure their volumes by pouring the liquid back into the measuring cups. Then weigh each cup of liquid separately on a scale. 

You'll find that the cup of hot water weighs less than the other, even though they hold the same volume. Now compare their densities: 0.941 g/cm3 (room temp) vs 0.998 g/cm3 (hot). While it may seem like warm water should be heavier, these numbers show us that the opposite is true. So next time you go swimming in some cool ocean waves, remember just how heavy that water really is!