Key Concepts: Aquatic Solutions; Cell Membranes--permeability & health/illness; Charge; Dynamic Equilibrium; Diffusion & Osmosis; Iso-osmotic (physiological) solutions; Transport (mostly passive) of materials through membrane.

Goals

Research goals:
This module is designed as a research tool. The goal of the research is three-fold: 1) to learn about the conceptual framework students use to explain atomic-scale phenomena; and 2) to assess the effectiveness of Molecular Workbench materials, both software and activities, by evaluating students reasoning with these models about phenomena they have not been taught; and 3) to research whether students are able to reason about the models themselves.

Learning Goals:
In this Solutions unit students will discover that there are special forces that contribute to the particular ways materials act in solutions. Molecules in liquids no longer act freely, as they do in a gas, but are constrained by strong and weak forces. Over the course of this mini-unit, students will be learning about the way water flows from the dilute side of a semi-permeable membrane to the concentrated side, and implications for the maintenance of the proper flow. Students will develop their model of materials in solution in a living cell.

The module is focused on the essential ability of water to dissolve and transport some substances and not others, and on the molecular properties underlying these phenomena. Students will be asked to reason using the kinetic molecular theory. The power of the kinetic molecular theory in understanding matter in solutions lies in helping students understand that matter does not simply disappear when it is dissolved, but rather remains in the form of hydrated ions and hydrated larger molecules. The central learning goal of this module, then, is to research whether these tools help students identify what happens to matter in solution, help them describe the role of water in hydrating matter, and help them to understand the molecular aspects of this process.

Those atomic (and molecular) scale phenomena are:
1. Aquatic solution (we do not deal here with gaseous solutions, or with organic materials dissolved in organic solvents, therefore we need to stress the water as a solvent).
2. The process of dissolving of inorganic (ionic) compounds (table salt) and organic compounds (sugar) in water.
3. The process of diffusion of dissolved materials in water.
4. The process of distribution of solvent (water) and dissolved materials when a container with previously freely diffusing materials is separated with a semipermeable membrane.

We expect students to be able to understand that water molecules can pull apart solids, ionic compounds, into charged atoms, or ions, (dissociation) and form a water shell around the ions (hydration), thereby dissolving them and moving them. They will also understand that ions disperse through a solution until they reach equilibrium, and that a semipermeable membrane acts to let through only ions of a certain size. Finally they will understand that after water hydrates ions with watery shells, free water in the region is reduced, thus creating a gradient towards which new water will flow. This dynamic movement in response to the concentration of ions has many implications in human biology.

By the end of the unit students should understand that the polar nature of water, dissolving and diffusion of dissolved materials and osmosis are the processes leading to the reaction of an erythrocyte to the changes in the salinity in surrounding water. Modeling this process should help students understand the role of a semipermeable membrane in controlling the flow of water and dissolved materials in and out of the cell. They will read several case studies that explain implications for health malfunctioning of a cell's membrane.

Prior Knowledge and Naïve Conceptions

Without an understanding of the bipolar nature of the water molecule and the hydrogen bond it makes with other water molecules and with other matter dissolved in water, students cannot grasp the nature of solutions. This molecular level understanding is easier to acquire with experience with dynamic models. This activity presents a sequence of models in which students can develop an intuition of the dissolving process and the transport of materials in solution across a membrane.

Students who distinguish pure substances from mixtures based on observable properties of substances may have difficulty understanding solutions. Based on color, taste or other observable properties some students may think that the water we get from a faucet is a pure substance.

Listed below is a set of concepts and the way students and scientists think about these concepts:

Issue: Conservation of Matter
Scientific: Matter is conserved in changes
Student: Matter not always conserved. The word "dissolve" sometimes is used as a synonym for disappear.

Issue: Size of molecules
Scientific: Molecules are too small to see
Student: Molecules are visible (e.g. crystals or "wavy lines" of sugar as sugar molecules.

Issue: Constant motion
Scientific: All molecules are constantly moving
Student: Molecules my sometimes be still ( e.g. sugar molecules go down and stay at the bottom of water.)

Issue: Different kinds of molecules
Scientific Molecules of one substance are different from molecules of another substance.
Student: All molecules are alike

Issue: Pure substance vs. mixture
Scientific: Pure substances are made of one kind of molecule; mixtures, two or more kinds of molecules.
Student: Distinction based on observable properties of substance, such as color, texture, taste etc.

Issue: Molecular explanation of dissolving
Scientific: Molecules of solute break away and mix with molecules of solvent.
Student: focus on observable substances or molecules themselves dissolve.

Some students think that sugar crystals are actually molecules.
Crystals of sugar are cube-shaped because of the rigid array of (trillions of) molecules in solid sugar.)

About Models

This module makes extensive use of multiple representations of atomic level models, including macro-micro models. The activities in the unit are organized around the development of specific science concepts but simultaneously students will be asked to analyze the models they are learning from. Students need to learn that models are tools and that they must be wrong in some respects or else it would be the thing itself. The trick is to see-with the help of a teacher-where it is right.

Unlike most models students think of (small versions of visible things), chemistry's tangible models are large metaphors for small, invisible things. Take, for example, a ball-and-stick model. To a chemist a molecule doesn't look like this model. Rather the model expresses our insight into the nature of the molecule. Joining the sticks and balls helps us visualize and learn about that molecule.

In this module students construct their own models. Students will:

- run a model and analyze its reasonability and fidelity by comparing model outputs to a target phenomena;
- understand the behavior of a model by varying parameters, testing limits, and examining the interrelationships between variables;
- apply model-based reasoning by grappling with a variety of explanations and phenomena.

In understanding the models students will need to reason using the key scientific concepts from this unit. It is our belief that students will gain a greater understanding of the concepts they will be studying and a facility with creating and understanding their own models as a result of these activities.

---

INDEX