What is Dissolving?

Why Explore Dissolving?

Because it's mysterious and interesting! Students learn about solutions in grade 7 but are not likely to acquire a good conceptual understanding. In grade 9 we focus on chemical and physical properties of elements and compounds, so exploring the process of dissolving is a great application of these ideas. The 9 curriculum does a terrible job of showing students that their science understandings are useful: in the context of science, "useful" means that their understandings can be used to generate new knowledge. Usually, ideas like physical or chemical properties are introduced in an encyclopedic way (i.e. in a lengthy list), presented with examples, and then dropped like a hot potato. Instead, we should deliberately use these ideas consistently after they are introduced. In this lesson, students use their understanding productively: they create a conceptual model for dissolving based on observations involving physical and chemical properties. Another bonus of this lesson is it helps students connect ideas from the kinetic molecular theory (KMT), which was also introduced in grade 7. In most student's minds, the ideas of the KMT, dissolving, and changes of state are a jumble because they have not consistently used good scientific reasoning, observations, and clear conceptual models to generate new knowledge from prior knowledge. This lesson (and the unit it is from) addresses these pedagogical problems. So let's explore dissolving!

Doing Science

The design of my dissolving lesson is inspired by the work of Eugenia Etkina (and others) at Rutgers University. She and her team have developed a fantastic pedagogical framework for science experiments that helped me understand why so many science experiments at school are both boring and unscientific. There is a lot I could mention here, but for now I will focus on the idea of "Testing Experiments". The purpose of a testing experiment is to discriminate between rival hypotheses that attempt to explain some phenomenon. This type of experiment is central to the scientific process for generating knowledge. You see, scientists don't this: "hmmm .... I think this mysterious phenomenon works like this. Here is my experiment that agrees with this. So, it must work like this." That produces a lot of bad "science". This is what they actually do: " hmmm .... here are all the possible ways this phenomenon could work that I can think of. Here are my experiments that eliminate all but one of these ideas. And here are follow-up experiments that show how the remaining idea explains lots of different things. So, it must work according to this remaining idea." That is good, proper scientific thinking.

Entertaining Multiple Hypotheses

In the dissolving lesson, students are given multiple hypotheses to consider that explain aspects of the dissolving process and multiple experiments to eliminate the excess hypotheses. This lesson is highly scaffolded: students are given the hypotheses and testing experiments. The lesson could be greatly enriched by asking students to generate both of these, however this is challenging and time consuming. Here is one example of what they are given:

Given this, students need to use each hypothesis to generate a prediction for the test. This involves a type of hypothetico-deductive reasoning that is challenging but very worthwhile (because science): assume the hypothesis is reliable, use it to predict what we will observe when the testing experiment is conducted, then repeat for the next hypothesis. Students are used to thinking of a prediction as an "educated guess" about what might happen or an expression of their opinion concerning which hypothesis is their favorite. No! It is a logical deduction based on a proposition new conditions, and everyone's should be essentially the same, with some wiggle room for the assumptions they might make. It takes practice to learn how to think this way. My students start this practice on day 2 of grade 9 science with the "Dancing Raisin" experiment. I have recently replaced this with a set of COVID-19 lessons that also introduce this model of scientific thinking.

9 Intro - Mad Science Skills.pdf

9 Workbook - COVID Lessons.pdf

Test Time

Once predictions are made, it is time to test! I have created videos for the three testing experiments, which you can find here. I particularly enjoyed making the first one. I measured the mass more carefully that I had done in the past and came across a measurement discrepancy! This gets scientists very excited. What's happening here? I added a follow-up testing experiment to try to figure out what happened to the missing mass. Does the mystery continue?