Boiling Acetone at Reduced Pressure

Acetone

Note to Teachers: Do the following demonstrations before asking students to read the text and answer the questions. Have them work in groups to answer the questions. First, boil some water in an open beaker on an electric heater and show how the temperature of the water remains constant due to the fact that the phase change of boiling is consuming heat. Second, boil acetone in a vacuum flask using an aspirator. Include some boiling stones to aid the boiling process. Keep all sources of ignition far from the acetone, especially when pouring it between containers. The temperature of the boiling acetone drops (and can drop as low as 10°C if the seal is tight enough). This happens because the internal energy of the acetone is used to drive the boiling process.

There are two ways to bring a liquid to a boil. First, put the liquid in a pot and set it on the stove. This works well with water to make tea or cook pasta. The added heat from the stove gives the water a high enough temperature to begin to boil and the continued addition of heat keeps the boiling going. The second way to bring a liquid to a boil does not require an input of heat at all. Simply place the liquid in a sealed container and connect the container to a vacuum pump to reduce the air pressure inside. When the pressure gets low enough, the liquid comes to a boil, no stove required.

This second way of bringing a liquid to a boil may not be familiar to most people. The kind of equipment needed for this experiment is not found in most kitchens. However, recipes and packaged foods are likely to be found in a kitchen and some of these include additional instructions for people who are cooking at high elevation. In Sante Fe, New Mexico (elev. 7,000 ft) the atmospheric pressure is about 0.8 atmospheres. At this pressure water boils at 93°C (199°F). La Paz, Bolivia (the highest capital city in the world) has an elevation of 11,980 ft (air pressure of 0.7 atm) and the boiling point of water there is 88°C (190°F). The higher you go, the lower the boiling point. This has to do with air pressure. At higher elevation, the atmospheric pressure is lower. At lower pressure, water boils at a lower temperature. Boiling is a phenomenon that depends not just on temperature but also on air pressure. In order for bubbles of vapor (molecules of the liquid in the gas phase) to form inside the liquid the gas pressure inside the bubbles must equal the external pressure. If the external pressure is higher then bubbles can’t form until the liquid is at a higher temperature. If the external pressure is lower then bubbles will form when the liquid is at a lower temperature. Water can even boil at room temperature if the pressure is low enough. At 17.5 torr (0.0231 atm) water boils at 20°C, which is room temperature.

In order to really understand what’s going on when a liquid boils a good understanding of heat and temperature is required. Molecules and atoms are in constant, random motion. Temperature is a measure of the average kinetic energy of molecules, their energy of motion. At low temperature molecules have a lower average kinetic energy, which means they have a lower average speed. If no phase changes or chemical reactions are going on then when heat is added to a material, its molecules will begin to move faster. Although the molecules are too small to see, it is possible to measure the temperature and the temperature rises as heat is added. There is heat in everything that has a temperature above absolute zero. For example, when an object with a higher temperature is placed in contact with an object with a lower temperature heat moves from the higher-temperature object to the lower-temperature object. In order to have heat, it is not necessary to burn some fuel or turn on a stove.

Although all changes in temperature involve heat moving into or out of a substance, not all transfers of heat cause the temperature to change. For example, when the water in the pot on the stove is boiling it has a constant temperature of 100°C (212°F) despite the fact that the burner underneath it is hundreds of degrees hotter. The burner provides heat to the water in the pot but the temperature of the water does not rise. Why should this be? The temperature of boiling water is constant because the heat that is added to the water is consumed in driving the phase change from liquid to gas. Instead of raising the temperature of the water, the heat breaks the bonds holding one water molecule in contact with its neighbors. All molecules experience a certain amount of stickiness (or intermolecular forces of attraction). This stickiness requires energy to overcome. Just as it takes a small effort to pull two magnets apart when they are stuck together, it takes a small amount of heat energy to pull two water molecules apart. The heat provided by the stove breaks the bonds holding water molecules together and these molecules then escape in the vapor phase, taking their additional energy with them. Heat not only gets used up breaking bonds but the energy is also carried away by the hot steam. Boiling, and melting, too, for that matter, require heat and consume it without allowing the heat to raise the temperature.

When sufficient heat is added to a boiling liquid its temperature remains constant. This raises an interesting question: when a liquid boils because of reduced pressure, what happens to its temperature? In the demonstration air pressure is reduced in a flask containing acetone. The acetone comes to a boil due to the reduced pressure but no external heat source is supplied. The temperature of the acetone drops—it drops further the longer it boils. Boiling requires heat and the heat has to come from somewhere. In this case, the heat is coming from the acetone itself. As the heat in its own molecules is used up in the transition from the liquid to the vapor state the temperature drops. In order for a molecule to escape into the vapor phase it must move fast enough to break the bonds to its neighbors. If the neighbors shove it hard enough then a molecule can join up with others in the vapor phase and make a bubble. But the neighbors of the now-escaped molecule gave up some of their kinetic energy when they shoved it away. A lower kinetic energy means they are moving more slowly. When molecules move more slowly we can detect that as a drop in temperature. In a way, this is like Dorothy in the Wizard of Oz. She had the ruby slippers on the entire time and could have gone home at any moment. The flask of acetone has the heat that it needs to boil within its own molecular motions. Acetone can be made to boil at 20°C with a pressure of 108 torr (0.141 atm). The normal boiling point of acetone (at 1 atm) is 56.5°C. As the acetone boils, its temperature can be reduced to about 10°C.

1. Endothermic changes consume heat energy. Exothermic changes give off heat energy. Classify the phase changes below as endothermic or exothermic. For each one, write one sentence that justifies your answer. Consider the formation or breaking of bonds between molecules in your answer.
   1. solid to liquid
   2. liquid to gas
   3. gas to liquid
   4. liquid to solid
2. When water is boiled on a stove the temperature remains constant at around 100°C. This is despite the fact that the electric burner is constantly adding heat to the water. Why does the temperature remain constant?
3. What happened in the demonstration when the flask of acetone was hooked up to the aspirator and its internal pressure was reduced? List all relevant observations.
4. When the temperature of a system drops during a physical or chemical change is heat being consumed or produced? Explain.
5. Prior to seeing this demonstration, what would you have said about boiling about whether it was exothermic or endothermic? Why? Have you changed your mind? Explain.
6. Heat is a form of energy and energy cannot be created or destroyed. Where does heat energy go when it enters a system and the system’s temperature stays constant or when it drops?
7. Does freezing water give off heat or consume it? Justify your answer in terms of molecular motion.

This demonstration is based on demonstration 1.1 on pg. 5 of Chemical Demonstrations, Vol. 1 by Bassam Z. Shakhashiri.

I made a video of this demonstration a few years ago. View it here: https://youtu.be/BxB2uVtL4rU.