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Understanding Water Phase Changes Through Heat Addition

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kara
16 diapositives · 313 Views · Oct 23, 2014

This article explains the water phase change graph and how temperature changes as heat energy is added to ice at -50°C. It illustrates what happens during different phases as energy is constantly input, leading to warming of ice, melting to water, and eventually vaporization. Key concepts such as specific heat capacity differences between ice and water are covered, emphasizing how this affects temperature changes during heating and phase transitions. The process culminates in understanding the energy dynamics involved in heating, melting, and evaporating water.

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  1. Understanding THE Water Phase Change Graph From RegentsEarth.com

  2. The graph below shows what happens when heat energy (Joules) is added to a block of ice at -50oC. Keep in mind that because energy is added at a constant rate (J/min) more and more energy is being pumped into the system as time goes by. You should also realize that although water freezes at 0oC, it doesn’t have to stay at that temperature. Ice can be chilled to any temperature below the freezing point. Our ice will begin at -50oC. 150oC 100oC Temp oC 0oC -50oC Time (minutes)

  3. As we add heat energy the first thing that happens is that the ice gets warmer and warmer. Not very surprising. Look carefully at what just happened. We started at A with solid ice at -50oC and we got to B where we still have solid ice which is now at 0oC. From point A to point B the only thing that happened was that ice got warmer. 150oC 100oC Temp oC B 0oC -50oC A Time (minutes)

  4. Now something very strange is about to happen! We are still adding heat energy at the same rate as before yet the temperature has stopped increasing! How could that be? What is the energy doing if it’s not warming the ice? 150oC 100oC Temp oC B C 0oC -50oC A Time (minutes)

  5. According to “Properties of Water” (ESRT page 1) it takes the addition of 334 joules of heat energy per gram to melt ice. All of the energy added from B to C is going to melt the ice. The temperature will not increase again until all the ice has melted! 150oC Whenever the slope of the line is flat it means a phase change is occurring! 100oC Temp oC B C 0oC -50oC A Time (minutes)

  6. After all the ice has melted and we have nothing but liquid water at 0oC the temperature will start to increase once again. But something is different! The slope of the line has changed! Why is the slope of the line from A to B so much steeper than the slope from C to D? Slope = rate! If the slope is steeper from A to B it means that the ice was warming faster from A to B than the water was from C to D. But why? 150oC D 100oC Temp oC B C 0oC -50oC A Time (minutes)

  7. It all has to do with Specific Heat, the energy needed to change the temperature of 1g of a substance by 1oC. (See ESRT page 1) The S.H. of ice is only 2.11 J/g•oC. But the S.H. of water is 4.18 J/g•oC Since the S.H. of water is so much higher, more energy is needed to warm the water than to warm the ice. 150oC D 100oC Temp oC B C 0oC -50oC A Time (minutes)

  8. Since we are always adding the same amount of heat energy (Joules) per minute, it’s going to take much longer to heat the water than to heat the ice. That why the slope of the line changes. Because water has a high Specific Heat it heats up VERY SLOWLY. More slowly than almost any other common substance! 150oC D 100oC Temp oC B C 0oC -50oC A Time (minutes)

  9. More strangeness! Once again the line goes flat. We are still adding energy but there is no temperature increase. Can you guess why? RIGHT! It’s another phase change. This time the change is from liquid water to water vapor, a gas. This process is called vaporization or evaporation. Either term is acceptable. 150oC E D 100oC Temp oC B C 0oC -50oC A Time (minutes)

  10. How much energy does it take to evaporate water? Once again look at the “Properties of Water” from ESRT page 1. It takes a whopping 2260 Joules of heat energy for each gram of water you want to evaporate! 150oC E D 100oC Temp oC B C 0oC -50oC A Time (minutes)

  11. Finally, after all the water has evaporated and turned into vapor, the temperature will once again begin to rise. Can you guess why the slope of the line is as steep as it was when ice was warming? Right! The specific heat of water vapor is small so it heats rapidly. F 150oC D E 100oC Temp oC B C 0oC -50oC A Time (minutes)

  12. Just a few more important things you need to know: As you go UP the line from A to B to C to D to E and finally to F, energy is being added. Where is that energy? The energy is in the H2O. It’s stored there as potential energy. The H2O has the least potential energy at A and the most at F. F 150oC D E 100oC Temp oC B C 0oC -50oC A Energy Being Added Time (minutes)

  13. But if the energy is stored doesn’t that mean we can get it back? Sure! As we go down the line from F towards A the same energy we put into the system is released into the environment as the H2O........ Cools from F to E. Condenses back into a liquid from E to D. Cools from 100oC back down to 0oC. Freezes, becoming a solid between C and B. And finally, chilling from 0oC ice at point B to -50oC ice at point A. The energy we put in is the same amount we get out. F 150oC D E 100oC Temp oC B C 0oC -50oC A Energy Being Released Time (minutes)

  14. Try this. For each description, you give the letter where it’s found. That’s letter D. 1) Liquid water at 100oC. 2) Solid ice at 0oC. Did you say letter B? 3) Water vapor at 100oC. That’s letter E. 4) How about liquid water at 0oc. Definitely letter C. OK, now let’s try something else. F 150oC D E 100oC Temp oC B C 0oC -50oC A Time (minutes)

  15. You provide the name of the process that occurs between the two letters given. Pay attention to the order of the letters! What’s going on between letters B and C? Ice is melting! How about between E and D? Water vapor is condensing! Between A and B? Ice is warming. What’s happening between C and B? Water is freezing! And lastly, between D and E. Liquid water is evaporating (vaporizing). F 150oC D E 100oC Temp oC B C 0oC -50oC A Time (minutes)

  16. A few more questions that get asked over and over again. Bye! Between which 2 letters is the most energy being added? Between D and E How can you tell where a phase change is occurring? The line is flat! When energy is released, where does it go? It enters the environment and warms the air immediately surrounding the H2O. F 150oC D E 100oC Temp oC B C 0oC -50oC A Time (minutes)