Solutions and Concentration

The solutions and concentration formative assessment (FA) task asks students to figure out what concentration means by comparing three solutions that they make. This FA targets structure-properties relationships because it intends to explore students’ thinking about what makes solutions behave differently. The FA focuses on the process of dissolution and the meaning of concentration.

In the FA task, students use the  Solutions and Concentrations sheet to guide their activity. They are first asked to sketch and explain what happens when CuSO₄ is dissolved in water. Then they are given three combinations of amounts of CuSO₄ and water and asked to predict which will be the most concentrated of these, and how they can tell. After this, they make the solutions and compare to their predictions. Finally, they consider two questions that further probe their thinking about concentration: how to tell which solution is most concentrated if salt were mixed into water instead of CuSO₄, and how they could make a solution at the same concentration of the most concentrated CuSO₄ solution if given a different amount of CuSO₄.

This FA task was tested with high school chemistry students who were in 11th and 12th grades. In the class in which it was tested, the students had recently finished a subunit in which they were studying moles. Earlier in the year, students had also studied the molecular structure of water and a little about crystal structures of ionic solids. In addition, the class had also talked a bit about why aqueous solutions conduct electricity when ionic compounds are dissolved in them, and they performed lab activities testing solutions for conductivity. This FA was situated just before the class began a unit on solutions and concentrations in relation to chemical quantities.

Teaching reflections

After going over the student work with another colleague I decided to make some changes in the FA in order to facilitate the process of students’ explaining their thinking.

Firstly, I decided to make clear my expectation that I want students to sketch the microscopic view of a solution. Although I clearly expected it, it was not clear from the directions, and students sketched a macroscopic view of the solution.

Secondly, I decided to change question 2 by asking students for a rule of formula that helps them with their prediction. This way I am hoping that they will start thinking about the concept of ratio as an indicator of concentration. Also, since most of the answers in question 2 were vague and not concrete, I am hoping that by using the term formula/rule students will be more focused in their thinking.

Finally, I moved the question about salt ( a colorless solution ) to the end, so that it doesn’t divert students’ focus from the solutions they created and makes it clear that we are talking now about a different case.

Examples of student work (names are pseudonyms)

Question 1: In this activity you will create three solutions by dissolving CuSO4 (a blue substance into water). What do you think happens when the solid is dissolved in water? Use both sentences and a sketch.
Tony	It will disperse and mix with the water and the ions will be spread so it can conduct.
Jacobi	I think that when they mix, the water will turn blue and the CuSO4 will dissolved.
Justice	Since Cu is a nonmetal and SO is a metal, it shows it’s ionic which means it will dissolve.

Question 2: In the first solution you will dissolve 1 g of CuSO4 in 100 mL of water. In the second solution you will dissolve 2 g in 200 mL of water.    And in the third solution you will dissolved 1 g in 50 mL of water. Which one do you think will be more concentrated?
Tony	I think 200 ml of water will be more concentrated because it has the most water and ratio of 200 ml and 100 ml of water.
Jacobi	The second [2 g in 200 ml] because it has less water and more substance. Also the other have more water but less solution.
Justice	Since 1 g is 100 ml and 2 g is 200 ml, for 1 g to get to 100 ml it will need to be timed by 2.