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Properties of Compounds

Investigation 3 – Lab

BE PREPARED

Supplies and Equipment:

Teacher Demonstration Materials:

92 g salt
160 g sugar
16 g baking soda
1 g cream of tartar
Ice cubes
2 ice cube trays
400 ml room temperature water
500 ml hot water
1 triple beam balance
2 thermometers
9 400 ml beakers
1 100 ml beaker
2 100 ml graduated cylinders
4 15 ml centrifuge tubes
4 weigh dishes
1 lab scoop
9 glass stir rods
2 liter pitchers
1 stopwatch

Teacher Preparation:

1. Prepare compound samples for each group, so that each group investigates two compounds. To do this, follow the table below and label the beakers with masking tape as you obtain the sample. Each group sample will contain some extra grams of the compound. This is to account for any spills or measurement errors that may occur.

2. Fill 2 liter pitchers with water. Allow water to reach room temperature.

3. For the first teacher demonstration, prepare 400 ml of ice water. To do this, fill one (1) liter pitcher with ice and water so that the temperature of the water is between 0°C and 8°C.

4. For the first teacher demonstration, prepare and label eight samples as follows:

Note: The cone portion of the centrifuge tube has additional markings from 0.5 ml to 1.0 ml. This scale should be used when preparing the falcon tubes with 0.4 ml of cream of tartar.

5. For the second teacher demonstration, fill a 400 ml beaker with 40 g of salt. This can be approximated by filling a 100 ml beaker to the 30 g line. Pour the salt into the 400 ml beaker and label the beaker as “40 g salt.”

6. Place materials at the distribution point as outlined in the table below.

7. Divide the class into five cooperative groups.

Instruction:

1. Assign each student group two compounds to test during this Investigation. Assign thecompounds as follows:

2. Direct each student group to obtain the following necessary materials from the distribution point: two (2) compound samples as assigned by instructor, one (1) liter pitcher, one (1) 100 ml graduated cylinder, one (1) stopwatch, two (2) glass stir rods, one (1) triple beam balance, 2 weigh dishes, two (2) lab scoops, one (1) 20 cm piece of masking tape, and paper towels. Instruct one student from each group to fill the liter pitcher with 400 ml of room temperature water from the liter pitchers at the distribution center. The water should be at room temperature.

Note: Sample data sheets for Trial 1 (sugar) and Trial 2 (salt) are included. The correct answers for cream of tartar, baking soda, salt, and sugar are summarized in a table in the KEYS section at the bottom of this page.

GET FOCUSED

Investigation Three introduces students to two factors that can influance the rate of solubility – temperature and stirring.

INVESTIGATE

1. During Trials 1 and 2, students will continue their exploration of solubility by investigating the effect of stirring on the two compounds that are assigned to them as shown on the following chart:

Note: Table salt contains silica to prevent the individual granules of salt from clumping together. Silica is not soluble in water. The silica may appear as a very fine, white film at the bottom of beakers after stirring. Any salt that does not dissolve will appear as clear granules at the bottom of the beaker.

Trials 1 and 2

2. Trial 1 will guide students in an exploration of the solubility of compounds. After adding 100 ml of water to two beakers containing one of the assigned compounds, students will stir the contents of one beaker while not stirring the contents of the other.

Trial 2 will guide students to perform the same procedure with two beakers containing the second assigned compound. By performing these trials, students will find that stirring increases the amount of compound that is dissolved in water.

In both Trials, students will find that less of the solute dissolves when water containing the solute is left alone and is not stirred. From these Trials, students should be able to conclude that stirring increases both the solubility (amount of solute that is dissolved in a solvent) and the rate of solubility of most compounds. These Trials are designed to encourage students to answer the questions:

How does stirring affect the solubility of compounds in water?

How does stirring affect the rate of solubility of compounds in water?

Do compounds with the same elements respond the same way to stirring?

Do compounds with the different elements respond the differently to stirring?

Trial 3

3. During this Trial, students will continue to investigate solubility by exploring the effect of the temperature of water on the solubility of compounds in water. To do this, students will observe two beakers of sugar, two beakers of salt, two beakers of baking soda, and two beakers of cream of tartar.

Note: Before beginning Trials 3 and 4, fill the remaining empty, liter pitcher with 500 ml of hot water from the faucet. The water should be between 40°C and 50°C.

Each beaker contains the same amount of compound that students used to explore solubility in Trials 1 and 2. As a class, students will add 100 ml of hot water to one beaker of sugar, salt, baking soda, and cream of tartar, and will add 100 ml of cold water to the remaining four beakers. Student volunteers will stir each beaker, and the class will come together to observe and compare each pair of beakers.

During this Trial, students will have opportunity to observe that increased temperature increases the solubility of all four compounds in water and increases the rate of solubility of each compound. Less of each solute dissolves in cold water than in hot water, and each compound dissolves more quickly in hot water than cold, allowing students to draw conclusions and answer the following questions:

How does temperature affect the solubility of compounds in water?

How does temperature affect the rate of solubility of compounds in water?

Do compounds with the same elements respond the same way to temperature?

Do compounds with the different elements respond the differently to temperature?

Trial 4

4. In this Trial, students will predict whether or not stirring and increased water temperature can always increase the solubility of a compound.

To test their prediction, students will observe a beaker that contains 40 g of salt. As a class, students will add 100 ml of hot water to the beaker. A student volunteer will stir the beaker for 2 minutes, and the class will come together to make observations.

During this Trial, students will have an opportunity to observe that for some compounds stirring and temperature only affect solubility to a degree. That is, if a solution has reached a certain level of saturation than even with stirring and a temperature increase, it may not be possible to dissolve all of the compound.

As students complete this trial, they should consider the following question: Can stirring and increasing temperature always increase the solubility of a compound?

5. Encourage students to summarize the results of their experiment by answering Problem 6 of their Student Data Record.

6. Upon completion of the experiment, permit enough time to clean up the lab and return the materials and equipment to their assigned locations.