Phase Changes of Water

A Yakima WATERS “5-E” Inquiry Lesson

Students investigate how water reacts to the addition of heat and how temperature behaves as the water is heated. Melting point and boiling point are introduced as characteristic properties of matter. The lesson is designed for 8th graders that use the NSRC STC/MS Properties of Matter curriculum, but it is suitable for other grade levels and other curricula with modifications. The lesson will take 2-3 forty-five minute periods.

Relevant Standards

6-8 INQC – Collecting, analyzing, and displaying data are essential aspects of all investigations.

6-8 PS2A – Substances have characteristic properties such as density, solubility, boiling point, and melting point, all of which are independent of the amount of the sample.

Outcomes

Knowledge:

  • Students will be able to describe two properties of matter – the melting point and boiling point.
  • Students will be able to describe and explain the behavior of temperature as water moves through its three phases.

Skills:

  • Students will further develop skills in data observation, recording, and analysis.

Materials and Equipment

Per lab group (3-4 students) -

  • 1 Bunsen burner
  • 1 beaker
  • 1 ring stand for the beaker
  • ~ 500 mL of ice
  • 1 thermometer
  • 1 timer

Prior Knowledge

The phase change component of this lesson draws on students’ general past knowledge of the different phases of water. It also draws on students’ knowledge of the distinction between heat and temperature. Students should know that heat refers to the transfer of energy from one body to another and that temperature is a measure of the average kinetic energy of particles within a body.

Safety

Bunsen burners are used in the phase change component of this lesson, so students should have already been exposed to them and understand the relevant safety concerns. These are:

  1. Gas should be turned off unless the burner is being lit. Gas should never be on for more than 3 seconds without a flame.
  2. Extra tubing should be tucked in a sink or away from the edge of the table so that it is not caught on something and the burner is not accidentally tipped over.
  3. Loose clothing should be removed or tied back.
  4. Long hair should be kept in a ponytail or hat.

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Engage (5 min) – Introduction and Demo

Engage students by first summarizing some of the knowledge gained thus far about the properties of matter, and ask them questions that draw on that previous knowledge. For example, “You have learned that all substances (including air!) have mass, that substances have density.” Ask if anyone can give the formula for density and ask what happens to a substance’s density as it is heated or cooled. “You have also learned about the relationship between volume, temperature, and pressure. However, there is a very important concept that we haven’t explored yet – phase changes. What do I mean by phase changes?”

Here it will be exciting to drop a piece of solid dry ice into a beaker of water which will get students’ attention and demonstrate different phases and phase changes. Explain exactly what is meant by the different phases – solid, liquid, gas – as the CO2 in the demo is rapidly turning from solid to gas. Emphasize that liquid CO2 is not seen in this demonstration, but that the CO2 is sublimating, or changing directly from solid to gas.

Explore (20 min) – Experimental Procedure

Students will likely have some idea of the 3 phases (they’ve all seen ice, water, and have recently learned about H2O gas), as well as how water moves from one to the other (they will obviously know what melting, freezing, and boiling are, at least in a non-scientific way). However, students will probably not know what happens to temperature as the water moves through the phases.

  1. Have students write a hypothesis about what temperature will do as heat is applied. It is likely that students will wrongly predict temperature to rise steadily through the whole procedure or jump quickly when water changes phase. Any hypothesis is acceptable if it is testable, so incorrect predictions should not be discouraged.
  1. Next, students should write a procedure to test the hypothesis that they just created. Review a few students’ procedures as a class so that a refined procedure may be drafted on the overhead projector. The procedure should look more or less as follows:
  1. Set up Bunsen burner and ring stand.
  2. Collect ~ 500 mL of ice in a beaker.
  3. Melt the ice over the Bunsen burner, recording temperature of the H2O every 30 seconds until the water boils.
  4. Have students record the data in a table that they design.
  5. Graph the data in a notebook or on the computer (if there is computer access).
  1. Complete the inquiry procedure.

Refer to the materials section above for information regarding materials. Refer to the safety section above regarding safety concerns.

Explain (20 min) – Results and Discussion

When students are finished recording data, initiate a discussion of the results. Have students from different lab groups report their data and compile on the overhead projector. The graph of temperature should increase to the melting point of the ice, flatten out as the ice melts, then increase again to the boiling point of the water, then flatten out again as the water is boiling. Collecting each data point from a group or groups will be very time consuming, so it is encouraged to have students plot their own graphs, describe the trend of the data, and plot a generalized graph on the overhead for discussion.

The purpose of showing this graph is to introduce and discuss the meaning of melting and boiling points as well as the flat parts of the graph – the latent heat needed to change molecules from one phase to the next. This should be done as group discussion. After the discussion, give students a few minutes to write an explanation in their own words why temperature behaves this way.

Assessment.Students should turn in their record of temperature data and graph for assessment. Students should also be assessed on their explanations of the behavior of temperature as the water is heated through its three phases.

Extend/Elaborate (45–90 min) – CO2 Extension, Water Cycle and Water Cycle Game

Once these concepts have been introduced, the knowledge may be extended in a discussion about how temperature would behave if a different substance were used, like CO2 (this will relate the inquiry back to the initial demonstration!). CO2 will still have a similar looking graph with a melting/ boiling point as well as latent heat of fusion/ vaporization, only the graph will be shifted because CO2’s melting and boiling points are much lower than that of H2O. Emphasize that melting and boiling point are characteristic properties of all substances. This extension will give students a chance to demonstrate that they understand the concepts, and it will give the teacher a chance to refine the points discussed in the previously if need be.

The knowledge of phase changes can also be extended by playing the “water cycle game.” See the reference to Tabitha Trosper’s Water Cycle Game below.

Evaluate (<5 min)

Students will be evaluated based on the quality of data collection and the graph of temperature and time. They will also be evaluated based on the level of understanding demonstrated on their written descriptions of the temperature trend and the melting/ boiling point.

Teacher Background Information:

All substances may exist as solids, liquids, or gases. These are called phases, and a phase change describes the transition of a substance from one phase to the next. We are all familiar with phase changes because we melt, boil, and freeze water everyday. We may not be as familiar with how temperature behaves during phase changes. When a substance is heated, kinetic energy is transferred to the molecules, which causes them to become more spread out. This is seen as a gradual rise in temperature. However, it takes energy for the molecules to change from solid to liquid, then from liquid to gas. This is called the latent heat of fusion and the latent heat of vaporization, respectively. During these transitions, temperature will stay constant. Once the phase change is complete, temperature will begin to gradually rise again. Different substances have different melting and boiling points, which are characteristic properties that may be used to identify the substance.

Performance Rubric

Element / Excellent
(5pts) / Good
(4pts) / In Development (3pts) / Needs Rethinking (2pts) / Not Scorable
(1pt)
Knowledge (1, 50%) / Student is able to provide a clear, concise, and accurate description of the melting and boiling point of water including the behavior of temperature as the water was heated. / Student is mostly able to provide a clear, concise, and accurate description of the melting and boiling point and behavior of temperature as water is heated. / Student is somewhat able to provide a description of the melting and boiling points as well as the behavior of temperature. / Student misses some of the main points when describing melting and boiling point and/or the behavior of temperature over time while heating water. / Student lacks any meaningful description of melting/ boiling point. Student cannot summarize the temperature trend over time.
Skill (1, 50%) / Student is able to provide a clear, organized, and useful data set of temperature over time. Student also plots the data correctly in a visually useful and meaningful graph. / Student is mostly able to record data that is clear, organized, and useful. Student also plots the data correctly, however the graph may not be as clear. / Student has recorded data in a useful way and can produce a plot of the data, however there are some mistakes or minor flaws. / Student is able to record and graph data, but shows major flaws. / Student lacks any meaningful data record and cannot produce a graph.

Resources:

Trosper, Tabitha. “The Incredible Journey: A Water Cycle Exploration.” Yakima WATERS Project, Central Washington University.

“It’s Just a Phase: Water as a Solid, Liquid, and Gas.”

By: Andy (James) Menking, Fall 2011, for Morgan Middle School

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