Classification of Matter
The Classification of Matter DCI is the first activity I use in my first semester introductory chemistry class. This activity introduces students to the following terms: pure substance or mixture; gas, liquid or solid; atoms or molecules; elements or compounds; heterogeneous or homogeneous mixture.
The activity consists of a page with 20 containers depicting matter at the particulate level. Before doing the activity students are asked to complete a Before Class Exploration Activity. The BCE asks students to define a number of different terms (atom, molecule, element, compound, solution), which they can look up in a textbook or Google. However, copying a definition does not mean the student completely understand the meaning of the term. So Question 2 asks students to apply their understanding of the classification of matter by considering a set of five containers depicting matter at the particulate level. Question 2 gets students to differentiate elements and compounds, and pure substances and mixtures (solutions). The activity continues with two images of a particulate level representation of water in the solid and liquid phase. In Q3 students are asked to determine which image has the lower density, followed by a question about which phase each image represents. Finally students are asked to identify the symbol for each atom in the representation. The results are very interesting.
So student complete and submit the BCE prior to class where the During Class Invention activity is discussed. Students are given 10 minutes to come up with categories that can be used to group each of the twenty containers, by looking at a subset of the containers specifically and than scanning the remaining containers. Everytime I do this activity students are successful in generating all of the important ways of classifying matter: phase, pure substance or mixture, element or compound, etc. Time is also used to identify container(s) that depict a physical change and a chemical change. We even write a chemical equation for the pair of containers that can be associated with a chemical change.
Students demonstrate misconceptions for elements and compounds, compounds and homogeneous mixtures, writing formulas, and writing chemical equations. So as not to confuse students about particulate level diagrams students also view a simulation that depicts the dynamic nature of a mixture of gases (from mw.concord.org).
While some ask the question most students quickly connect a different sized circle, or a different shape of a circle with different atoms.
The contents of the different containers range from very straight forward, a pure substance in the form of an element in the gas phase to the more complicated, a pure substance subliming, an ionic compound in the solid phase or a heterogeneous mixture of two liquids.
Following class students are expected to complete an After Class Application activity which asks specific questions associated with what was just discussed in class. Some of the questions try to cover slightly more complicated examples of the content. The ACA provides new examples of the contents of two containers with choices of how to describe the container contents. There is also a question that shows a chemical reaction in a container at two different times and the students are to select from a list of possible chemical equations that best describe the reaction that is happening. The results on this 3rd question are very interesting. In Q4 student complete a table that contains information about three different elements to clarify the difference between an element's symbol, name, formula and phase. Finally in the last question students watch a video showing the macroscopic reaction between sodium solid and chlorine gas. The students are once again asked to select from a possible list of chemical equations the best equation that describe the reaction. The student responses are very interesting.