PRISMS: Phenomena & Representation for Instruction of Science in Middle School

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This is a representation reviewed for the following learning goal:

In solids, the atoms or molecules are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely held together, and can slide past one another. In gases, the atoms or molecules have still more energy and are free of one another, except during occasional collisions.

Summary

The set of 3 diagrams, with accompanying text explanations and an activity that creates models of ice, liquid water, and water vapor, shows the arrangement of water molecules in each of its three states of matter. It addresses the part of the key idea about the atoms or molecules in solids being closely locked in position and only vibrating, being more loosely held together in liquid and sliding past one another, and being free of one another in gases. It is likely to be comprehensible to middle school students.

Limitations:
The resource inaccurately represents the idea that molecules in ice (solid water) are packed more closely together than molecules of water in the liquid state. There is no mention that the molecules in the solid state vibrate while held in a nearly rigid structure.

Suggestions to improve effectiveness:
It is important that students recognize that not all molecules behave like water in the solid and liquid phase. It is also important to explain that the arrangement and motion of water molecules in the gas state can be generalized to other molecules.

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Detailed Review

Description

The representation is a set of 3 diagrams, with accompanying text explanations, showing the arrangement of water molecules in each of its three states of matter and an activity that uses BB's and petri dishes to create models of ice, liquid water, and water vapor.

  • Resource Copyright Owner: University Corporation for Atmospheric Research
  • Type of Resource: image/gif


Content alignment

The resource addresses the actual substance of the learning goal rather than just the topic.

Explanation

The resource addresses the arrangement and movement of molecules in each of the three phases of matter. The diagrams address arrangement and the supporting text and model building activity addresses both arrangement and movement.

Scope

The resource addresses the indicated part of the learning goal:

Explanation

"In solids, the atoms or molecules are closely locked in position and can only vibrate. In liquids, the atoms or molecules" have higher energy, "are more loosely held together, and can slide past one another. In gases, the atoms or molecules" have still more energy and "are free of one another", except during occasional collisions.

Sophistication

The resource correctly reflects the level of sophistication of the learning goal.

Explanation

The diagram and model address the arrangement and movement of water molecules at the molecular level, including the anomoly of water in the solid state having more space between the molecules than the liquid state (in the activity). While the teacher background information details the polar nature of water molecules and describes some of the unique properties of water as a result of hydrogen bonding, students should not be expected to use this information to explain the molecular arrangement of water in various phases. Middle school students are expected to be able to use their understanding of phase change at the molecular level to explain phenomena such as freezing, melting, evaporation, and condensation.

Notes for Teachers

The resource addresses the arrangement and motion of water molecules. It is important that students recognize that not all molecules behave like water in the solid and liquid phase. It is also important to explain that the arrangement and motion of water molecules in the gas state can be generalized to other molecules.

Quality of instructional support

Does the representation accurately represent relevant aspects of the learning goal and/or brings out limitations that are not accurate?

Partial—The representation accurately represents most, but not all of the relevant aspects of the learning goals. It may or may not bring out inaccuracies.

The resource inaccurately represents the idea that molecules in ice (solid water) are packed more closely together than molecules of water in the liquid state. Yet, the text uses the example of ice floating in water to describe the density relationship. When you look at the second representation of liquid water, the picture shows the molecules of water as being much further apart than they are in the previous solid state and they do not appear to be sliding past one another since they all look as if they are separated from one another. The learning goal idea that atoms and molecules of a liquid are less tightly held and can slip past one another is not evident in this representation. Furthermore, there is little distinction in the distance between molecules in the third diagram (water vapor). The BB model diagram shows a representation of how students are likely to represent the arrangment of water molecules in all 3 states. The teacher notes describe the correct arrangement in terms of distance between the particles, but the representation of motion may lead to a misconception that molecules in solids do not move. The text explicitly states (see #7) that the molecules in the ice model do not move. The act of gluing the bb's in place may also imply to students that they do not move, although it is true that they are held in place. There is no mention that the molecules in the solid state vibrate while held in a nearly rigid structure. There is also a danger that students may develop a generalization that all molecules are arranged and move in this way.

Is the representation likely to be comprehensible to students?

Partial—The representation might make the learning goal comprehensible to students.

The model relies on students using knowledge of density to represent the arrangement of molecules in ice. If students lack a concept of density in relation to arrangement of molecules, then this representation would be difficult for students to come up with on their own.

Does the representation make clear which aspects of the real thing are represented and which are not?

Partial—The representation usually, but not always, makes clear which aspects of the real thing are represented and which are not.

The resource instructs students to "imagine that each dish is a VERY enlarged part of a phase of water: one dish is ice, one is liquid, and one is vapor. The BBs represent the water molecules themselves." While middle school students are not required to understand bonds, the drawing of the lines between the molecules in the ice may imply to students that there is a "stick-like" structure holding them in place, rather than the lines representing an attraction between molecules. When jiggling the molecules on the overhead projector, it does not really show the water molecules being able to slip past one another nor the gas molecules moving rapidly and further apart with occasional collisions. Instead, it shows back and forth movement in one direction at a time with frequent collisions.

Summary Judgment

Useful with modification

Notes to teachers

This activity could be useful with middle school students to represent the anomaly of water in the solid and liquid phase. However, students should also have an opportunity to contrast this arrangement with other molecules, besides water in order to develop a broader understanding of arrangements of molecules in the 3 phases that is consistent with the learning goal.

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