Directions: Read the material and follow instructions in the Feature. Then answer the questions. SCIENCE Cool Vapors Remember going swimming last summer? liquid as by heating it, the molecules overcome While you were in the water, it felt great. When these attractive forces and move out of the liquid you got out, it probably was a little cool, espe- state to form the gas state. cially if it was windy. As water evaporates from The refrigerant in a vehicle's air conditioning your skin, it changes into a gas. When a liquid system follows the same principles. If the refrigevaporates to become a gas, heat is absorbed. erant has a hilgh heat of vaporization, it absorbs Therefore, the heat is absorbed from your body, more heat from its surroundings as it changes making you feel cool. from the liquid state into a gas. In selecting refrig- The amount of heat absorbed depends on erants, the boiling point (bp) is also important varying factors. One of these is a physical prop- because the lower the bp, the lower the tempererty of a substance called the heat of vaporization. ature at which heat absorption can occur. At its This is the amount of heat absorbed by a sub- bp of 212F[100C], water absorbs 970Btu stance as it changes from a liquid to a gas without (British thermal units) per pound. The refrigerant changing its temperature. In a chemical sense, R12 absorbs 158Btu at its bp of 21.6F the process of boiling and evaporation are the [29.8C], while the newer refrigerant R134a same. The molecules of a liquid are closer absorbs 210Btu at its bp of 15.7F[26.5C]. together due to attractive forces between the The following experiment will show you how the molecules. If as much energy is added to the heat of vaporization affects cooling. Apply it! Producing the Heat of Vaporization Meet MatEF Science Standards for demonstrating an understanding of the effect of heat in automotive systems, and explaining the three states of matter. \begin{tabular}{|l|} \hline Materials and Equipment \\ - Water \\ - Rubbing alcohol \\ - Cotton balls \end{tabular} (1) Place some water on a cotton ball and rub it (4) Would it be possible to pour liquid R134a across your left forearm. out on a workbench in your lab? Explain (2) Place some rubbing alcohol on a cotton ball your answer. and rub it across your right forearm. (5) From what you know about the boiling Results and Analysis process, how could you raise the bp of (1) Did one substance feel cooler than the system? other? (6) Where does the heat come from that (2) Which liquid do you think had a lower heat changes the refrigerant from a liquid to a of vaporization? gas? (3) Explain how a liquid like R134a can boil at (7) In what component of the air conditioning a temperature below 0F. system does this change occur? 1. Why has the refrigerant R134a replaced R-12 in automotive air conditioning systems? 2. In theory, water could be a refrigerant since it absorbs 970 Btu per pound when it changes from water to steam. Why isn't water a good choice? 3. What is the heat of vaporization? 4. Why is the boiling point important in the selection of a refrigerant? 5. What benefit does a refrigerant gain by having a high heat of vaporization
