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Prior Knowledge Questions
- A family from Minnesota turns off the heat and flies to Florida for a winter holiday. When they come home, all of their water pipes have burst. What do you think happened?
Answer: the water in the pipe froze and made them burst
- Spaghetti takes about 9 minutes to cook at sea level, but about 14 minutes in the mountains. Why do you think this is so?
Answer: boiling point of water is lower there which makes the water cooler so makes the pasta cook slower
- In which phase(s) are the molecules held rigidly together?
Answer: together, solid
- In which phase(s) do the molecules move freely?
Answer: gas, liquid
- In which phase(s) are the molecules held in a defined shape?
Answer: solid, liquid
- In which phase(s) do the molecules take the shape of their container?
Question: How is temperature related to phase changes?
- Predict: Based on your prior knowledge, predict the following:
A. At what temperature will water change from a liquid to a solid (freeze)? Answer: 0
B. At what temperature will water change from a solid to a liquid (melt)? Answer: 0
C. At what temperature will water change from a liquid to a gas (boil)? Answer: 100
- Investigate: Use the Gizmo to explore phase changes. Use the Add/remove heat energy slider to control the water temperature. Record your observations in your notes, then answer the questions below:
A. At what temperature does water freeze? Answer: 0 This is the freezing point.
B. At what temperature does ice melt? Answer: 0 This is the melting point.
C. At what temperature does water boil? Answer: 10 This is the boiling point.
- Observe: Set up the Gizmo to observe freezing. What do you notice about the temperature while the water is in the process of freezing?
Answer: stays the same
- Explore: Use the Gizmo to investigate melting and boiling. Does the temperature change while either of these phase changes is occurring?
Answer: no, it doesn’t
- Interpret: Select the GRAPH tab to see a graph of temperature vs. time. Click the “–” button until the whole graph is visible. What does the graph look like during a phase change?
Answer: line is horizontal
- Extend your thinking: Why do you think the temperature does not change much during a phase change? If possible, discuss your answer with your classmates and teacher.
Answer: the energy is being used to change the state of matter
Questions: Why do phase changes occur?
- Compare: Set the Water temperature to 0 °C and click Play. Observe the water molecules. Click Reset, set the Water temperature to 100 °C, and click Play again. What do you notice?
Answer: there is no difference so nothing
- Observe: Click Reset. The mean molecular speed of the water molecules is displayed below the container. Set the Water temperature to 0 °C and Add/remove heat energy to 400 J/s. Click Play.
A. How does the mean speed of the water molecules change as they are heated?
Answer: increases the hotter water gets
B. Does the mean molecular speed change as much as the temperature as the water heats up? Explain.
Answer: Yes, because the more temperature increases, the mean molecule speed changes
- Explain: How is temperature related to the motions of molecules?
Answer: higher the temperature, the faster the molecules are
- Observe: Click Reset. Set the Water temperature to 20 °C and the Ice volume to 50 cm3. Set Add/remove heat energy to 0 J/s. Click Play. How do the molecules in the liquid interact with the molecules in the solid?
Answer: the molecules in the liquid crash into the molecules of the solid causing the bonds to break
- Observe: Click Reset. Set the Water temperature to 100 °C and the Ice volume to 50 cm3. Click Play. How does this situation compare to the previous one?
Answer: the ice melts faster
- Propose a theory: Based on what you have observed, explain why you think phase changes occur. If possible, discuss your theory with your classmates and teacher.
Answer: It happens because the amount of kinetic energy they have within them exceeds what their bond can hold causing them to change forms
- Apply: Use your theory to explain what happens at the molecular level in each of the following situations. Also, list the temperature at which each transition occurs.
A. Ice is warmed to the melting point.
Answer: Atoms are fast enough to break bonds and turn into liquid. Temperature: 0 C
B. Water is warmed to the boiling point.
Answer: Water is warmed to the boiling point. The atoms go fast enough and are hot enough to become vapor. Temperature: 100 C
C. Water is cooled to the freezing point.
Answer: Water is cooled to the freezing point. The atoms slow down till they are stable enough to start forming bonds. Temperature: 0 C
- Extend your thinking: Click Reset. Set the Water temperature to 0 °C, the Ice volume to0 cm3, and Add/remove heat energy to -400 J/s. Click Play and wait until all of the water freezes.
A. What volume of ice is created from 200 cm3 of water?
B. Why do water pipes sometimes burst in the winter?
Answer: When the water freezes, it expands so it bursts.
Question: The altitude of a location is its vertical distance above sea level. How does altitude affect phase changes?
- Form a hypothesis: As altitude increases, the air pressure decreases. How do you think the lower pressure will affect the following? (Highlight your answers.)
Freezing point: Increase Stay the same Decrease
Melting point: Increase Stay the same Decrease
Boiling point: Increase Stay the same Decrease
- Experiment: Use the Gizmo to find the freezing, melting, and boiling points of water at 5,000 meters (16,404 feet). Write these values below.
Freezing point: 0 Melting point: 0 Boiling point: 83
- Analyze: How did altitude affect the freezing, melting, and boiling points of water?
Answer: Freezing and melting stayed the same but boiling point decreased
- Challenge: Try to explain these results based on the fact that air pressure decreases with altitude. If possible, discuss your ideas with your classmates and teacher.
Answer: With less air pressure, it requires less kinetic energy to break the bonds as less pressure is on it.
- Apply: Why does pasta take longer to cook in the mountains?
Answer: The water can’t reach as high as a temperature so the pasta cook slower
- Apply: A pressure cooker allows food to be cooked under high pressure. Why is this useful?
Answer: Higher pressure makes it reach a higher temperature so it cooks faster