Phase Change Graphs Heating and Cooling H2O

PHASE CHANGE GRAPHS Heating and Cooling H O 2 Use the following diagram to answer questions 1-9. 1. Is heat being added or removed from this substance? Explain how you determined this. a. Heat is added because the temperature of the material is rising (from left to right along the x-axis) as time goes by, and the material melts and freezes. 2. There are two flat sections in the above graph. What is happening to the temperature during these two time periods? Why is this happening even though a heat source is constantly present? a. When something melts or boils (vaporization), the temperature doesn't change. When a phase changes, the temperature doesn't change because the extra heat energy has to be used to break the bonds that hold the water molecules together. b. Remember that temperature is a measure of how fast particles are moving; when temperature goes up, it means that particles are moving faster. When heat is added, it speeds up the particles at a, c, and e. The particles at b and d don't use heat to move faster. It is moved to separate the particles so that a phase change can happen. 3. What is happening to the substance during segments a, c, and e? a. Heat is being added, which makes the particles move faster. This is shown by a rise in temperature. 4. In what state of matter is the substance during segment a? a. Solid 5. Which segment represents the point where a liquid is turning into a gas? What is this process called? a. "d" is the best answer because this is the point at which the substance starts to boil. b. It's possible to say that "c" includes some evaporation, so that could be an answer, but "d" is the best choice. 6. Which segment represents a time when the substance is all in the liquid phase? c. “c” 7. Which segment represents the point at which a solid is turning into a liquid? What is this process called? a. "b"—that's the melting point, and the process is called melting. 8. In what state of matter is the substance during segment e? a. gas 9. Imagine this diagram represented the changes that H O molecules went through over time. What temperatures would you expect segment b and segment d to be associated with? 2 a. As well as 0 degrees Celsius and 100 degrees Celsius. Use the following diagram to answer questions 10-18. 10. Is heat being added or removed from this substance? Explain how you determined this. a. Heat is leaving the system (exothermic) because the temperature is going down over time. 11. What is the substance doing between minutes 1 and 3? a. As the substance cools, it changes from a gas to a liquid. 12. What state of matter is the substance in at the end of segment d (just before the beginning of segment e)? a. Part "d" is where the freezing point is. The best answer is "solid," since the material is likely to have frozen by the end of segment "d." 13. What is happening during segment a, c, and e? a. The temperature is going down because the particles are losing speed and heat is leaving the system. This process gives off heat. 14. In what state of matter is the substance during segment a? c? e? a. “a” = gas; “c” = liquid; “e” = solid 15. Which segment represents the point during which the gas is turning into a liquid? What is this process called? a. “b”--the process is called condensation 16. Which segment represents the point during which the liquid is turning into a solid? What is this process called? a. “d”--the process is called freezing 17. How long did it take for the substance to condense completely? (From the beginning of the condensation process to the end – don’t start w/ minute 0!). a. 2 minutes: t = 3min minus t = 1 min 18. Does this graph represent a cooling curve for water or for a different substance? How do you know? a. It has to be something other than water. The changes in phase don't happen at the same temperatures as water.