Evaporation (Liquid to a Gas)

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SWBAT: Predict, investigate and explain how water evaporates

Big Idea

Students will be able to investigate what causes water to disappear.

Do Now

5 minutes

In this lesson, students are building an understanding of evaporation and the role that thermal energy has on molecular motion and phase changes.  Up until this point, students have investigated the role that temperature has on molecular motion.  This lesson sets out to help students make connections between molecular motion and evaporation.  

I instruct students to look at the beaker with water and food coloring from lesson 1.2 in the front of the room.  What might explain the change in water level from the top line to the present level of water?

Note: I have intentionally left the beaker filled so that it could initiate our investigations on evaporation at the beginning of this class.  I will keep it on the desks so that we can refer to it as we continue to learn about matter.  

Activating prior knowledge and identifying misconceptions:

  • When taking student responses most of them knew the word evaporation, but they couldn't explain how it was happening.  
  • They knew that heat was involved, but couldn't explain the process.  
  • Some students even thought that the water vapor was sucked out of the beaker by the air.

My approach:

I knew that students would benefit from experiencing a concrete example of evaporation and then developing deeper understanding from participating in their own investigations.  I asked students how they could test the following question: "Does the temperature of water affect the rate of evaporation?"

Science and Engineering Practice 3: Planning and Carrying Out Investigations

15 minutes

To help promote the development of SP3 (Planning and Carrying Out Investigations), I give students this Graphic Organizer and a list of the materials that they will have access to, including:

  • Paper towels
  • Ziploc bags (as many as they need)
  • Timer or clock
  • Water (hot or cold)  I have warm water from an electric tea kettle ready to go before they arrive.  Cold water from the tap.
  • Graduated Cylinders
  • Thermometers
  • Other items upon asking (I don't want to steer too much or squash their thought process, so if they have a great idea and need other materials, I will try to meet their requests)
  • Various other heat sources (hands, different parts of a room, heaters (non-electric, of course), etc.)  Sky's the limit.

As sixth graders, my goal is not to have them independently maneuver around a lab knowing exactly what materials to use, so I guide them through the materials portion.  I do expect some serious deliberations on how they plan to use the given materials to execute a scientific investigation that provides evidence to support our guiding question. 

You will be amazed with the ideas that your students come up with!  As they fill in their graphic organizers, you should circulate around the room facilitating learning.  Students will have ideas that will need further development.  That's where you come in--remind them about the IVCDV  lesson from our Nature of Science Unit and how that information can we leveraged to develop scientific investigations.

Common Student missteps:

  • Students will only want to perform one test, not considering the temperature change highlighted in the guiding question.  Teacher action: Ask students if testing only one temperature if going to result in meaningful results that address our inquiry.  I then walk away and come back so that the group has time to process and problem solve.  Facilitation is key, not steering.  Yes, this takes time--so be it.
  • Students will not consider having constants, such as same amount of water or size of cups, etc., in their experimental and control groups.  Teacher action: responses are specific to each group, but you may consider stating, "How will we know if it is temperature or amount of water being evaporated?"  This will get kids thinking about constants and their importance in isolating the independent variable--"The one they I (they) change in an experiment."  
  • How will they collect their data?  Teacher action: Ask students how scientists collect and record evidence.  If they don't remember, have them turn back to other experiments and how they recorded their data.  If they are struggling with setting up the columns and rows, ask them to first consider what they're measuring and then go from there. 


Carrying Out Investigations and Collecting Data

25 minutes

Students came up with several investigations to test our question.

Some groups decided to use their hand as the heat source and the desk as the control.

Other groups decided to use Ziploc bags with the same amounts of different temperatures water in each one.

Here is a sample of one student's observations.

Once students performed these experiments, I encouraged them to quickly design and test other ideas.

Some groups tested what happened when the wet paper towel is placed under each plastic bag.  Another group tested what would happen if they changed the amount of water that was placed on the paper towels, maintaining a controlled experiment.  Other groups decided to change the temperatures (Independent variable) to see if that altered the results. 

Analyzing Results and Forming Evidence-based Conclusions

10 minutes

After giving students a few minutes to clean up their areas and organizing supplies, it is time to analyze results.  I don't particularly think that it's necessary for kids to graph their results, as they're easily interpreted just by looking.  I ask them to look at their results and compare them to our guiding question: "Does temperature affect the rate that water evaporates?"  

You may need to extend this section until the next class period, possibly assigning some parts for HW.  It all depends on your class and how quickly they can plan and carry out an investigation.  

I ask students to provide at least 2 pieces of evidence to support their conclusions.

I then ask them to reconsider how the water level in the large beaker keeps going down.  Is it getting sucked out, as some believed at the beginning of class?  Although we can't rule out that it isn't getting sucked out by the air from our experiments, we can agree that when water warms up faster then it's molecules move faster and change into water vapor faster than water that is colder.  

I also like to ask students, as I walk around from group-to-group, how does this experiment compare to what we learned in lesson Molecules in Motions?  Students make connections that liquid molecules that are warmer move faster than colder liquid molecules.  

A great homework assignment--to grade for completion, not level of correctness--is to ask students to explain how they think the water evaporates.  Remember, the NGSS is all about developing deeper conceptual understanding, not making kids feel bad because they are struggling to explain a phenomenon.  That is why evidence-based understanding is crucial.  If students don't have evidence to support their claims then we must give them the opportunities to plan, execute and learn from those investigations.  Students will be asked to develop an elaborate, evidence-based model upon completing their condensation lesson(s).

Key potential misconceptions to lookout for:

1.) A large vacuum sucking water into the air :)

2.) Beware of students who continue to mix up the direct relationship with an inverse one in regards to temperature and molecular motion.  If this occurs, I suggest that you sit down with these students and review their results.  

3.) Many students think that air evaporates.  They need to understand that water is evaporating, not air.  The water vapor is going into the air which relates to relative humidity in an area.  I like to use the idea of crusty boogies.  I know--gross!  It helps them relate the role of temperature on evaporation, though.  In the winter, especially here in New England, we often wake up with dry noses.  The lack of heat means that the water freezes and therefore is not in the air as water vapor, leading to dryness.