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Solubility Unit
Introduction:
In this unit the students will learn about solubility. Students should already be familiar with the basic chemistry concepts. They should know that some substances are soluble in water and others are not. The unit begins with a review of solubility and terms such as solvent and solute by way of a class experiment. The experiment demonstrates three ways to decrease the time it takes to dissolve sugar in water (stirring, increasing the temperature of the solvent, and increasing the volume of the solvent). Students will also compare how quickly sugar dissolves in different solvents. On Day 2 they will use data that is provided to show how temperature affects the solubility of the salt, ammonium chloride. As a part of this lesson the students will review graphing (L 1, 2, and 3) and unit rate of change/slope (L 2 and 3) and linear equations (L3). On Day 3 and 4 the students will conduct an experiment in which they compare the rate at which sugar and salt can be dissolved in a given amount of water.
ILS Core Curriculum – Major Understandings Standard 4 Physical Setting 3.1a, 3.1b, 4.2e
Objectives:
After completing this lesson students will be able to:
Define solute, solvent, solution, soluble, insoluble and solubility. Describe three ways to increase the rate of dissolving. Collect data. Create and interpret a graph depicting the change in the rate of dissolving of salt with changing temperature. Describe how temperature changes affect dissolving rate. Compare rate of dissolving for various solutes.
Day One - Observing Dissolving Rate
The lesson begins with a demonstration of factors that affect the rate at which a given substance will dissolve in a given volume of solute. In this case you should use sugar and water. Give the students Worksheet #1. Have 4 beakers and eight containers with 1 g of sugar available for the experiments. Add 200 ml of room
temperature water to the first beaker and ask the students to predict how long it will take to dissolve 1 g of sugar in the water. Write down the range of “guesstimates.” Add the sugar, start a timer and show the students the crystals of sugar sitting at the bottom of the beaker.
Begin reviewing the concepts the students have already learned and given new definitions: solute, solvent, solution, soluble, insoluble and solubility. Check the beaker occasionally. It will take more than 20 minutes for the sugar to dissolve.
After 10 minutes start the rest of the demonstration. Show the students that the sugar has not dissolved and ask them what you could do to speed up the rate of dissolving. They should be able to come up with the following answers: stir the water, increase the temperature, and increase the amount of water.
[Alternately, you may challenge the students to a contest. Give groups of students a large, pre-weighed (~ 10 g) amount of salt, sugar or other solute of your choice and a container (all containers must be of equal size). The goal is to see which group can dissolve the most solute in a given amount of time (5-10 minutes). Have available stirring rods, running hot and cold water. Let the students try to dissolve as much of the solute as possible in the specified time. At the end of the contest time have each group weigh their remaining solute. Penalties should be given for undissolved solute left in the container. The group that dissolves the most solute wins! Follow-up by having the students discuss the methods they used to increasing the rate of dissolving. Ask the students why this was not a scientific experiment. They should be able to recognize two problems with the experiment:
- more than one variable was changed at the same time and 2) final measurements were imprecise because undissolved solute already in the container could not be measured.]
Conduct the following experiment. Divide the students into 3 groups: STIR, HEAT, and VOLUME. Give each group a container with 1g sugar. STIR group will get 200 ml room temperature water and a stirring rod. HEAT group will get 200ml hot tap water (measure this last). VOLUME group will get 400 ml of room temperature water. Students must wait to add sugar to their water on your signal.
(Continue to check the original sugar water beaker and note the time when it has completely dissolved.)
Continue your discussion of solubility. Include a discussion of the properties of solutes and solvent that increase solubility (size of solute and solvent molecules, whether or not the molecules come apart (ionize), etc.) Also ask the students how a solute can be separated out of the solution. Show the class a beaker containing water from the beach. It should contain sand as well. This will provide the opportunity to compare soluble and insoluble materials. Ask students to brainstorm in small groups a way to separate the sand, salt and water. Ask what can be done to separate another mixture, such as salt, iron filings and water.
Level 1 Finish class by creating a solubility graph using Solubility Worksheet #2. Level 1 students may need help constructing and scaling the axes. The students’ graphs should resemble the graph below. Note that the solubility curve for ammonium chloride is nearly linear in the temperature range chosen. Different salts produce less linear data. You may want to show your students other solubility curves such as those below.
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0 20 40 60 80 100 120
Maximum
mass
dissolved
(g)
Temperature ˚C
http://galileo.phys.virginia.edu/education/outreach/8thgradesol/TempSolubility. htm
QUESTION OF THE DAY:
Have students draw a diagram of the beaker containing sand and salt water that was used earlier in the class. Given a word bank on the board, have students identify the solvent, solute and solution. Which material is soluble? Insoluble?
directed to compare their extrapolations that were performed mathematically to their graph.
Students should be able to finish the graph for homework and complete the analysis questions if necessary.
QUESTION OF THE DAY: Have students draw a diagram of the beaker containing sand and salt water that was used earlier in the class. Given a word bank on the board, have students identify the solvent, solute and solution. Which material is soluble? Insoluble?
Using the graph you finished on Worksheet 2, write an equation for the line. Depending on student ability you may provide formula for the class.
Day 3 and 4: Solubility Lab
Overview In this lab the students will compare the dissolving rate of salt and sucrose in water at room temperature. They will do this by collecting data on the time needed to dissolve each substance one gram at a time in the water. Solubility is the total amount of a certain substance that can dissolve in a specific amount of solvent at a given temperature. This is difficult to measure because determining the end amount before saturation is a difficult process. You should explain to the students that they are not measuring solubility.
Objective: To collect and record data To construct a dissolving rate curve To compare dissolving rate curves of two solutes
Materials: (per group) 2 - 500 ml beakers Cup/container Sugar and kosher salt measured in 1 gram amounts (pill cups make good containers) Stirring rod A stopwatch
Procedure: Students should work in groups of 2-4. Have each group of students test both solutes. Students will follow Solubility Worksheet #3. Students will add the solute, 1 gram at a time, to the water. Each gram must totally dissolved before the next is added and the students will keep a running time log of total number of minutes needed to dissolve 1, 2, 3 and 4 gram of each solute in the 100 ml of water. They will dissolve by stirring for 15 sec, after which they stop and let the solution settle so they can check for undissolved solute in the bottom of the beaker. If the added solute has not totally dissolved, they should continue stirring for additional 15 sec intervals until all of the added solute is dissolved. Then they can add the next gram of solute. Salt should dissolve within 3 minutes and sugar will take about 5 minutes
When all of the data are collected, the students should be directed to graph their results plotting the grams of solute on the X-axis and the time on the Y-axis. The amount of direction given will depend on the level; less direction should be needed because the students now have practice from Worksheet #2. In this experiment, solute is added in discrete amounts but will dissolve as a continuous series of amounts of solute. Therefore, it will be appropriate to draw a best-fit line through the data points for each solute. The students should add a key to distinguish the different solutes lines on the graph. The students can then compare the line for steepness at Level 1, for unit rate of change at level 2 and as a linear function at Level 3.
Solubility Unit Assessment – Day 5 Administer assessment (Solubility Worksheet #4).
