Getting to Know: Density
Why does a helium balloon float away into the sky when you let it go? You probably know that these balloons are not filled with air. Instead, they are filled with the element, helium, which is usually a gas.
You might have heard someone say, “Helium is lighter than air.” However, it is more accurate to say that helium is less dense than air. Density and weight are different but related properties. Weight is a measurement of gravity’s pull on an object. Density is a measurement of the amount of matter in a given volume. What is density?
Density is a measurement of the amount of
matter that is in a specific amount of space. To
These balloons are filled with helium, a gas
calculate density, you have to know an object’s
that is less dense than air.
mass as well as its volume. Remember that the
mass of an object is how much matter is present
in the object, whereas the volume of an object is how much space it takes up. To calculate the
density of an object, you divide the mass of the object by its volume.
Misconception 1: A heavy iron rod has a greater density than a small iron nail.
Actually, despite differences in size, weight, mass, and volume, as long as temperature
and pressure are the same, objects entirely made of iron will have the same density.
A good example of how density affects objects is one you see nearly every day: some objects
float when placed in water, whereas others sink. The reason for this is very simple—objects
float on water when they are less dense than water, and they sink when they are denser than
water. This principle also holds true for liquids other than water. An object may float on water,
but if you place it in a liquid that is less dense than water such as alcohol, it may sink!
Concept: Density
Getting to Know
www.discoveryeducation.com
1
© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.
Misconception 2: I measured 50 mL of
oil and 50 mL of water. This means the
densities of the oil and the water are the
same, right?
What you measured was the volume of the oil
and the water, not their densities. Volume is not
the same as density. In fact, if you combined the
oil and water into one container, you would see
that the oil forms a layer on top of the water.
This indicates that the oil is actually less dense
than the water.
Gases all have the same density,
don’t they?
Just because you can’t see most gases doesn’t mean they don’t have density. Gases have mass—that is, they are matter. They also have volume—that is, they take up space. Therefore, gases have density. Cooking oil floats on water
because water is denser than
the cooking oil.
Think about the helium balloon that rises up to the sky until you can’t see it any more. The air
around us is made up of a mixture of gases, including nitrogen (approximately 78%), oxygen
(approximately 21%), and carbon dioxide (approximately .03%). The density of this mixture is
greater than the density of pure helium gas, which is why the helium-filled balloon rises when
you let it go. However, other gases such as carbon dioxide are denser than air. That is why a
balloon filled with carbon dioxide would fall to the ground if you released it.
What units are used to express density?
There are several different units used to express density. Mass is often measured in grams or kilograms, whereas volume can be measured in liters, milliliters, or cubic centimeters. As a result, density can be expressed in a variety of ways. A common way to express density is in g/cm3. Sometimes scientists refer to the specific gravity of a substance. The specific gravity of a
substance is the density of a substance compared with the density of water. It is calculated by
dividing the density of the substance by the density of water. Because the density of water is
1 g/cm3, the specific gravity of a substance compared with water is equal to the density of the
substance, except that specific gravity is expressed as a number without units.
Keep on reading to find out more about how density relates to the properties of substances!
Concept: Density
Getting to Know
www.discoveryeducation.com
2
© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.