It's Just a Phase: Water as a Solid, Liquid, and Gas

In this activity, students will construct models of the way water molecules arrange themselves in the three physical states. Students will understand that matter can be found in three forms or phases (solid, liquid, and gas). Using physical models, students will be able to explain the molecular behavior of ice, water, and water vapor.

Background

Water is an essential part of the earth system. Water is special not only because it covers over 70% of the earth's surface, but also because it is the only known substance that can exist in gaseous, liquid, and solid phases within the relatively narrow range of temperatures and pressures found on earth.

Water's special qualities come from the unique shape of the water molecule. Each molecule contains two atoms of hydrogen and one atom of oxygen, arranged such that one side of the molecule (nearest the hydrogens) is positively charged while the other side (nearest the oxygen) is negatively charged. If two water molecules come together, the positive side of one is attracted to the negative side of the other, making the molecules cling together. This simple fact accounts for the high heat capacity, surface tension, cohesion, adhesion, and other characteristics that make water so important to the earth's biosphere.

In general, when considering the states of matter, solids are more dense than liquids and liquids are more dense than gases. Water is a bit of a contrarian in this regard.

When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape. Ice has a very regular pattern with the molecules rigidly apart from one another connected by the hydrogen bonds that form a crystalline lattice. These crystals have a number of open regions and pockets making ice less dense than liquid water. This is why ice floats on water. Ice forms when the temperature is below freezing (0°Celsius or 32°Fahrenheit).

When ice is warmed above freezing, it melts and becomes liquid water. As a liquid, the attractive forces between molecules weaken and individual molecules can begin to move around each other. Because the molecules can slip and slide around one another, water takes the shape of any container it is in.

The third state of water is the gaseous state (water vapor). In this state, water molecules move very rapidly and are not bound together. Although we cannot see water in its gaseous state, we can feel it in the air on a hot, humid day. Commonly, water boils at a temperature of 100°C or 212°F, forming water vapor. Many people believe that the visible plume of steam from a boiling kettle is water vapor. However, the steam that you see consists of very small water droplets suspended in the air, while water vapor is the invisible gas that results when water evaporates. We can "see" water vapor through the electromagnetic eyes of infrared-sensing instruments.

Water cycles endlessly throughout the atmosphere, oceans, land, and life of planet earth, taking each physical state at one time or another.

In this activity, students will construct models of the way water molecules arrange themselves in the three physical states. This should help them understand some of the workings of the water cycle. At this stage in the students' learning, a precise understanding of the molecular forces and energy interactions that lead to phase changes is not necessary.

Learning Goals

1. Students will understand that matter can be found in three forms or phases (solid, liquid, and gas).
   
   
2. Using physical models, students will be able to explain the molecular behavior of ice, water, and water vapor.
   
   

Alignment to National Standards

National Science Education Standards

• Unifying Concepts and Processes, Grades K to 12, pg. 117: "Models are tentative schemes or structures that correspond to real objects, events, or classes of events and that have explanatory power. Models help scientists and engineers understand how things work. Models take many forms, including physical objects, plans, mental constructs, mathematical equations, and computer simulations."
  
  
• Physical Science, Grades 9 to 12, Structure and Properties of Matter, pg. 179, Item #4: "The physical properties of compounds reflect the nature of the interactions among its molecules. These interactions are determined by the structure of the molecule, including the constituent atoms and the distances and angles between them."
  
  
• Physical Science, Grades 9 to 12, Structure and Properties of Matter, pg. 179, Item #5: "Solids, liquids, and gases differ in the distances and angles between molecules or atoms and therefore the energy that binds them together. In solids the structure is nearly rigid; in liquids molecules or atoms move around each other but do not move apart; and in gases molecules or atoms move almost independently of each other and are mostly far apart."

Benchmarks for Science Literacy, Project 2061, AAAS

• Common Themes, Models, Grades 6 to 8, pg. 269, Item #1: "Models are often used to think about processes that happen too slowly, too quickly, or on too small a scale to observe directly, or that are too vast to be changed deliberately, or that are potentially dangerous."
  
  
• The Physical Setting, Structure of Matter, Grades 6 to 8, pg. 78, Item #1: "All matter is made up of atoms, which are far too small to see directly through a microscope. The atoms of any element are alike but are different from atoms of other elements. Atoms may stick together in well-defined molecules or may be packed together in large arrays. Different arrangements of atoms into groups compose all substances."
  
  
• The Physical Setting, Structure of Matter, Grades 6 to 8, pg. 78, Item #3: "Atoms and molecules are perpetually in motion. Increased temperature means greater average energy of motion, so most substances expand when heated."
  
  

Grade Level/Time

• Grade level: 6 to 9
  • Note that at grades 9 and above, the activity would be more useful as a quick introductory demonstration that leads to discussion of the molecular and energetic basis of the phases.
    
    
• Time:
  
  
  • Teacher introduction to phases: 20 minutes
    
    
  • Student model building: 30 minutes
    
    
  • Discussion: 15 minutes
    
    

Materials

• Petri dishes (three per team - plastic ones work best)
  
  
• Tape
  
  
• BBs
  
  
• Overhead projector
  
  
• Chart of the water cycle
  
  

Procedure

1. Start with a short lecture on the nature of solid, liquid, and gaseous water. Your goal is to give some understanding that water molecules themselves don't change when the state of the matter changes, but that the interaction between molecules changes. You might wish to use copies of the graphics above to explain the phases. Explain that a water molecule is made up of two hydrogen atoms and one oxygen atom. To illustrate what a molecule looks like, ask each student to visualize his or her head as being the oxygen atom with their two fists representing the hydrogen atoms.
   
   
2. Put students into teams of three. Give each team three petri dishes, a supply of BBs, and some tape.
   
   
3. Challenge them 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.
   
   
4. Ask what the water molecules look like in each phase. Give no more instructions than that and let the students puzzle over the assignment for a few minutes.
   
   
5. If they need hints, ask: In which form are the molecules most tightly held together? What would that look like in the petri dish? This will likely "jump-start" some imaginations.
   
   
6. Typically, students will arrive at similar models within a short time that look something like this:
   
   
   
   

   Note: Most students will produce models similar to those illustrated in the graphic above. While generally accurate for liquid water and water vapor, the model is wrong for ice, though it reflects a very common misconception about the structure of ice. Most students (and most people) would explain that solid things are always denser than liquid and gaseous things, forgetting that water is a powerful and important exception to this rule. Allow the students to produce the models as they see fit, but then challenge them to explain, from their model, how ice floats in water (presumably the students will be familiar with the concept of density, and with the fact that less dense things float on more dense things). They should recognize that the packed BBs in the ice model are too dense to float in water!

   You may wish to have a petri dish previously prepared to illustrate the density of ice, with drops of superglue molding the BBs onto the dish such that the BBs are spread out more than in the water. Next you should draw lines between the BBs on the dish, indicating that the BBs are bonded together as illustrated below.
   

   
   
   
7. Place each dish on an overhead projector and jiggle it to show how the molecules move, or in the case of the ice model, don't move.
   
   
8. Weather permitting, take your class outside and divide them into groups of six to eight. Have the students demonstrate how their group would look if they were water molecules in the three different phases.
   
   
9. Back in the classroom, discuss what the water molecules looked like in each phase of the activity. Students (of all ages) often think that "liquid water molecules" are different from "ice molecules" or "vapor molecules." Emphasize that the molecules DO NOT change, only their arrangement, and that the same holds true for all other matter as well.
   
   

Assessment Ideas

• Observe the groups as they work in the lab or outside to identify students who are having conceptual troubles.
  
  
• At the end of the lesson, give students yet another model idea to diagram and explain individually. For example:
  
  
  
  • Imagine that the closet is an ice cube and that the water molecules are red balloons. If you open the closet door, what will you see?
    
    
  • Now, using red balloons as molecules again, imagine that the closet is full of liquid water. What will be different when you open the door?
    
    
  • How will it differ if there's water vapor in the closet instead?
    
    
  • Have students draw diagrams and/or explain on a separate sheet of paper and turn in.

Modifications for Alternative Learners

• Make sure that students can use labeled diagrams without abundant text for any written report or assessment.