In this demonstration, students will witness the relationship between temperature and volume as well as temperature and pressure. This is a great substitute for the imploding soda can demonstration that doesn’t always work.
Middle School, High School
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
- MS-PS3-5: Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
- Scientific and Engineering Practices:
- Developing and Using Models
- Developing and Using Models
By the end of this demonstration, students should be able to:
- Understand that temperature and volume are directly related.
- Understand that temperature and pressure are directly related.
This demonstration supports students’ understanding of:
- Gas laws
Teacher Preparation: 15 minutes
Lesson: 30 minutes
- 200-mL Erlenmeyer flask
- 12-inch balloon
- Hot plate
- Tongs or heat resistant gloves
- Always wear goggles when working in a lab setting.
- Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
- A fun way to build interest around this activity is to have an example of the balloon in the flask sitting on your desk at the beginning of a unit on gases or states of matter. Students may ask how the balloon got in there, or you could point it out to them and ask them to think about it, letting them know that you will be working on it later in the unit when you do this demo. Or, you could do the demo at the beginning of the unit and revisit it a second time with the student handout after you have taught students about the relevant concepts (gases having changeable volumes depending on temperature and pressure, liquids having a fixed, much smaller, volume).
- This activity is designed as a demonstration, but it could be done by students in groups of two.
- Be sure to pre-stretch the balloon by blowing it up and letting the air out a few times before putting it on the flask. If it is not pre-stretched, it may rip, especially if you cool the flask rapidly with cold water (see notes below).
- I discuss the procedure with the class and ask them to predict what will happen to the balloon and support their answer with a reason. This usually leads to an interesting discussion, which exposes students to misconceptions. The prediction and discussion result in the students being more interested in the final result.
- Oftentimes, students will say a vacuum is created inside the flask which sucks the balloon inside. This is a good opportunity to discuss what a vacuum is – in a very simple definition, a vacuum is a volume of space that contains very few or no atoms. It is also a good chance to explain that the balloon is not so much “sucked in” as it is “pushed in” – the air outside the flask is at a much higher pressure than the gas particles inside the flask, since most of them have condensed to the liquid phase as the flask cools.
- For a more dramatic version of the demo, after the students have observed the balloon beginning to be pushed into the flask by external air pressure, finish the process quickly by rapidly cooling the flask in a cold water bath or under a cool stream of water from the faucet. There is a video version of this you could use as well. Since the cooling and condensing of particles from the gas phase into the liquid phase happens more rapidly, the balloon is quickly and dramatically pushed into the flask. If you do this, hold the top of the balloon directly above the opening of the flask when you put it under cold water so the balloon is not pulled at an angle/over the edge of the flask, or it could tear and you would need to start over.
- Always use borosilicate glass flasks for this demonstration if you are going to put it in cold water to rapidly get the balloon into the flask. The sudden change in temperature can cause other kinds of glass to shatter. Never use a flask (of any kind) that is cracked or damaged in any way.
- This is a much more reliable way to demonstrate temperature and pressure relationships than a soda can in water.
- As an extension, you could try reversing the process by putting the flask in a hot water bath to re-boil the water that condensed as it cooled. As the water molecules return to the gas phase, the pressure inside the flask would increase again and the particles would push the balloon back out of the flask. (There is a possibility that the balloon will tear during this re-heating process.)
- A more reversible (but less dramatic) way of doing this activity is with a water bottle instead of a flask and using a hot water bath and cold water bath instead of boiling it. Start with an empty water bottle with a balloon over the top. Put it in the freezer or refrigerator for several hours. Take it out at the beginning of a lesson and show it to students. Then you can use a hot water bath and a cold water bath to increase or decrease the volume of air in the balloon. It won’t get pushed into the bottle like it does with the flask, but it demonstrates the same principle that gases (this time air, rather than water vapor) take up more space when they are warmer than when they are colder. See pictures below for examples.
For the Student
- What happens to gas particles when energy is added to them? When energy is removed?
- Read the procedure and predict what will happen. Justify your prediction.
- Pour about 25 mL of water into the flask.
- Heat the water on a hot plate until it boils. Allow it to boil for about two more minutes. Turn off the hot plate. There should still be water in the flask.
- Using tongs or heat resistant gloves to protect your hands, remove the flask from the hot plate and pour the water out.
- Immediately place a balloon over the mouth of the flask.
- Place the flask on the table and record observations.
Record your detailed observations at each stage of the demo:
Before the water is heated
When the water is boiling
When the balloon is added, after the water boils
When the flask is cooled
- What is in the flask when the balloon is placed over the mouth of the flask?
- What is happening to the contents of the flask as they cool? What does this do to the balloon?
- Based on your answer to the previous question, what effect does temperature have on the volume of a gas?
- What causes the balloon to go inside the flask? (Hint: Think about pressure!)
- What would happen if the flask had been filled with normal air, rather than water vapor, and that air had been heated and cooled? Would the balloon have gone into the flask? Explain.
- Using arrows, sketch the molecules inside and outside of the flask that make up the water vapor and air during four points in this experiment (see below). Use one color for air particles and another for water vapor and include a key. Add the balloon and what it looks like, when appropriate. Use arrows to indicate particles, and longer arrows to indicate faster moving molecules.
What did you learn from this experiment about temperature, pressure, and volume of gases?