Heat and Phase Change

Transcription

1 Heat and Phase Change In the last lesson we looked at heat and how heat can be moved from one object to another. In this lesson, we are going to explore how, even though heat can move from one object to another, it doesn t necessarily mean that the temperature of the objects will change. You may ask, What? Heat can move from one object to another without temperature changing one little bit?!?! Yea...confusing huh? In this lesson we re going to take a look at one of the ways heat can move while the thermometer doesn t. Changing Phases When things change phase (change from solid to liquid or liquid to gas or...well, you get the picture) the temperature of those objects don t change. If you were able to take the temperature of water as it changed from a solid (ice) to a liquid you would notice that the temperature of that piece of ice will stay at about 32 F until that piece of ice was completely melted. The temperature would not increase at all. Even if that ice was in an oven, the temperature would stay the same. Once all the solid ice had disappeared, then you would see the temperature of the puddle of water increase. (By the way, as the ice is melting, from where is heat being transfered? Heat is being transfered, by conduction, from the air.) Experiment 1 On The Plateau Caution: This experiment involves boiling water. An adult needs to be involved. You need: Thermal Energy: Heat and Phase Change 1

2 A thermometer that can reach over 212 F or 100 C. A pot (the smaller the better) Water Timer or watch 1. Put a cup or so of water into the pot. 2. Put the pot on the stove. 3. Turn on the stove. 4. Take the temperature of the water every minute. 5. Once the water begins to boil continue taking the temperature once a minute for five minutes. (In other words, five more times) 6. If you wish, you may want to record the temperatures. Feel free to make a line graph of the results you see. So what s going on? Why doesn t the temperature change as the water is boiling? Think about what s happening as things change. The relationship between the molecules in the objects are changing. In a solid to a liquid change, the molecules go from being held very tightly to one another to being able to slide and wiggle and jiggle around one another. The bonds between the molecules change. It takes energy for those bonds to change. Where does that energy come from? Heat! So as heat continues to flow into or out of the object, as long as the bonds are changing, the temperature is not. This state of constant temperature as the phases are changing is called a temperature plateau. In our experiment, the water went from liquid to gas. As it did, you noticed that the temperature hit a plateau at about 212 F. What else was happening at 212 F? The water was boiling, right? Changing phase from a liquid to a gas. The bonds between the water molecules were breaking apart and this takes energy. So, all the heat that was being transfered into the water through conduction and convection was used by the molecules to break the Thermal Energy: Heat and Phase Change 2

3 bonds and change state. In essence, 212 F is liquid water s speed limit. The liquid molecules can t go any faster then that without breaking apart and becoming a gas. On the flip side, water molecules can t go any slower then 32 F without becoming a solid. As long as the bonds are changing, the temperature (the speed of molecules) won t change. Freezing points, melting points, boiling points and condensation points are the speed limits of the phases. Once the molecules reach that speed they must change state. Experiment 2 Heat Sucker You need Rubbing alcohol Skin (your arm will do fine) 1. Put a small amount of rubbing alcohol on your arm. 2. Wait How does your arm feel? Warm or cold? What happened there? Well, alcohol is a liquid with a fairly low boiling point. In other words, it goes from liquid to gas at a fairly low temperature. The heat from your body is more then enough to make the alcohol evaporate. As the alcohol went from liquid to gas it sucked heat out of your body. For things to evaporate, they must suck in heat from their surroundings to change state. As the alcohol evaporated you felt cold where the alcohol was. This is because the alcohol was sucking the heat energy out of that part of your body (heat was being transfered by conduction) and causing that part of your body to decrease in temperature. As things Thermal Energy: Heat and Phase Change 3

4 condense (go from gas to liquid state) the opposite happens. Things release heat as they change to a liquid state. The water gas that condenses on your mirror actually increases the temperature of that mirror. This is why steam can be quite dangerous. Not only is it hot to begin with, but if it condenses on your skin it releases even more heat which can give you severe burns. Objects absorb heat when they melt and evaporate/boil. Objects release heat when they freeze and condense. Do you remember in the heat lesson, when I said that heat and temperature are two different things? Well, I hope this lesson makes that even more clear. Heat is energy. It is thermal energy. It can be transferred from one object to another by conduction, convection, and radiation. In the next lesson, we heat things up even more as we explore heat capacity and specific heat. Thermal Energy: Heat and Phase Change 4

5 In a Nutshell -Heat is movement of thermal energy from one object to another. -When an object absorbs heat it does not necessarily change temperature. -As objects change state they do not change temperature. -The heat that goes into something as it s changing phases is used to change the bonds between molecules. - Objects release heat as they freeze and condense. -Objects absorb heat as they evaporate and melt. -Freezing points, melting points, boiling points and condensation points are the speed limits of the phases. Once the molecules reach that speed they must change state. Thermal Energy: Heat and Phase Change 5

6 Did You Get It 1. An object s temperature always drops when it loses heat. True or false? 2. What happens to molecules as they change from one state to another? 3. When objects evaporate do they absorb heat or release heat? 4. Why do we sweat when we re hot? 5. Why doesn t temperature change when things are changing state? Answers 1. False. 2. The bonds between molecules change. They can either tighten up or loosen up depending on whether the energy is increasing or decreasing. 3. They absorb heat. 4. The sweat absorbs excess heat from the body as it evaporates and cools us off. 5. The energy that s entering the object is being used to change the bonds between the molecules. The molecules have reached their speed limit. They cant go any faster or slower without changing state. Thermal Energy: Heat and Phase Change 6