Introduction: Modeling the Chemistry Around You
One practice that people can use while they are learning to understand what is happening in our physical world is by developing, testing, and refining a model of what is happening based on the evidence that they observed. I do this with my students in my chemistry classes, but anyone can use the same process to better understand their surroundings in terms of what is happening with the matter and energy in the universe around them.
First I work with the the whole class to build a simple model together, then each time we investigate chemical reactions or thermodynamic situations in the lab, the students can design, test, and redesign their own models of the situations.
Step 1: How to Construct a Model of the Physical World
The components that I ask students to include in their model are:
- a diagram of the situation
- a legend indicating what you are using to represent specific particles an indication of the flow of matter
- an indication of the flow of energy
- a verbal description of how to interpret your model
I use a simple example of a situation with which all of the students will be familiar as the first example of constructing a model. One such situation is a cold soda can placed on a counter in a warm room where it will begin to "sweat." The question the model is designed to answer is, "Why does the outside of a cold soda can get wet on a hot day?"
I start with a blank screen each time I develop the model with the class, and add details and representations based on suggestions from the students so it looks different each time I create it. I have included an example of the visual portion of the model from one of those model creations.
Step 2: Student Model Examples of Cat's Meow (milk, Food Coloring, and Soap Interaction)
The first interaction I asked the students to observe was a very common experiment called the Cat's Meow. The physical apparatus is a shallow dish of milk, I used the bottom of a petri dish, drops of food coloring are placed in the center of the dish, and then a single drop of dish soap is placed in the center of the food coloring. I have found that Dawn dish soap gave the best results. Here is a link to some visuals of the experiment:
I presented it to the students as an investigation in which they compared the effect of the amount of fat in milk to the type of results they saw. The milk choices were 2%, whole milk, and half and half. I did not give the students any specific details about how many colors or food coloring to use or how to orient it in the center. I left those decisions to the student groups to optimize.
After observing several different trials, students created models based on their observations. I included some examples of the student models explaining what they hypothesized was happening on a microscopic level in their experiment.
Step 3: Student Models of Three Different Flasks
In this experiment, at the beginning of the chemistry course I asked students to combine chemicals in three different ways, and then compare and contrast their observations of the flasks with the idea that they can determine whether or not a chemical reaction occurred in each case.
The physical apparatus was the same in each situation. One reactant (ingredient) was measured into a small Erlenmeyer flask, I used 125 mL flasks, and the other inside a balloon. Then the balloon was stretched over the mouth of the flask and the reactant inside the balloon tipped into the flask. The balloon and flask then create a closed system to capture any products created during the interaction of reactants.
Flask 1: sodium hydrogen carbonate and acetic acid (a.k.a. baking soda and vinegar)
Flask 2: ammonium nitrate and water
Flask 3: calcium chloride and water
These reactions all use small amounts of the chemicals involved. For safety, students are all required to use chemical splash safety googles and lab aprons.
Step 4: Rubric to Evaluate Models
To keep the scoring of student models uniform and easily understood by the students, I use the same rubric throughout the semester. It is called a "claim, evidence, reasoning" rubric. The students are evaluated on whether they indicate the claim they were investigating, the evidence they collected/observed/measured during their investigation of the claim, and their reasoning explaining how the evidence relates to the claim.
Here is an excellent example of such a rubric:
We have a be nice policy.
Please be positive and constructive.