The rain puddle formative assessment (FA) asks students to evaluate four submicroscopic representations of evaporated water and say whether they agree or disagree that each is an accurate representation, and explain why. This FA targets chemical mechanism, which is the question, “How do chemical processes occur?” The focus of this FA is the process of evaporation as a physical change to matter. This FA makes visible students’ thinking about what happens to water molecules after evaporation.
In the FA task, students are presented with the task on the Rain Puddle handout that guides their activity. They are asked to work in pairs to read everything out loud, discuss, and come to a consensus on whether they agree or disagree with each microscopic view.
This FA task was tested with students in 11th and 12th grades in a chemistry class, during a unit on intermolecular forces. The school is a large urban public school with a student population, the majority of whom are English language learners and native Spanish speakers. This particular group of students are in a sheltered English immersion science classroom, and are at the Emerging and Developing levels of English language development, as defined by World-Class Instructional Design and Assessment, WIDA.
These students had spent a few weeks on intermolecular forces, with evaporation as the anchoring phenomenon of the unit. They had learned how to identify a substance as polar or nonpolar, predict the intermolecular forces experienced by different substances given the molecular formula and electronegativity values, and predict the physical properties of different substances based on intermolecular forces. Students had also completed an independent experimental design project investigating the factors that affect the rate of evaporation of water. They set up an experiment to test a specific independent variable, such as temperature, dissolved substances, etc., collected data for 5 days, analyzed their data, and presented their findings to the class.
By the time students were asked to engage in this FA, they were very familiar with evaporation and its connection to intermolecular forces. The unit had started about two weeks prior, with a conversation about evaporation in which the students were presented with questions such as, “What happens to water after it evaporates?” “Where does the water go?” and “Is it still water after it evaporates?” The FA was used as a way to return to that conversation to see new ideas students had about evaporation.
While examining student work, I looked for evidence of chemical thinking, specifically, chemical identity, by looking to see whether or not students were able to identify that evaporated water still contains water molecules, and no new materials are made. I expected to see that students, due to their tendency to use specific physical characteristics as the way to determine the identity of a substance, would say that water, once evaporated, would be a new substance.
I noticed that most students were able to correctly identify the microscopic view that represents evaporated water, which shows the evaporated substance as water molecules on a microscopic level.
I noticed that one student, Josue 129, incorrectly identified the microscopic images with separated oxygen and hydrogen atoms. However, puzzlingly enough, the student’s word that he used to explain his reasoning is consistent with the physical changes underlying the process of evaporation. To this student, as a follow-up, I would ask him whether or not he thinks that evaporation is a chemical change. I would ask further questions such as:
- Is evaporation a reversible process?
- Is there any difference between the molecules of the evaporated puddle and the air particles in the surroundings? What could those differences be?
- Is evaporation a chemical reaction?
- How could we test to gas particles that result from evaporation to see if it is still water?
Although the other students were able to identify the correct drawing, I would ask them further questions to better understand their thinking about chemical identity. I would ask questions such as:
- Label the parts of each drawing
- Recreate the image of the original rain puddle and label the intermolecular forces and bonds in your drawing
- Describe the image of the rain puddle before evaporation. What do you see in this image?
The reason for these questions is to further make chemical identity the focus of the formative assessment by asking students to identify the parts of the pictorial representations included in the assessment
Examples of student work
|Agree or disagree
|I disagree because I don’t think the molecules separate when they evaporate.
|I disagree because I don’t think the molecules separate from each other when they evaporate.
|I agree with this because it’s showing how it’s evaporating but the molecules are still together and it’s evaporating slow because water is polar.
|I disagree because water can’t just disappear.
ElTommy and Nation’s responses
|Agree or disagree
|Because I just can see 2 water molecules and the three oxygen and hydrogen are separated.
|Because there is not water molecules.
|Because in here we can see the two water molecules and how the intermolecular force disappear.
|Because we don’t see even water and molecules so there is nothing.
|Agree or disagree
|I’m agree with this picture because the molecule are separing from each other.
|I’m agree because the molecules are separated from each others.
|I’m disagree because the molecules still together and when the water evaporates the molecule separate from each other.
|Because they don’t see nothing.