How Heavy Is Your Air?

Introduction

If you’ve ever accidentally let go of a Helium balloon while outside, then you know that some gases are less dense than others. In the case of your Helium balloon, it most likely floated away before you could catch it, because Helium is much lighter (or less dense) than the air in our environment. We don’t often think about gases having density, but they do! In this activity you’re going to explore the different densities of some common household gases, including the air that you exhale! 

Background

All of the materials we encounter on a daily basis, from our toothpaste to the plate we eat our dinner from, all of these objects are made up of different types of atoms. Atoms are the smallest unit of matter that retain the properties of their chemical element. The type and arrangement of these different atoms accounts for the different characteristics of all the solids, liquids and gases in our environment. 

These characteristics include properties known as density, mass and volume. The density of an object is the relationship between the mass (weight) of the object and the amount of space it takes up (the volume of the object). The mass of the object’s atoms, their size and how they are arranged all determine the density of the object. If we know the mass and volume of an object, we can figure out the object’s density using the equation:

From this equation we can also observe that if 2 objects have the same volume, but one weighs more than the other, than the two objects have different densities. If you have a die made from plastic, and another die of the same size made from lead, the one made from lead will feel heavier. Since the two dice are the same size, we know that the lead is denser than the plastic. 

Differences in density have to do with the size of the atoms, as well as how tightly the atoms are packed together. The atoms that make up metal are generally heavier than those that make up plastic, and they are packed more closely together. 

In this activity you will be observing the density of different gases, and how differences in density affect how an object behaves. 

Materials

Preparation

Procedure

Observations and Results

In this activity you created the gas carbon dioxide (CO2) by combining baking soda and vinegar. Baking soda and vinegar are known as the reactants, in this reaction, because undergo a change while taking part in the reaction.  Vinegar is weakly acidic, while baking soda is a bicarbonate. When they are combined, a 2-step reaction takes place. The first step of the reaction is an acid-base reaction, while the second step is a decomposition reaction. When both steps are complete, the final products are carbon dioxide (CO2) and water (H2O). When you added the baking soda to the vinegar, you should have observed bubbling and foaming in the bottle, this was the CO2 gas being produced and released. However, the reaction produced too much CO2 for the bottle to hold. As a result, the CO2 gas escaped into Balloon A, and the balloon expanded. Once all of the baking soda mixed with the vinegar, the contents of the bottle were carbon dioxide (CO2) and water (H2O). Since there was nothing left to react, the reaction ended.   

Your next step in this activity was measuring the rate that the two different balloons dropped to the ground. As you know, Balloon A contained carbon dioxide (CO2) from the baking soda and vinegar reaction. In contrast, Balloon B contained the air you exhaled while blowing it up. The air we exhale is mostly nitrogen and oxygen, and only about 4-5% carbon dioxide. Therefore, you were measuring whether the mostly pure carbon dioxide in Balloon A  dropped more quickly than the mostly nitrogen and oxygen gas in Balloon B. 

You should have found that Balloon A dropped more quickly to the ground than Balloon B. You probably noticed that Balloon B encountered greater air resistance on the way to the floor. Balloon B’s path might not have been straight down, instead the balloon may have floated around as it fell. Since both balloons were the same size (or volume) but one fell more quickly than the other, our results tell us that the CO2 gas in Balloon A is more dense than the combination of gases found in Balloon B. 

More to Explore

• Can Water Float On Water? from Science Buddies
• How Much Weight Can Your Boat Float? from Science Buddies 
• Science Activities for All Ages! from Science Buddies

Credits