In this lesson students will collect data for a phase change graph by applying heat to ice water and graphing the results, and they will apply the scientific vocabulary associated with phase change to their graph. Students will use this experience and vocabulary in subsequent lessons to analyze how boiling points can vary between different liquids and to distinguish between chemical and physical change.
This lesson relates to the NGSS Science Practice 5: Analyzing and interpreting data as students gather a series of data points and then analyze them to see what they can understand about how temperature and phase changes take turns occurring. It relates to the NGSS Crosscutting Concept of Energy and matter by giving students an introduction to how adding energy into a system affects it.
Students may have some prior knowledge for this lesson. What may differentiate it from middle school content is the vocabulary, and the accuracy of their phase change diagram.
The materials you will need to do this lab are:
In this lab students will put ice water on a hot plate, record the temperature change every minute, and then graph the results. They will use this experience to reflect about the nature of phase change.
Mini-lesson: A student projects their cartoon for the class to observe. Here is an example of a student's phase change cartoon. Going through each square I use the technique of cold call to randomly call on students. For each square, a student can either agree with the square based on the depiction that they have, or the student can suggest a modification based on what the directions indicate.
Parts of the procedure that I make sure are covered are:
I remind students that one of the basic questions for today can be found at the top of the lab: If you heat a beaker of ice water until it is boiling, will the temperature change constantly or only at certain times?
I ask for opinions, but remind them that a scientist answers questions based on data, and it is time to collect some. Student opinions vary on this topic. Some students think that the change is gradual and constant because their cold drinks gradually get warmer, while others guess that it must be that temperatures change only at certain times, because otherwise why would we be asking the question!
Students conduct the lab. I walk around making sure that students are consistently stirring and that all students in the group are simultaneously recording data. I am very vigilant about safety issues. students need to wear safety goggles and pay attention to their personal space. Students need to be reminded to record data.
One of the challenges is getting an uncovered beaker to boil. Students will think that their water is boiling when they see bubbles, and that happens in the high 80's or low 90's, and this fact leads students to think that water boils at that temperature. I explain to them that while there are some bubbles, stirring the water will slow down the bubbles at these lower temperatures. I also show them that in fact water can get hot--up to 100oC. In this temperature countdown video, I am agitating some students who have been waiting and waiting to get their water up to that hottest temperature. I do it in about 2 minutes, and call out my temperatures as I do. This leads to an interesting conversation about how boiling is a form of evaporation, and evaporation is a cooling event. The hottest water molecules, the ones with the most energy, are constantly leaving the beaker, which means that it is difficult to heat it to its fullest potential. covering it makes it more difficult for the molecules to leave the beaker, and this barrier means that more of the most energetic molecules stay in the beaker, thus raising the temperature.
When all students are done recording their data I ask them to answer the first six conclusion questions.
At this point in the lesson I review the first six questions with students. These questions are designed to connect the students' experience at the lab bench to the theoretical underpinnings of phase change. My hope is that students come to appreciate that increased thermal energy leads to increased kinetic energy, which is actually what temperature measures. I project a student's graph and discuss these questions to provide schema for students. The answers I expect are found in the Phase Change Lab Answer Sheet.
Here is a typical student phase change graph from this experiment. While it is not exactly like the graph in our textbook, I do explain to students that it does approximate the graph, and that if we had ideal conditions, in which the water was constantly being stirred in a closed system, then the two graphs would be even more similar. Still, the fact that our graph stays relatively cold until all the ice melts and the evens out at the boiling point is a close approximation of the textbook example of a phase change graph.
I assign the remaining questions for homework and review them at the start of the next class, which will be a class that explores the boiling points of different liquids.