Flying Tea Rocket
Launching an empty tea bag reveals the power of density differences and convection currents.
The Tea Bag Liftoff is an adaptation of a classic science demonstration called the “Ditto-Paper Rocket.”* Each two-page Ditto master had a sheet of tissue paper separating the two pages. It was this discarded tissue paper that teachers used to make the “rockets.” Since Ditto paper is – thank goodness – a thing of the past, science enthusiasts have found a simpler, albeit smaller, replacement: an empty tea bag.
*Ditto paper was used in schools many years ago (ask your grandparents). It printed purple ink on paper and produced an unforgettable smell from freshly printed copies. Come to find out later, however, that both the Ditto machine solvent and the ink were highly toxic to humans. Apparently, no one seemed to care back then. Go figure.
- Tea bag (full to begin with)
- Nonflammable surface (a dinner plate or shallow pan)
- Matches or lighter
- Safety glasses
- Magnifying glass (optional)
- Adult supervision
Use scissors to cut off the top of the tea bag. You don’t need the string and label or any staples on it but you do need to be able to empty the bag.
Empty the contents of the tea bag into the trash. They probably smell pretty good depending on the tea.
Unfold and straighten the empty bag. Open it so that it looks like a cylinder and is pretty straight. It doesn’t need to be perfectly round but it does have to be opened up. Place it upright on the plate or pan.
Light the top rim of the bag on fire all the way around the edge. As you watch it burn, think about what the heat is doing to the air both around and inside the burning bag.
Let the bag burn completely to ash and watch it liftoff the pan and rapidly rise into the air. You should be able to watch it rise quickly and then descend slowly as it cools. If there are any remains, look at them closely to determine what’s in them.
How Does It Work
There are actually three principles at work on the burning paper cylinder that make this activity happen.
First: There’s the density difference between the air inside the cylinder compared to the air outside of the cylinder. As the flames move down the bag, they heat the air trapped in the cylinder. The heat causes the molecules to move very quickly and to spread out inside and above the cylinder. These molecules are much farther apart than those outside the cylinder. That means the air inside the cylinder is less dense than the air outside the cylinder. Warmer, less dense air rises above cooler, denser air.
Second: The burning bag generates hot, less dense air inside it and above it. This leads to a “thermal” or a convection current of rapidly rising hot air above the flames. The larger volume of space generated by the hot rising air inside the cylinder needs to be filled. The cooler denser air outside of the burning cylinder moves in from the bottom to fill the space under the heated air.
Third: As the bag burns, it becomes both ash and smoke. The hot smoke rises, lifts away, and dissipates into the air. A delicate ash frame of the bag is all that remains but since the ash frame is so lightweight, the force of the rising, hot air is strong enough to quickly lift it upward. As it rises, it cools and then falls back into the dish… if you’re lucky.
Real World Connection
While NASA launch vehicles use rocket propulsion engines to achieve an incredibly powerful vertical “liftoff,” hot air balloons are way more subtle. They use a method similar to your burning tea bag to launch into the cool, morning air. Hot air balloons use a burner to rapidly heat and expand the air inside the balloon. This creates the same air density differences you made with your burning bag. However, in a balloon, there is no mass change like there was in the bag when it turned to ash. That would not be a good thing in a hot air balloon. Instead, the air inside the balloon is heated much hotter than the outside air which creates an envelope of trapped, less dense air within the denser morning air. As a result, the balloon lifts off the ground and rises.
Check out our Solar Bags to see first hand the power of hot, rising air on a large, very visible scale. Two types of the Solar Bag expand to either 25 or 50 feet (7.6 to 15 m) when inflated. Both harness solar energy to expand and then float into the air. It’s truly a sight to behold as a giant, black “solar sausage” slowly lifts off of the ground powered by nothing but sunshine and controlled by you!