Exploring Phase Changes

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Objective

SWBAT relate the temperature of a sample to the speed of its molecules, and therefore its state of matter.

Big Idea

Visualizing molecular speed helps students understand the microscopic properties of matter in different phases.

Background

This lesson builds on the previous day.  In the previous lesson, students became familiar with molecular arrangement and behavior in each state of matter separately.  Today, students will use the ExploreLearning site to investigate how matter changes states, and how energy is involved in making those changes.

The reasons for choosing this simulation can be read in detail here, or viewed in this brief video.

 

This lesson is aligned with the following standards:

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.   While students are not planning the investigation, identification of how electrostatic attractions are overcome to cause changes in state are investigated.

Science and Engineering Practice 2: Develop and use models.  Students will use the computer models to gather information and construct their explanations of what is happening.

Science and Engineering Practice 6: Constructing Explanations.  Students will use the evidence from the model to explain why substances change states.

High School Energy and Matter Cross Cutting Concept 4.Energy drives the cycling of matter within and between systems.

Introduction

13 minutes

When students enter the room, I pass out two papers: their assignment from the previous day identifying the characteristics of solids, liquids and gases, and "Is it a Solid?" from page 25 of Uncovering Student Ideas in Science, Vol 3 from NSTA Press.  Due to copyright restrictions, I cannot share this page or copies of student work, but following the link above will allow you purchase it for yourself.

On "Is It a Solid?" there are 21 various items for students to categorize as either solid or non-solid.  After deciding what items from the list are solid, students are asked to identify what definition, rule, or reason did they follow in deciding what was a solid.  I really like this particular assessment probe to find the misconceptions about what is and isn't a solid.  These misconceptions may apply to more than one state of matter, so early identification and remediation is particular crucial.

When students are done with their individual classifying and own reasoning, I have them share at their table, and then we discuss the ones they disagreed about as a whole class.

Students struggle with classifying rubber band ("it's not hard"), baby powder and dust ("it's small grains") as solids.  Students are conflicted on how to classify melting wax, either including it because it is both solid and liquid or excluding it for the same reasoning.

If a student doesn't bring up the work from the previous day, I ask them to look at it.  I ask what we know about the molecules in a solid compared to a liquid and gas.  Students will respond that "They are packed together" or "They are barely moving" and then I ask how that affects what we can observe about solids.  Students will respond "They hold their shapes" or "they don't move around," which we re-direct into flowing.

I then inform them that today we will be focused on the transitions between solids, liquids, and gases.  I ask them to go back and log into the computers with a partner while I pass out the activity sheet.

Computer Simulation

25 minutes

As students transition onto the computers, I pass out their Exploration Sheet, which is slightly modified from the ExploreLearning site.  For the reasoning behind my modifications, please see my reflection.  I remind students that a link is available on my SchoolFusion page, or they can go to ExploreLearning.com and log in and search for the Gizmo.

While the computer is logging in, I remind students they can complete Section 1: Comparing Phase Changes from what they had done the previous day.

As students progress, I am circulating the room to answer questions and help keep them on task.  I quiz the students about what they are seeing, as many students don't identify the block in Activity A as ice, so I make sure they recognize it as they move on.

Activity A is concerned with how temperature and molecular motion changes at the melting point of water.  Students can observe that while the temperature is not changing, the molecules go from the slow-moving, connected solid state into the liquid state with increased motion.

Activity B where students model the effect of temperature on the freezing of water is a direct analog to the lab we will be doing in two days with Vernier probe ware.  Students may repeat this section, as they are surprised the temperature levels out while the ice is melting.

In Activity C, students will observe the behavior of the water particles as they heat and boil the water.  After doing parts A and B, they are no longer surprised at the temperature plateau, but may not realize that you can continue to heat steam.

Finally, in part D, students directly observe how temperature correlates with the speed of the water molecules.  They finish this section with explaining the direct relationship between temperature and molecular speed.

As students finish, or when there is 5 minutes left in the period, I ask all the students to log out and return to their tables at the front of the room.

Closure

4 minutes

When students return to the front of the room, we don't have much time left.  However, I want to cement the last piece of the activity in their head.  I ask them "Which state of matter for water has the fastest moving particles?" and call on a student with a raised hand.  Two possible responses occur:

  • The gas
  • The hottest

If I get the latter response, I follow up with "Which state is hottest?"

Now that we've identified it as a gas, I ask "How do we know steam has the fastest moving particles?"

Students will respond with either:

We can measure the temperature, and it was hottest

or

Particles in a gas always move faster than in the liquid or solid phase

I remind students to turn in their paper as they leave.  This is a particularly well done student example.  The student is detailed throughout, and precise in what they observe and why they believe it is happening.

At this point in the semester, the students have acclimated to using the simulations in lieu of notes.  Coupled with the PhET sim the previous day, students did very well on their explorations.  The quiz two days later showed strong understanding of the differences in motion between the particles in a solid, liquid and gas of the same substance.