If you’re learning science, you’ve likely come across the definition: “Density is the mass per unit volume of a substance.” In simpler terms, density tells us how much “stuff” is packed into a certain space. Think of it as the weight of a specific amount of a material. For water, a common way to express density is grams per milliliter (g/ml) or grams per cubic centimeter (g/cm³).
You’ll often hear that the density of water is 1 g/ml. While this is a good approximation, the actual density of pure water is slightly less, precisely 0.9998395 g/ml at 4.0° Celsius (39.2° Fahrenheit). For most everyday situations, using the rounded value of 1 g/ml is perfectly acceptable.
How Temperature Affects Water Density
Water density isn’t constant; it changes with temperature. To illustrate this, consider a fun anecdote. Imagine a kid tasked with carrying water uphill on a hot day. A gallon of water at 70°F (21°C) weighs about 8.329 pounds. Now, what if that same gallon of water was closer to boiling? Surprisingly, a gallon of water at the boiling point weighs less, around 7.996 pounds! This difference in weight is due to the change in water density at different temperatures.
The table below shows how water density changes with temperature. Notice that density is highest at 39.2°F (4°C) and decreases as temperature moves away from this point, whether hotter or colder.
| Temperature (°F/°C) | Density (grams/cm3 | Weight (pounds/ft3 |
|---|---|---|
| 32°F/0°C | 0.99987 | 62.416 |
| 39.2°F/4.0°C | 1.00000 | 62.424 |
| 40°F/4.4°C | 0.99999 | 62.423 |
| 50°F/10°C | 0.99975 | 62.408 |
| 60°F/15.6°C | 0.99907 | 62.366 |
| 70°F/21°C | 0.99802 | 62.300 |
| 80°F/26.7°C | 0.99669 | 62.217 |
| 90°F/32.2°C | 0.99510 | 62.118 |
| 100°F/37.8°C | 0.99318 | 61.998 |
| 120°F/48.9°C | 0.98870 | 61.719 |
| 140°F/60°C | 0.98338 | 61.386 |
| 160°F/71.1°C | 0.97729 | 61.006 |
| 180°F/82.2°C | 0.97056 | 60.586 |
| 200°F/93.3°C | 0.96333 | 60.135 |
| 212°F/100°C | 0.95865 | 59.843 |
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
Why Ice is Less Dense Than Liquid Water
One of water’s most unique and crucial properties is that its solid form, ice, is less dense than its liquid form. This is why ice floats. If you’ve seen an iceberg, you know that only a small portion is visible above the waterline. In fact, about 90% of an iceberg’s volume is submerged because ice is less dense than the surrounding liquid water.
This phenomenon is due to the molecular structure of water. In liquid water, molecules are packed relatively close together in a somewhat random arrangement. When water freezes, the molecules arrange themselves into a crystalline structure called a lattice. This lattice structure is more open, meaning the water molecules are further apart in ice than in liquid water. Because the same number of molecules occupies a larger volume in ice, ice is less dense.
This property is vital for aquatic life. If ice were denser than liquid water, it would sink to the bottom of lakes and rivers. Water bodies would freeze from the bottom up, potentially killing aquatic organisms and preventing lakes from completely thawing in summer due to water’s insulating properties. Instead, ice forms at the surface, insulating the water below and allowing life to survive through winter.
Density and Dissolved Substances
The density of water is also affected by what’s dissolved in it. Pure water in nature is rare; it usually contains dissolved minerals, salts, gases, and other substances. The more material dissolved in water, the denser it becomes. This is why seawater is denser than freshwater – the dissolved salts increase its mass for the same volume.
There’s even a type of ice called “heavy ice” that’s denser than normal water. This ice is made from “heavy water,” where the hydrogen atoms are replaced with deuterium, a heavier isotope of hydrogen. Heavy water ice will actually sink in normal water, demonstrating the effect of molecular mass on density.
How to Measure Water Density
Density of liquids, including water, can be measured using a device called a hydrometer. This simple instrument is often made of glass and looks a bit like a thermometer with a weighted bulb at the bottom. When placed in a liquid, the hydrometer floats higher or lower depending on the liquid’s density.
Hydrometers are marked with a scale, often with the mark for pure water (at a specific temperature like 4°C) labeled as “1.000,” representing its specific gravity. They are used in various applications, from science education to industries like dairy (to estimate milk fat content) and brewing (to monitor sugar levels in beer and wine making).
Want to test your knowledge of water properties? Take our interactive water-properties true/false quiz!
