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.

These balloons are filled with helium, a gas that is less dense than air.

What is density? Density is a measurement of the amount of matter that is in a specific amount of space. To calculate density, you have to know an object’s 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!

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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.

Cooking oil floats on water because water is denser than the cooking oil.

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.

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

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Discovery Education is a subsidiary of Discovery Communications, LLC.