Why Do Balloons Float?

Next Generation Science Standards

PS1.A: Structure and Properties of Matter
  • The periodic table orders elements horizontally by the number of protons in the atom's nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states. (HS-PS1-1),(HS-PS1-2)
PS1.B: Chemical Reactions
  • The fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. (HS-PS1-2),(HS-PS1-7)
PS2.B: Types of Interactions
  • Newton's law of universal gravitation and Coulomb's law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects. (HS-PS2-4)
Some chemicals are heavier than others.

The term "lighter than air" refers to something that is less dense than Earth's atmosphere. Air has a density of 1.29 grams per liter. This is because of several factors, gravitational pull being one of them. We're going to focus on the molar masses of the chemicals that make up air. Air is made up of several chemicals: Oxygen, Argon, and mostly Nitrogen. There are some others in very low concentrations. (You can read more about it in my article, “How is Mars' Atmosphere Different from Earth's”.)

Balloons float for one reason. The gas inside the balloon is less dense than the air surrounding it. This causes the balloon to float. We can achieve a lesser density in two ways. One way is heat up the gas in the balloon. Temperature is really the average kinetic energy of the particles that make up a substance. Therefore warmer particles move faster, colliding with the walls of the balloon harder, pushing the walls out. Since the walls are pushed out, the balloon takes up more volume for the same mass: less dense.

The second way to have a balloon with less density is to have a lower mass for each particle. Avogadro's principle says that at the same temperature and pressure equal volumes of gas have equal particles. If I choose a gas with a lower mass per particle, I can have a less dense gas. The way that we determine the mass of a particle is using the molar mass from the periodic table.

The chemical formula of a substance tells the elements and amounts of each element in a substance. For instance glucose (sugar) has a chemical formula of C6H12O6. That means that there are 6 Carbon atoms, 12 Hydrogen atoms, and 6 Oxygen atoms in each molecule of glucose. We can calculate the molar mass by taking the molar mass of each element, multiplying it by the number of atoms in each molecule, and adding that product for each element together. For glucose we would be concerned with the molar masses of Carbon (12.011 g/mol), Hydrogen (1.0079 g/mol), and Oxygen (15.999 g/mol) can all be found on the periodic table. The molar mass for the compound would be (6 × 12.011) + (12 × 1.0079) + (6 × 15.999) = 180.16 g/mol.

Bromine and ethane are both gaseous at room temperature. Bromine gas has a chemical formula of Br2, which means its molar mass can be calculated to be twice the mass of one mole of monoatomic bromine. The mass is 159.8 g/mol. The molar mass of ethane C2H6, however, is 30.07 g/mol. Since air is made up primarily of Nitrogen, N2, the molar mass of air is close to, but heavier than 28.01 g/mol. Therefore an ethane balloon could sort of float, not high like a Helium balloon, but in a boring way, like a balloon a day after its been blown up. A Boron balloon, far heavier, would fall to the ground somewhere between a feather and a rubber ball. For a great balloon you want something super light, like Hydrogen.

Early scientists like Robert Boyle used gas laws to blaze the trail for chemistry. They were able to use gases often react quicker than liquid or solid because there is more contact between various particles. They were able to notice that if two masses of of different gasses were put together, they would react leaving the same amount of material even if it became something new. This is called the law of conservation of matter. Matter is never created nor destroyed, it just changes form from one thing to the next. So if two moles of Hydrogen gas (H2), 8 g, is added to 1 mole of Oxygen gas (O2), 32 g, the result will be 40 g of water. This has to measured in a form like mass or volume, because the particles are too numerous to count. Different forms of measurement make science possible.

Common Core Science Standards

CCSS.ELA-Literacy.RST.9-10.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.
1. What is the molar mass of Carbon Monoxide gas?
2. What is the molar mass of Carbon Dioxide gas?
3. Which is heavier, Carbon Monoxide or Carbon Dioxide?
4. Water can be produced from a synthesis, chemical joining of hydrogen and oxygen. If 16 g of Oxygen react with 2 g of hydrogen, what is the mass of the balloon's contents before reaction?
5. What will be the mass of the balloon's contents after the reaction takes place?

Science doesn't happen all at once. Even balloons were invented and improved over time. Science Friday had an interview with an author about the history and science of balloons. Read or listen to this article about balloon exploration.
Answer the following questions.

Common Core Science Standards

CCSS.ELA-Literacy.RST.11-12.7 Integrate and evaluate multiple sources of information presented in diverse formats and media (e.g., quantitative data, video, multimedia) in order to address a question or solve a problem.


Complete three of the following four sentence starters for the essay. When your neighbor has finished share your comments with him or her.
Make a Prediction
I predict that . . .
Ask a Question
Why did . . .
Clarify Something
Oh, I get it . . .
Make a Comment
This is good because . . .
Make a Connection
This reminds me of . . .
6. What were the earliest “balloons” made out of?
7. What is the thickness of the Earth's atmosphere?
8. What makes it harder to breath at a higher elevation? (Give a scientific explanation.)

Go to the molar mass digital minilab. See what happens to the balloons when placed in different atmospheres.

Common Core Science Standards

CCSS.ELA-Literacy.RST.11-12.9 Synthesize information from a range of sources (e.g., texts, experiments, simulations) into a coherent understanding of a process, phenomenon, or concept, resolving conflicting information when possible.
9. What is the molar mass of methane gas?
10. Is the molar mass of methane greater than or less than that of chlorine?
11. Which of the materials in balloons has the greatest molar mass?