Condensation: Gas to a Liquid (Part 1)
Lesson 3 of 9
Objective: SWBAT Predict, investigate and explain how water condenses
The lesson on evaporation brought to light several ideas that I want students to think about:
1) What happens to water when they can no longer be seen on a paper towel?
2) What is causing the water to disappear from view--is is gone/where did it go?
3) What effect does changing the amount of energy available to the water have on how quickly it evaporates?
I want to make sure that students are reflecting on this information to deepen their conceptual understanding.
I ask: How does evaporation happen? Explain at the molecular level.
To get kids thinking about condensation, I show this condensation cup video (from ACS) depicting an experiment where one cup with water and ice is exposed to the air and one is placed in a sealed Ziploc bag.
I ask kids to:
1) Predict what you think is going to happen in the experiment
2) Record pictures of the experiment and the results. (The cup outside the bag has moisture on the outside, while the one in the bag doesn't.)
To promote an inquiry-based, student centered learning environment, I ask students what questions they have about the results from the experiment.
Some common questions include:
- Why does the cup in the bag not have water on the outside?
- Why does the cup without the bag have water on the outside?
- Does this have anything to do with heat?
These questions are not addressed during our discussion when students share them, rather the questions can be revisited later by the students to reflect on what they have learned.
I now want students to experience condensation by participating in an demonstration where they will have 2 cups. One cup gets hot water poured 3/4 full and the other cup is inverted over the top and rests on the rim of the other cup. I also give them two cups for a similar experiment having cold water in the cup, as opposed to hot water.
ACS doesn't have students experiment with both cups, but by adding the other demonstration, I hope to have my student realize that heat is responsible for heating the air that eventually cools and condenses on the inside of the cup with warm water. Since water vapor will most likely not condense, in typical classroom conditions, in the cup with cold water, I want students to observe and record that information.
Picture: American Chemical Society
The driving question for this experiment is: "Does the temperature of water affect the rate of condensation in a cup?"
I instruct students to record observations in their notebooks and any questions that they might have about what they're seeing.
We then reflect as a class on our experiments and share our questions.
I mainly want students to share their ideas that temperature of water, in this case, plays an active roll in condensation. Evidence supports that warmer water promotes the creation of condensation faster than colder water.
Teacher tips: I have a plug-in tea kettle, set at the lowest temperature setting, that I use to warm up water quickly. If the water is too hot, simply add some cold water prior to distributing the water to each group. It is helpful at the end of class if you have students dry out the top cup, so that there isn't any condensation in the cup. I used snow from outside for the air temperature experiments that I stored in a Styrofoam cooler. You could use a cooler.
Now that students have experienced a primarily teacher led experiment, it is time for them to take over. Prior to this students have learned about experimental design in this lesson, so I encourage them to look in their notebooks if they are struggling to develop a controlled experiment.
I explain that we are going to investigate the following question:
"Does making water vapor colder affect the rate of condensation?"
I explain that they will use the general set up of the previous demonstration but will be held responsible for establishing a few things:
1) They will explain how they will set up the experiment
2) They will create an IVCDV chart
3) They will devise a way to quantify their results--wiping a paper towel on the inside of the cup isn't very accurate and won't help answer our question.
Here is a clip of students working hard determining how to fill in their IVCDV chart:
As students are developing their investigation, I am circulating around the room asking probing questions. Here are some common issues that you can anticipate:
Issue 1: Students will want to put the snow or ice in the water.
Solution: Ask students what phase of matter water vapor is and then wait for their responses. Simply asking them the question is enough for them to realize that they want to directly cool gas, not liquids.
Issue 2: Students may struggle with setting up the IVCDV chart.
Solution: I teach my students the IV (Independent or manipulated variable) is the part that is being changed, or "I" am changing this part of the experiment. I then ask what am "I" --I being you--changing about the experiment. They are encouraged to look back at the question, if they continue to struggle. Resist telling them, instead ask questions until they get it correct. The same is true for the Dependent variable. Refer back to the Variable and Constant Scenarios and discuss the fertilizer and plant height experiment.
Teacher note: If your students are struggling with this part of the lesson, please don't feel the urge to rush this step so that you can finish the experiment. NGSS lessons are messy and often unpredictable, so utilize this time to fortify your students' ability to Plan and Carry Out Investigations (SP3). In the long run they will be better prepared to plan future investigations. These are not neatly packaged, cookie-cutter lessons--accept that realization and adjust your future lessons accordingly.
Let Students Investigate
Students now perform their experiment and record their results.
Findings: Students find that the cup with snow on top had condensation form faster than the cup without the snow on top.
I then ask students to write a reflection about this experiment:
What effect does cold water vapor have on the rate on condensation? Supply evidence and reasoning.
Student finish this for HW, if they don't finish in class.
Check For Understanding
I then take a few minutes to ask students to reflect on what they learned from the demonstrations and investigations.
- A difference in temperature is required to make condensation in our cups.
- If water is colder, than it condenses slower or not at all.
- If air is warmer, than water condenses slower inside the cup.
- If air is colder, than water condenses faster on the inside of the cup.
I then explain that tomorrow, we will model how condensation and evaporation form through various modeling techniques.