Density is the mass per unit volume of a substance. The density of water is most given as 1 g/cm3, but below is the density of water with different units.
|Density of water g/cm3||1 g/cm3|
|Density of water g/mL||1 g/mL|
|Density of water kg/m3||1000 kg/m3|
|Density of water lb/ft3||62.4 lbs/ft3|
It's no coincidence that water has a density of 1. Density is mass divided by volume (ρ=m/v), and water was used as the basis for establishing the metric unit of mass, which means a cubic centimeter (1cm3) of water weighs one gram (1g).
So, 1g/1cm3 = 1 g/cm3, giving water its easy-to-remember density. However, water's exact density depends on both the air pressure and the temperature of the area.These variations in density are very slight though, so unless you need to know very exact calculations or the experiment takes place in an area with an extreme temperature/pressure, you can continue to use 1 g/cm3 for water density. You can look at the chart in the next section to see how water's density changes with temperature.
Note that these water density values are only true for pure water. Saltwater (like the oceans) has a different density which depends on how much salt is dissolved in the water. Seawater density is typically slightly higher than the density of pure water, about 1.02g/cm3 to 1.03g/cm3.
Below is a chart that shows the density of water (in grams/cm3) at different temperatures, ranging from below water's freezing point (-22°F/-30°C) to its boiling point (212°F/100°C).
As you can see in the chart, water only has an exact density of 1 g/cm3 at 39.2°F or 4.0°C. Once you get below water's freezing point (32°F/0°C), the density of water decreases because ice is less dense than water. This is why ice floats on top of water and, when you put ice cubes in a glass of water, they don't just sink straight to the bottom.
The chart also shows that, for the range of temperatures typical for indoor science labs (about 50°F/10°C to 70°F/21°C), the density of water is very close to 1 g/cm3, which is why that value is used in all but the most exact density calculations. It's not until the temperature is very extreme in one direction or another (close to freezing or boiling), that the temperature of water changes enough that 1 g/cm3 would no longer be acceptably accurate.
|Temperature (°F/°C)||Density of Water (grams/cm3)|
So know you know what the density of water is at different temperatures, but what if you want to find the density of something that isn't water? It's actually pretty easy to do!
You can find the density of any substance by dividing its mass by its volume. The formula for density is: ρ=m/v, with density represented by the symbol ρ (pronounced "rho").
There are three main ways to calculate density, depending on whether you're trying to find the density of a regularly-shaped object, an irregular object, or a liquid, and if you have any special tools like a hydrometer.
For regular objects (those whose faces are standard polygons, such as squares, rectangles, triangles, etc.) you can calculate mass and volume fairly easily. The mass of an object is simply how much it weighs, and all regular polygons have an equation for determining their volume based on their length, width, and height.
For example, say you have a rectangular piece of aluminum that weighs 865g and has measurements of 10cm x 8cm x 4cm. First you'd find the volume of the piece of aluminum by multiplying the length, width, and height (which is the equation for volume of a rectangle).
V = 10cm x 8cm x 4cm = 320 cm3
Next, you divide the mass by the volume to get density (ρ=m/v).
865g/320cm3 = 2.7g/cm3
So the density of aluminum is 2.7g/cm3, and this is true for any piece of (pure and solid) aluminum, no matter what its size is.
If the object has an irregular shape and you can't easily calculate its volume, you can find its volume by placing it in a graduated cylinder filled with water and measuring the volume of water it displaces. Archimedes' Principle states that an object displaces a volume of liquid equal to its own volume. Once you have found the volume, you'd use the standard ρ=m/v equation.
So if you had a different, irregular piece of aluminum that weighed 550g and displaced 204mL of water in a graduated cylinder, then your equation would be ρ = 550g/204mL = 2.7g/mL.
If the substance you're trying to find the density of is a liquid, you can simply pour the liquid into the graduated cylinder and see what its volume is, then calculate density from there.
If you're trying to calculate the density of a liquid, you also can do so by using an instrument known as a hydrometer. A hydrometer looks like a thermometer with a large bulb at one end to make it float.
To use one, you simply gently lower the hydrometer into the liquid until the hydrometer is floating on its own. Find which part of the hydrometer is right at the surface of the liquid and read the number on the side of the hydrometer. That'll be the density. Hydrometers float lower in less dense liquids and higher in more dense liquids.
Water density is typically rounded to 1 g/cm3 or 1000 kg/m3, unless you are doing very exact calculations or conducting an experiment in extreme temperatures. Water's density changes depending on the temperature, so if you're doing an experiment close to or past water's boiling or freezing point, you'll need to use a different value to take into account the change in density. Both steam and ice are less dense than water.
The equation for density is ρ=m/v.
In order to measure the density of a substance, you can calculate a regularly-shaped object's volume and proceed from there, measure the volume of a liquid or how much liquid an irregular object displaces in a graduated cylinder, or use a hydrometer to measure the density of a liquid.