Lesson 7 of 11
Objective: Students will be able to define solubility as well as name and demonstrate some of the factors that effect a substance's rate of dissolving.
Solubility is an important topic in chemistry. Many reactions happen in solutions. This lab-based lesson is designed to help students experience the fact that how and whether a solution can be made is determined in large part by the chemical composition of the solute and solvent. How fast the solution can be made is determined in part by the temperature at which the mixture is created.
Studying solubility also gives students another lens with which to peer into the nanoscale. A discrepant event occurs for many students when they discover that adding a certain volume of solute does not necessarily increase the volume of the solution by a corresponding amount. They also see evidence of particle movement.
My students have some prior knowledge about different scales, as they have just completed a lesson on metric prefixes and scale. However, prior knowledge of this type is not necessary and this lesson can stand alone as the concepts presented are straightforward and the mini-labs reinforce the concepts nicely. All students will benefit from being able to think about substances at the different scales.
This lesson incorporates the NGSS Crosscutting Concept of Structure and Function by relating the structures of substances at the nanoscale to solubility, an observable macroscale property. Students learn about solubility through the practice of carrying out investigations.
The following materials are required to do these labs:
3 cups or 250 ml beakers
100 ml of hot water, room temperature water, and ice water
food coloring (3 different colors )
Mini-lab # 2:
25 grams of sodium chloride (NaCl)
A 100 ml graduated cylinder
An electronic balance
50 ml of water
Mini-lab # 3:
1 Alka-Seltzer tablet
1 electronic balance
Mortar and pestle
2 100 ml beakers
Mini-lab # 4:
15 grams of calcium carbonate
15 g of baking soda
15 g of magnesium sulfate
mortar and pestle
100 ml graduated cylinder
250 ml beaker
Students work alone by reading an excerpt from a text similar to this website with the goal of recording some key vocabulary and definitions related to solutions. This warm-up activity should also give students time to think a little bit about solutions and the vocabulary used to describe them. They record their work in Part 1 of the Solubility Notecatcher. I use this time to take care of administrative tasks such as taking attendance, and to check-in with students who may have been absent.
After students have had a chance to so some reading and recording, I project the Solubility Notecatcher Answer Key for students to compare their notes and ask questions.
This is an important part of the lesson--students use the vocabulary during the rest of the lesson. This copy of a student's Solubility notes was typical of what student's produced and it captured the important vocabulary I hoped they would get from this part of the lesson.
I briefly discuss each of the mini-labs in the handout Solubility Minilab, but I do not go into too much detail. The directions are straightforward, and one of our class’s safety rules is when in doubt ask the teacher. Here is what I say:
"For the first lab, you will be putting a drop of water into three different cups that each have a different temperature. You will record what happens in one-minute intervals, being careful not to jostle the cups.
In the second lab, you will be doing a few things. See how adding a known volume of salt contributes to the volume of water, and see how much salt you can add to 100 ml of water. Note that I figured out how much 1 ml of salt was by looking at its density. 1 ml of salt has a mass of 2.16 grams.
In the third lab, you will compare how quickly a crushed tablet dissolves compared to an uncrushed tablet. You will know it is dissolved when the bubbles are no longer being created.
In the final lab, you will see how well different substances dissolve in water.
What questions do you have about any of the labs?"
To avoid some obvious questions, I make sure that students understand where all of the materials are located prior to the start of the lab. I also make sure that students know what group they are in, and where each group works in the lab. Groups share hands-on activities, but a class rule is that every student must record data and answer questions in real time; in other words, students should not wait to copy data later.
During this time students conduct the various mini-labs. I compliment team work, adherence to the procedures, and I stop class if there is a safety violation or common confusion about a procedure. The Teacher's Guide in the reflection section of this lesson offers some additional comments and breaks down some of the choices I made when designing this lesson.
At this point in the lesson I project my answer key to the second part of the Solubility Notecatcher, giving students the chance to volunteer and record answers. Temperature, surface area, and possibly concentration were fairly easily observed. Here is a solubility debrief video that shows how students and I debrief each of the labs.
I explain that chemical composition is not obvious from simple observation. Knowledge about how the substances are made at the nanoscale make this an easier concept to comprehend, and that will come later in the course. students took notes during the debrief section as evidenced in this student's post-lab questions.
The question of why adding 1 ml of solute not increase the solvent by 1 ml may not be immediately apparent to students. If there is time I let students discuss this using a think-pair-share protocol, or if time is short I provide them with the answer by showing them a dissolving particles slide while explaining that at the nano scale there is a lot of empty space, and the salt can actually fill in some of this empty space. I note that we do not observe these empty spaces at the macroscale because they are too small.