Defining Phase Changes

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Objective

SWBAT define kinetic energy and its relationship to temperature and explain how kinetic energy and intermolecular attractions combine to determine the state of a sample.

Big Idea

States of matter can be changed by altering the levels of kinetic energy, processes we refer to as heating and cooling.

Background

The previous day, students used the ExploreLearning simulations to begin to explore the relationships between energy and phase changes.  Today I utilize PowerPoint to draw things together for the students.

In the past, I have asked students to take notes in their own fashion, and it has slowed down some lessons significantly.  Today I am attempting a different strategy, providing them with the slides with certain terms removed, and space at the side for students to write additional comments.

I find the easiest way to do this is to save an additional copy of the presentation, and then go back and delete key terms.  The difficulty lies in keeping the versions consistent if I choose to revise one or the other.  Often, I end up with the two in split screen to ensure that they are consistent before printing student copies and presenting in class.

This lesson continues our work towards HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.   We continue to stress Coulomb's law that the closer the particles are, the stronger their attraction is.

We access the High School Energy and Matter Cross Cutting Concept: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.  Our focus is on the flows of energy in and out of the system to drive the changes in state.

The final section of the lesson finds us in Science and Engineering Practice 3: Planning and carrying out investigations.  While this is not a full-blown inquiry investigation, it is important for students to plan out their role in lab ahead of time to maximize the lab experience and work safely.

Opener

5 minutes

When students enter the room, I pass back their Exploration Sheets from the previous day's ExploreLearning lesson. I also have a student pass out the student copies of the PowerPoint slideshow.

I explain that we are going to summarize today what we have learned about the different states of matter, and learn about the changes within a single substance. I explain the new notes format, as students are excited to be provided the slides. I point out that each slide has some missing parts for them to fill in.  I show them the lines on the side of each slide and encourage them to write any other important information from each slide in that space.

Once all the papers are returned and students have their writing utensils out, we begin the notes.

Whole Class Notes

25 minutes

The slideshow begins with a definition of energy.  I reference the title of the Xbox "Kinect" as it tracked your motion as being a good way to remember Kinetic Energy.  I ask the students where they were able to see the kinetic energy of the particles on the simulations and they refer to the speed of the various particles.

On the next slide I define temperature as "the average kinetic energy of a sample" with a graphic of an "excited, hot atom" and a "laid back, cool atom".  At the end of the slide, we refer to how different matter will show their speeds differently, and I ask students if the air molecules we are breathing or the steel molecules of their chair legs are moving faster.  Students respond that the air is faster, and when pressed to explain why point out "Gases move faster than solids."  I congratulate them and explain that steel being different from oxygen, will have different temperatures and speeds for its various states of matter.

The next slide is a review, with particulate level pictures of the three states of matter.  I ask students to answer the questions on their notes.  Once everyone has written, I ask someone to answer the first question and explain why.  I call on a volunteer, who answers "The solid, because its particles are closest together."  I ask if anyone disagrees, and if so can they explain why.  In this case, no one responds, so I ask if someone can follow up with the weakest attraction.  Another student will volunteer that, "the gas has the weakest attractions because its particles are the furthest from each other."

The next slide shows the erupting volcano from the lesson image.  I ask students where they can see all three states of matter.  Responses include:

  • The mountain is solid
  • The lava is the liquid
  • The steam off the lava is a gas

I particularly like this image, as it shows a non-water example of the states of matter.  Oftentimes, we get very caught up on the single example, and it can limit student thinking.  I ask "Does the lava has to be cold to freeze into solid rock?"  Students pause, and usually one will volunteer "No, but it has to be colder than when it is a liquid" which is the perfect transition to our discussion on phase changes.

The next slide describes the flow of energy, and I ask students "How does your freezer at home work?"  Many don't know, so we discuss the cooling coils at the back of the freezer and how it takes heat away from the food and moves the energy to the outside.  I include the recommendation to clean them to help the freezer be more efficient and cost less to run.  We then review the Law of Conservation of Energy, and refer back to the forms of energy from the first slide.  Depending on student feedback, we can also go back and ask where the lava loses heat to in order to cool off and freeze into solid rock.

The next two slides cover the names of the state changes, first going through warming changes.  We focus on the matter gaining energy from somewhere in the surroundings.  If you have a sample of gallium metal, it makes a good demonstration here to hold it in your hand with a rubber glove on and let students observe it melting at body temperature, and then freezing back in the bottle at room temperature.  Unfortunately, we could not locate our bottle of gallium this year, so it's on the order list for next year.

Next we go through the cooling changes.  Students are unfamiliar with depositing, but as we just had our first hard frost of the year, I ask "Where does the frost on your windows this morning come from?"  "The air"  "Does it condense and make liquid water first?"  "No, and when I use the defroster it doesn't melt, it just disappears.

We stress that when objects cool off they are losing energy to something else in the process.  I talk about Ralphie from A Christmas Story getting his tongue frozen to the flagpole, and describe how the metal pole carries heat away very quickly, allowing the saliva to freeze and fuze his tongue to the metal.  Students then ask "How do you get unstuck?"  We talk out the opposite change, until they realize they need to add a lot of heat to melt the frozen saliva.

Finally, we summarize that the chemicals are not changing when they change state.  This is hard for students sometimes, especially when we over-use water as our example because ice, water, and steam have different names, so it is difficult at times for students to realize they are all H2O.

At this point, I ask if there are any other questions before we move to the pre-lab.  This year, there were not, and I mention that I will post the PowerPoint on my Schoolfusion page later for students to view if they felt they missed anything.

Pre-Lab

20 minutes

Preparing for lab is important in terms of both safety and efficiency.  I have stolen this Filmstrip pre-lab technique from my fellow BetterLesson master teacher, Keith Wright.  I highly recommend you check out his lessons to see this technique in further action.

I have two student volunteers pass out the Freezing of Water Lab Filmstrip and the Vernier Freeze Water lab paper.  I ask students to sit with their lab partner if they are not already doing so.

I explain the first step in preparing for lab is to decide on roles, with one partner running the computer and temperature probe and one partner handling the chemicals and glassware.  Once students decide their roles, I ask them to write it on their filmstrip paper.

Now we start looking at the purpose and procedure of the lab.  Although students modeled the freezing of water the day before with the ExploreLearning simulation, the idea of discovering the freezing point of water does not sound like a repeat to them.  We begin to look at the procedure steps, and determine if they are a computer step or a chemical step. 

When students get the hang of analyzing the procedure, I then instruct them to record their own responsibilities in the filmstrip.  Students can choose to re-write the procedure, or to sketch out the steps in drawings.  I personally prefer them to sketch it, but many of the students plead a lack of art skills.  Some students skip boxes so that the numbered procedure matches the original, whereas others just list their own steps from one to the end.

The remainder of the period is spent completing their filmstrips to familiarize themselves with their jobs in the lab so that the lab runs safely and efficiently.

The filmstrips allow me to see if students have a handle on the procedures, and in this case, their specific parts of it.  Even if there is some lack of understanding, it forces them to pre-read the procedure, which automatically makes for a smoother lab than when students walk in cold.