Key concepts
Physics
Chemistry
Density
Gas

Introduction
If you've ever accidentally let go of a helium-filled balloon while outdoors, 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’ll 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 our dinner plates, are made up of different types of atoms. Atoms are the smallest units of matter that retain the properties of their chemical element. The type and arrangement of these different atoms account 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 its mass (weight) and its volume (amount of space it takes up). The mass of an object's atoms, their size and how they are arranged all determine its density. If we know an object’s mass and volume, we can figure out the density using the equation: density = mass/volume

From this equation we can also observe that if two 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. Because the two dice are the same size, we know the lead is denser than the plastic.

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

• Yardstick
• Two balloons
• Four tablespoons baking soda
• Stopwatch or timer
• One cup white vinegar
• Clean 16-ounce plastic water or soda bottle
• Small plastic funnel (If unavailable, use tinfoil or parchment paper to make a temporary funnel.)
• A partner to help
• A sink
• Pencil or pen
• Sheet of paper

Preparation

• Create a table on your sheet of paper with three columns and three rows.
• Label the left column: “Balloon.” Write: “Balloon A” in the middle box and “Balloon B” in the bottom box. Label the middle column: “3-Foot Drop”; label the right column: “6-Foot Drop.”

Procedure

• Carefully pour the vinegar into the water bottle.
• Carefully pour the baking soda into one of the balloons using your funnel. Do not tie it closed. Hold it carefully so that the baking soda does not spill out.
• Secure the baking soda–filled balloon to the top of the water bottle. Avoid letting the baking soda fall into the bottle. To do this, hold the mouth of the balloon and shake the baking soda down to the bottom. Keep holding it this way while you gently cover the top of the bottle with the mouth of the balloon, not allowing the contents of the balloon to drop into the bottle.
• Ensure that the mouth of the balloon covers the bottle top as completely and securely as possible.
• Place the bottle in the sink. Note the size of the balloon and the appearance of the vinegar in the bottle.
• Carefully tip the balloon vertically so the baking soda spills into the bottle. What happens when the baking soda contacts the vinegar in the bottle? Is the liquid in the bottle changing? What happens to the balloon? What do you think is causing the balloon to change shape?
• When the reaction slows, you can gently shake the bottle and tap the balloon, to ensure that no baking soda is stuck in the balloon or on the sides of the bottle.
• When the reaction is complete, ask your partner to help you remove the balloon from the bottle without allowing gas to escape the balloon. While the balloon is still attached, have your partner tightly squeeze the balloon closed just above where its mouth meets the bottle. With the balloon held closed, you can gently remove it from the bottle. (Don’t be surprised if a puff of gas escapes from the bottle when you remove the balloon!)
• Tie off the balloon. This is “Balloon A.”
• Take the second balloon and blow it up to the same size as Balloon A and tie it off. This second balloon is “Balloon B.”
• Have your partner hold the yardstick vertically, resting the end on the floor. They should also hold your stopwatch or timer.
• Stand next to your partner and hold Balloon A at the top of the yardstick, so that it is exactly three feet off the floor.
• Drop the balloon and at the same time have your partner start the timer.
• Note how long it takes the balloon to fall to the ground. Record this time in your table.
• Repeat the balloon-drop steps with Balloon B. Which balloon took longer to drop to the ground?
• Have your partner use the yardstick to measure six feet from the ground. (It’s easier if you do this against a wall.)
• Repeat the balloon-drop steps, dropping each balloon from six feet. Record your results in your table.
• Extra: Test to see the maximum distance that you can throw each balloon. Is one easier to throw than the other? Why do you think that is?

Observations and results
In this activity you created the gas carbon dioxide (CO2) by combining baking soda and vinegar. Both are known as the reactants—in this reaction because they undergo a change while taking part in the reaction. Vinegar is weakly acidic whereas baking soda is a bicarbonate. When they are combined, a two-step reaction takes place. The first step of the reaction is an acid–base reaction, and the second step is a decomposition reaction. When both steps are complete, the final products are 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. The reaction produced too much CO2 for the bottle to hold, however. 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 CO2 and H2O. When 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 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 to 5 percent 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. Because both balloons were the same size (or volume) but one fell more quickly than the other, our results tell us the CO2 gas in Balloon A is denser than the combination of gases found in Balloon B.

More to explore
Showing Science: Watch Objects in Free Fall, from Scientific American
Can Water Float on Water? from Science Buddies
Stacking Liquids, from Scientific American
Science Activities for All Ages! from Science Buddies

This activity brought to you in partnership with Science Buddies