Lesson 1: 4.1 Introduction for Reactions

What macroscopic observations serve as definitive proof that a chemical change has occurred rather than a physical change?
How does the Law of Conservation of Mass dictate the coefficients used in a balanced chemical equation?

Chemical Change
Physical Change
Precipitate
Coefficient
Law of Conservation of Mass
Reactant
Product

Learning Objective 4.1.A; Suggested Skill 1.A (Models and Representations)

This topic establishes the boundaries between physical changes (retaining intramolecular composition, like phase changes or dissolving sugar) and chemical changes (breaking and forming intramolecular bonds to create new substances). Students learn to identify evidence of a chemical change (temperature change, gas production, precipitate formation, or unexpected color changes) and balance chemical equations to satisfy the conservation of mass.

Support: Use a balancing scale digital simulation or a color-coded physical balancing mat where students must physically match the number of atomic tokens on the reactant and product sides.
Extension: Provide students with a list of ambiguous processes (such as dissolving NaCl in water versus dissolving HCl in water) and have them argue whether the molecular transformations are best classified as physical or chemical.

Consider the two processes described below:

• Process 1: Solid iodine (I2) is heated gently in a flask, transforming completely into a purple vapor.

• Process 2: A solution of silver nitrate (AgNO3) is added to a solution of sodium chloride (NaCl), resulting in the immediate formation of a white solid.

1. Classify each process as a physical change or a chemical change.

2. Justify your classifications by describing what happens to the intramolecular bonds or forces during each process.