Name: __________________ Period: ____

Phase Changes Lab Purpose: Students will investigate phase changes and their temperature dependence. Ultimately learning that the temperature stays constant during all phase changes. SOLs: PS.7 The student will investigate and understand temperature scales, heat, and thermal energy transfer. Key concepts include b) phase change, freezing point, melting point, boiling point, vaporization, and condensation; Essential Knowledge, Skills, and Processes: - analyze a time/temperature graph of a phase change experiment to determine the temperature at which the phase change occurs (freezing point, melting point, or boiling point). - distinguish between heat and temperature. Materials: 1. Test tubes with naphthalene or dichlorobenzene 2. Thermometers 3. Hot plates 4. Beakers with water Safety: Students should wear googles and be careful with the hot glassware. Engage: (5 min) 1. Put out some ice cubes on the tables.

a. What’s happening to the ice? b. What temperature does ice melt at?

i. So this ice and water is 0 degrees celsius right now? ii. Even though the room is 22 degrees celsius?

c. Do all solids melt? 2. Put out some dry ice. (Tell students not to touch it)

a. What’s happening to this solid? i. Wo wait it’s going from solid directly to gas? ii. Does anyone know the name of that process? (You’ve talked about it before)

b. Does CO2 ever exist as a liquid? 3. Demo CO2 triple point (watch video in this case - no blast shield) 4. In your groups you’re going to study the melting and freezing of another compound.

This lab, like the density of carbon dioxide lab requires careful and exact measurement. The instructions are clearly given in your lab handout. We’re not going to walk through the instructions today, instead you’re going to depend on your lab

Name: __________________ Period: ____

partners to determine what to do. Be sure to read through all procedural steps before starting so you don’t accidentally do something that ruins the experiment.

Explore: (30 min) Imagine you have an ice cube that starts at -10 ℃ and is heated to 110 ℃ over 10 minutes. Draw a graph modeling how the temperature changes over time. Be sure to label the phase changes (melting, and boiling) and their temperatures on your graph!

Lab Procedure: 1. Record the temperature of your unknown solid. 2. Turn on the hot plate to high. 3. Record the temperature in the data table below every 30 seconds.

a. Include observations in the appropriate column. b. Record temperatures for 3 minutes after the phase change completes.

4. Turn off the hot plate. 5. Have Mr. Mach or Ms. Mauney remove the hot beaker. 6. Continue to take temperature readings every 30 seconds until the sample is

completely frozen again.

Name: __________________ Period: ____

a. Ask Mr. Mach or Ms. Mauney when you think this is the case to be sure. 7. Graph your temperature data on a separate sheet of graph paper. Make your graph

as big as possible!

Time (min)

Temperature (℃)

Observations Time (min)

Temperature (℃)

Observations

0 15

0.5

1 16

1.5

2 17

2.5

3 18

3.5

4 19

4.5

5 20

5.5

6 21

6.5

7 22

7.5

8 23

8.5

9 24

9.5

10 25 10.5

Name: __________________ Period: ____

Time (min)

Temperature (℃)

Observations Time (min)

Temperature (℃)

Observations

11 26

11.5 12 27

12.5 13 28

13.5 14 29

14.5

Explain: (10 min) Students will answer the following questions in their lab groups. 1. Look back at the phase change graph you made for water earlier.

a. How is it similar to the graph for your unknown substance? b. How is it different? c. Did you draw plateau’s on your original water graph? Why or why not? d. What would it mean about phase changes if there weren’t plateau’s in the

temperature? 2. Does adding thermal energy (heating) a substance ALWAYS result in a temperature

change? a. Circle the regions of your graph where it does. b. Box the regions where it doesn’t. c. What’s happening when the temperature isn’t changing? (Think molecular

structure, motions, and attractions) 3. What portion of the graph corresponds to the phase change?

Name: __________________ Period: ____

a. What temperature did the phase change occur at? (Pick the approximate average temperature for the transition between phases)

Elaborate: (5 min + Homework) 1. Are phase changes physical or chemical changes? How do you know? 2. Label the following for the phase change curve below:

a. Solid b. Liquid c. Gas d. Melting e. Boiling f. Condensing g. Freezing

3. What substance do you think this graph is for? What evidence leads you to that conclusion?

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Evaluate: Rubric for graph portion of activity.

Criteria Present?

Temperature data graphed legibly and completely.

Phases labeled correctly.

Phase Changes labeled correctly.

Temperature change regions properly circled.

Static temperature regions properly boxed.

Energy input sections colored RED.

Energy output sections colored BLUE.

Axes labeled.

Graph has a title.

Name: __________________ Period: ____

Standards INTASC Standard #6: Assessment The teacher understands and uses multiple methods of assessment to engage learners in their own growth, to monitor learner progress, and to guide the teacher’s and learner’s decision making NSTA Standards: 2c) Design instruction and assessment strategies that confront and address naive

concepts/preconceptions. 3c) Plan fair and equitable assessment strategies to analyze student learning and to evaluate if the

learning goals are met. Assessment strategies are designed to continuously evaluate preconceptions and ideas that students hold and the understandings that students have formulated.

5a) Collect, organize, analyze, and reflect on diagnostic, formative and summative evidence of a change in mental functioning demonstrating that scientific knowledge is gained and/or corrected.

5b) Provide data to show that P-12 students are able to distinguish science from non-science, understand the evolution and practice of science as a human endeavor, and critically analyze assertions made in the name of science.

5c) Engage students in developmentally-appropriate inquiries that require them to develop concepts and relationships from their observations, data, and inferences in a scientific manner.

Reflection

Engaging student prior knowledge is very important for meaningful student learning.

Knowing students preconceptions and providing lessons that activate this prior knowledge are

the first steps to conceptual change. Learning requires cognitive disequilibrium followed by a

personal or group struggle to restore equilibrium. Discrepant events based on student

misconceptions are an excellent way (especially if they're exciting) to engage prior knowledge

and create this cognitive dissonance.

Showing the video of liquid carbon dioxide was not only engaging, it provided a

discrepant event that was linked to a couple alternative conceptions, addressing NSTA Standard

2c. Some of the students interviewed were adamant that nothing goes straight from a solid to a

gas, it must be a liquid in between. When pressed for further information about this the students

started to come back from their statement and decided that maybe under extreme temperature

conditions (like the surface of the sun) things could go directly from solid to gas. Even when

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conceding this the discomfort with that statement was obvious, they were really attached to the

solid to liquid to gas phase change pattern. One student even drew a diagram (Figure 1) of the

cycle to show their thinking. Many other students during the class discussions thought this also

and justified it by saying that the amount of liquid may not be visible, or that it is there for a very

short time, but it is definitely there. Others thought that dry ice does not have a liquid form, only

solid and gas. They justified this by saying that carbon dioxide can be made a solid and it is just

returning to its natural state of being a gas. These discussions enabled me to assess the entire

classes alternative conceptions and whether they agreed with those that the interviews had

uncovered or were slightly different and shift the emphasis of the lesson accordingly, which

aligns with INTASC Standard 6 and NSTA Standard 3c.

Name: __________________ Period: ____

Figure 1: Student Explanation of the Phase Change Cycle.

The lesson also included another discrepant event that most students observed during the

freezing portion of their data collection. The groups were basically in two camps, one already

knew that the temperature plateaued during phase change and the other had no idea. The groups

that didn't know were often distressed that something was wrong, their thermometer wasn't

working anymore, or they must be measuring wrong, or made a procedural mistake because

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the temperature should be descending steadily the entire time. This idea of constant temperature

during phase changes was a key component of the desired learning outcomes for this lab. To

further emphasize this idea the students were asked to draw a temperature versus time graph

predicting how temperature changed when heating ice through a temperature range that included

the melting and boiling points. After completing the lab activity they were asked to compare and

contrast their prediction and the graph of their collected data. They were also explicitly asked

whether they drew plateaus in their prediction and why or why not. Many of the students

answered "No" with most explaining it by saying that they had no idea it would have a plateau.

They were quite candid in their responses simply stating that they didn't know. This reinforced

that the behavior they observed and were concerned about was the proper behavior and that

their previously held beliefs needed to be altered to fit this new information. Students having

preconceptions about phase changes is well documented in the education literature. According to

Kind and Modic there are a wide range of student misconceptions about what occurs during

phase changes including ideas of molecular decomposition, i.e. water changing into hydrogen

and oxygen, molecular weight and size changes rather than the true speed and distance changes

that occur, and whether phase changes are chemical or physical changes. Baker and Piburn also

give evidence that student misunderstandings of phase changes (and other concepts) relate to

their fundamentally inadequate understanding of the concepts of heat versus temperature, and the

particulate nature of matter and the interaction of those particles. It was not immediately obvious

to me that these ideas come into play when dealing with learning about phase changes at the

middle school level. However, after developing the lesson, working through it with the students,

and reviewing their responses to the analysis questions I think that this is the main area that

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students are lacking in. The idea of the temperature plateau while heating and its implications for

phase changes was lost on many students. After answering the above mentioned question on the

prediction of plateaus students were asked what it would mean about phase changes if there were

no plateaus. Very few student responses showed any consideration of temperature, heat, or

energy concepts. Even more indicative that this is the underlying problem was student responses

to the next follow-up question: "Does adding thermal energy (heating) ALWAYS result in a

temperature change?" Despite this question following questions about the plateau the

majority of students answered "yes". There is obviously a disconnect between students idea of

temperature and heat that makes understanding phase changes difficult. Reading the student

responses to the analysis questions allowed me to determine the level of student understanding

and comparing their predictions to their collected data and analysis of that data helped me to see

which conceptions had been altered. This reflection meets NSTA Standard 5a, and the follow-up

discussion in class with the students about the potential errors in their data and why some of

them didn’t see the plateau that they should have in their temperature graphs met NSTA

Standard 5b, demonstrating to the students that science is indeed prone to errors and human

influence. Having the students answer these questions based on their collected data met NSTA

Standard 5c.

Teaching the lab on one day worked, but it would have been nicer to have two days to

spread it out more and really get into the ideas. During the engage activity we often started really

getting into good discussions about what was happening with the temperature and molecular

interactions when ice was melting. Due to time constraints I had to stop the discussions short to

ensure enough time to complete the phase change data collection. It would also have been good

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to take some follow-up time to discuss in lab groups and as a whole class the data we had

gathered and what was interesting about it and what those things meant. The students did discuss

these things with my cooperating teacher the next day, but unfortunately I was unable to be there.

The analysis questions for the lab probed student thinking on these ideas in depth, perhaps too

much depth for the amount of time spent working with them in class. They gave me a good

formative idea of where they were at conceptually, but were not easy to grade in a summative

manner because student thinking was often not correct or complete. Rather than marking things

write or wrong for points, I gave the students points for their effort and gave written feedback to

every student probing their knowledge further and asking them follow-up questions or explaining

where their thinking was in error, again in alignment with INTASC 6 and NSTA 3c. The activity

was a good kick-off activity for phase changes, but the students need more attention to this topic

to really understand and remember how phase changes happen and what is occurring on the

particle level.

References:

Kind, Vanessa. Beyond Appearances: Students' misconceptions about basic chemical ideas.

2004. 2nd Edition. Royal Society of Chemistry.

Modic, Amiee L. Student Misconceptions - Identifying and Reformulating What They Bring to

the Chemistry Table. 2011. Montana State University.

Baker, Dale R., and Piburn, Michael D. Constructing Science in Middle and Secondary School

Classrooms. 1997. Chapter 3.