Temperature Measurement and Phase Change

  1. Introduction

    In this session, we watch ice melt, measure time, temperature and devise a hypothesis.

  2. Objectives

    Our objectives are to introduce the student to the concepts of measurement and record keeping by measuring temperature and time. We will introduce the scientific method, a recurring theme in this course. We will introduce graphing. Finally, we will briefly introduce the concept of phase change by melting ice and boiling water.

  3. Materials
  4. Vocabulary
  5. Presentation

    This may be the first class of this course so there are lots of things to do. This discussion covers only the course material. The instructor will have to weave all other classroom management tasks into this presentation. Maybe this will take two class sessions.

    As the students arrive, put a large amount of ice into a beaker. Put a little water in the beaker and then start heating the beaker with a Bunsen burner or electric hot plate. Place a thermometer in the water. The instructor should have tested this demonstration before class to find how long it takes to change the ice into steam. It should take less than half the class period (15 to 20 minutes). Adjust the amount of ice so the demonstration is complete within the allotted time.

    Have one student stir and read the temperature of the ice/water mixture at one minute intervals. Have another student record the time and temperature. Also record what is happening and the state of the water and ice within the beaker.

    While the ice/water mixture is heating, discuss the scientific method. The diagram from Chapter 0 might be helpful in this discussion. In preparation for this discussion, the instructor should read Wikipedia discussion of the scientific method as well as the University of Rochester discussion.

    Create a graph of temperature as a function of time on the blackboard. Annotate it with the state of the mixture. Emphasize the fact that we almost always analyze data with a graph. We often use horizontal axis to represent one changing parameter (often the one over which we have some control) and we use the vertical axis to represent the parameter indicating the system response. From a mathematical standpoint, we are graphing a function of one variable. The argument of the funtion is the independent variable (horizontal axis, abscissa, or x value) and the result of the function is the dependent variable (vertical axis, ordinate, or y value).

    Compare what we are doing with the flow chart of the scientific method. We are near the hypothesis section of the flow chart so encourage students to create a hypothesis about what is happening. Whatever the students hypothesize is fine because we will design experiments in the next step to test the hypothesis.

    If the students fail to notice the relationship of phase change and temperature, gently suggest that they are missing something and try to guide them into observing what happens. That is, as long as ice is present, the temperature of the water should remain at the freezing point. After the water begins to boil, it should remain at the boiling temperature as long as water is present.

    Spend some time talking about measurement (time, and temperature), its accuracy and the need for accuracy in the demonstration. Hint: We don't know how to measure the "quantity of heat" we are adding to the ice/water mixture. We can guess that we are adding some "quantity of head" at a fairly uniform rate. Also, the important features of this experiment are the two phase changes: solid to liquid, liquid to gas. Fortunately, there is one important thing we can measure while the phases are changing. The important thing is the temperature of the water.

    Have the students devise some experiments to test their hypotheses. Guide them in evaluating the experiments. The students will probably never have an opportunity to do these experiments in a classroom setting so set a good spirited example of how to helpfully criticize experimental design. Invite other students to offer helpful suggestions (criticism) of experimental designs. Revisit measuring time and temperature and the need for accuracy in an experimental setting. Estimate how accurate the measurements must be for the experiments.

  6. Evaluation

    Devise a short problem set that reviews the basic ideas of phase change and the change in temperature when a substance is heated, both in a single phase and when a phase change is occurring. Due next meeting. Below is a suggested framework for homework questions: