We're going to be honest: we do Physics. That's why this is called The Physics Classroom website. And when we do the Teacher's Notes section for our Concept Builders, we typically have a lot to say ... and a lot of resources to point you to. We're not claiming to be ignorant of chemistry; we just don't have a lot of resources here at The Physics Classroom to point you to. And so this page is going to be a lot shorter than our usual page that accompanies our Physics Concept Builders. That's our honest confession.

Most Chemistry courses have a unit on the topic of reversible reaction systems and chemical equilibrium. A common goal of such units includes developing the ability to write the equilibrium product expression for a given reaction. This Concept Builder addresses that goal. Students are provided a balanced chemical reaction and must identify (from a set of 5 to 8 answer options) the correct equilibrium constant expression. The state of every reactant and product is given. Coefficients are also given. Students must apply the law of mass action in order to identify the proper expression for the given reaction. The typical problematic areas are addressed by the answer options. Such problems include including solid and liquid concentrations in the expression, failing to raise concentrations to the coefficient power, inverting the ratio of products over reactants, and confusing the subscripts of formulas with the role that is served by the coefficients in the chemical equation. All equilbrium constant expressions are K_c expressions where the concentrations (not pressures) are included in the expression.

There are three difficulty levels that can be engaged in through the Concept Builder. Those three difficulty levels are differentiated as follows:
 

• Apprentice Difficulty Level: Question Groups 1-4 ... Each question includes a chemical equation. Learners must identify the correct equilibirum constant expression from the five to eight possible answer options.
• Master Difficulty Level: Question Groups 1-8 ... Each question includes a chemical equation. Learners must identify the correct equilibirum constant expression from the five to eight possible answer options. Four of the Question Groups are the same four Question Groups from the Apprentice Difficulty Level.
• Wizard Difficulty Level: Question Groups 5-12 ... Each question includes a chemical equation. Learners must identify the correct equilibirum constant expression from the five to eight possible answer options. Four of the Question Groups are from the Master Difficulty Level.

Teachers are encouraged to view the questions in order to judge which difficulty levels are most appropriate for their classes. For teachers using this Concept Builder with their classes, we recommend selecting the number of questions that you wish students to do - either four, eight, or 12. Also decide on the difficulty level of questions. We believe that the questions in the Wizard Difficulty Level are more difficult than those in the Apprentice Difficulty Level. So for a teacher wishing to provide eight difficult questions, they should assign the Wizard Difficulty Level. If you wish your students to have eight easy to moderate questions, then assign the Master Difficulty Level. Select both the Apprentice and the Wizard Difficulty Level for the experience of 12 questions. The questions can be viewed on a separate page. 

There are two similar questions in every Question Group. If a student misses a question from within a particular Question Group, then they will have to answer two different questions correctly (without any further miss) from that same Question Group. This strategy provides students extra practice on their most troublesome questions.

Like all our Concept Builders, this Concept Builder utilizes a variety of strategies to make each student's experience different. The ordering of questions is random. The Question number assigned to each question is scrambled. For instance, two side-by-side students will not have the same question for question number three. And questions are organized into "groups" with questions within the same group being very similar (for instance, they have the same type of representations) but not identical.

The Concept Builder also keeps track of student progress. It requires that students demonstrate a mastery of questions in each Question Group. If they miss a question from one group, then they will have to answer two consecutive questions correctly in order to demonstrate mastery. Progress is displayed in the progress report on the right side of the Concept Builder. A star indicates a demonstration of mastery. A question with a red background indicates that the student has missed the question. And a question with a yellow background means that the student must get one more question from that Question Group correctly answered in order to obtain a star. When an activity is completed, the student will be awarded a Trophy. This Trophy is displayed on the Main Menu screen. These strategies make the Concept Builder an ideal addition to the 1:1 classroom and other settings in which computers are readily available. 


In order to complete a difficulty level, a student must correctly analyze each question of that difficulty level. If a student's analysis is incorrect, then the student will have to correctly analyze the same or very similar question twice in order to successfully complete the activity. This approach provides the student extra practice on questions for which they exhibited difficulty. As a student progresses through an activity, a system of stars and other indicators are used to indicate progress on the activity. A star is an indicator of correctly analyzing the question. Once a star is earned, that question is removed from the que of questions to be analyzed. Each situation is color-coded with either a yellow or a red box. A red box indicates that the student has incorrectly analyzed the question and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the question must be correctly analyzed one time in order to earn a star. Once every question of a difficulty level has been analyzed, the student earns a Trophy which is displayed on the Main Menu. This system of stars and trophies allows a teacher to easily check-off student progress or offer credit for completing assigned difficulty levels.

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise.