formative assessment

6. What determines the outcomes of chemical changes?

“What determines the outcomes of chemical changes?” is a question of dynamic behavior and probability. When the particles that make up different substances interact with each other, their interactions may lead to a variety of structural changes (e..g., some atoms may change positions or separate from a molecule). Which of these random changes are more likely to occur depends on the relative potential energy between the original particles and the new particles that are formed. More stable particles (with lower potential energies) are more probable to form and will determine the path and outcome of the reaction. Understanding the relationship between structure, stability, and reaction mechanism allows us to predict, explain, and control the products of chemical processes.

1. What types of matter are there?

“What types of matter are there?” is a question of classification. Classification is a very important tool for predicting and explaining the properties of substances in our surroundings. For example, classifying a material as a metal versus a nonmetal allows us to predict that it may conduct heat and electricity quite well. Similarly, identifying a substance as an ionic compound allows us to explain why its aqueous solution conducts electricity. Classifications are often loose categories with gray areas, but they support chemical thinking when seeking to synthesize new substances, determine the identity of a material, or control a chemical process.   

3. What properties of matter types emerge?

“What properties of matter types emerge?” is a question of the origin of properties. Predicting or explaining properties of substances often requires analysis of structural rather than compositional aspects of substances, and involves reasoning about emergence rather than arguing based on a central cause. Explaining behaviors of substances involves examining what influences energetic stability and how behaviors on one distance scale emerge from dynamic interactions between structural components on a smaller scale. There are many different scales at which these structure-property relationships are built (from multiple entities in mixtures down to electronic structure). This chemical thinking question is often central to predicting properties of substances, e.g., which oil is best for lubricating a transmission or frying plantains or making soap.

The Rain Puddle

A formative assessment designed by an ACCT cohort member designed to investigate student understanding of chemical mechanism.

2. What cues are used to differentiate matter types?

“What cues are used to differentiate matter types?” is a question of differentiation, or telling matter types apart. Differentiation in chemical thinking is based on the assumption that every chemical substance has at least one differentiating property that makes it unique. Good differentiating properties do not depend on the amount of substance under analysis and have unique values for different materials. Examples include boiling points, solubilities in water, and molecular structure. The characterization of these differentiating properties is critical for the design of methods to separate substances, identify them, detect them in our surroundings, or quantify their amounts.