Thermal Energy/Heat and Phase Changes

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thermal energy

Key Ideas from the Lessons:


  • Thermal energy is the total kinetic energy (energy of motion) of all the particles in an object.
  • Heated molecules have a lot of kinetic energy and thus move a lot while cold molecules do not have as much thermal energy and move very little.
  • When the motion of atoms or molecules increases, so does the temperature. Therefore, as matter in any particular state is heated, stirred, shaken, etc., the average speed of its atoms/molecules increases, which is reflected in an increase in its temperature.
  • As the temperature of a substance increases, the average distance between the atoms/molecules of the substance typically increases, causing the substance to expand.
  • As the temperature of a substance decreases, the average distance between the atoms/molecules typically decreases, causing the substance to contract.
  • When the temperature of a liquid decreases, the average speed of the atoms or molecules decreases and, as a result, the pull that exists between the atoms or molecules is strong enough to link them together as a solid.
  • When the temperature of a solid increases, the average speed of the atoms or molecules increases and the pull between the atoms or molecules is no longer strong enough to hold them together as a solid; the atoms or molecules are now more loosely connected as a liquid.
  • When the temperature of a gas decreases, the average speed of the atoms or molecules decreases and, as a result, the pull that exists between the atoms or molecules is strong enough to loosely connect them together as a liquid.
  • When the temperature of a liquid increases, the average speed of the atoms or molecules increases and the pull between the atoms or molecules is no longer strong enough to hold them together as a liquid. In this case, the pull between atoms or molecules is so weak that they are no longer connected to each other, but rather they exist as a gas.
  • At the boiling point and freezing point, atoms or molecules from anywhere in the substance can enter the gas state and solid state, respectively. Because of this, the bubbles that form when the substance is boiling are atoms or molecules of that substance in the gaseous state.
  • Evaporation or condensation can also occur independent of temperature; i.e., at any temperature there are some atoms or molecules that may move from one state to another at the surface of a substance. This also includes atoms or molecules on the surface of a solid that can enter the gas state.
  • A substance is made up of the same type of atom or molecule regardless of whether it is in the solid, liquid, or gas state. There is no change in the identity of the atoms or molecules during a change of state; only the arrangement, motion, and interaction of the atoms or molecules change.
  • Atoms or molecules are not destroyed during a change of state.
  • Any change of state is reversible.
  • Heat describes the transfer of thermal energy from a warmer object to a colder one.
  • Coldness is not transferred, warmer things transfer heat to colder ones, and as a result they get cold.
Lessons’ Summary

Lesson 1: Warm up:  What is heat? What does it mean to be cold? What does it mean to be hot? Discuss and accept all answers. Students will explore what they think it means to be hot and cold at the molecular level by completing the Hot colors lab. Students observe and describe what happens when food coloring is added to hot, cold, and room temperature water. They should notice that the it moves slowest in the cold and fastest in the hot. Ask them to discuss with their partner if that changes their original idea of “hot” and “cold”. Discuss as a class their responses. Then, write thermal energy on the board (the students write in their notebooks). Make a “hot” and “cold” spectrum. It will eventually look like this:

thermal energy spectrum


Either as a whole group, small groups, or individually depending on technology available, student explore this concept and how it relates to phase change with this PhET simulation. I use an assignment that is in the teacher resources. Finish by watching Study Jams: States of Matter Although I do not like how they interchange the terms “heat” and “thermal energy” so I make sure to emphasize the differences during the video. And the narrator says the molecules stop moving around when freezing happens–point out that mistake as well.

Lesson 2: Heat is not thermal energy. Stress that heat is the transfer (movement) of this energy and it always moves from more energy to less energy until there is equilibrium (explain that term). I explain this as me holding 10 m&m’s and a student holding 5.  In order for us to have equal m&m’s, who has to lose m&m’s and who has to gain? I show this video on thermal equilibrium. Pass out the Ice-Cold Lemonade assessment probe found in Uncovering Student Ideas in Science, Volume 2: 25 More Formative Assessment Probes. In this probe, students have to decide if the lemonade is getting cold because the coldness of the ice moved into the lemonade; the heat from the lemonade moved into the ice; or the coldness and the heat moved back and forth until the lemonade cooled off. I modify this probe by directing students to draw a model of what is happening at the molecular level and to use arrows to show the transfer of energy.

Usually the students need some more examples of heat transfer and why things feel hot or feel cold. I want them to understand that heat is flow of energy. If something is hot it means it gained thermal energy from something else and that “something else” had to lose energy. So, I give different scenarios and each time they model why something became hot or cold using arrows to show the flow of energy (heat).

Example:  Did you ever put a metal spoon hot chocolate and then touch the spoon to your mouth? What do you think is happening between the molecules in the hot chocolate and the molecules in the spoon and then your mouth?

Example: Tape a thermometer to a metal board and another one on a wooden board (or piece of cardboard). Let them sit there for awhile to ensure that they are at room temperature. Read the temperatures out to the students  (digital thermometers are the best for this demonstrations). The temperatures should be the same. Then go around and let the students touch the metal and the wood. Ask them which one feels colder? Why don’t they feel the same temperature? As you are letting students touch the materials, the temperature will stop being the same. The metal will most likely have a higher temperature than the wood because it is a conductor but it will still feel colder than wood. Have students model what is going on. Introduce the terms conductors and insulators. Some materials allow energy to transfer easier than others. Your hand feels colder on the the metal because more of your energy is transferring to the metal. Losing energy–>colder. How do you know this is happening? The metal is gaining energy. You can prove that by seeing how the temperature is rising.

Example: More ideas in this link:


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