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Exploring Density

Investigation 5 – Lab

Back to Investigation 5

BE PREPARED

Supplies and Equipment:

Class Materials:

1 100 ml graduated cylinder 325 g salt
10 400 ml beakers
1 roll of masking tape
1 marker

Group Materials:

1 100 ml graduated cylinder
3 400 ml beakers
1 liter pitcher
1 triple beam balance
1 glass stir rod
1 acrylic cylinder
1 calculator
300 ml of water
3 paper towels

Individual Materials:

1 Student Data Record

Teacher Preparation:

1. Fill 5 400 ml beakers with 20 g of salt each. Label the beakers “20% salt solution.” This can be approximated by filling a 100 ml graduated cylinder up to the 15 ml mark and transferring the salt into the beaker.

2. Fill 5 400 ml beakers with 45 g of salt each. Label the beakers “45% salt solution.” This can be approximated by filling a 100 ml graduated cylinder up to the 35 ml mark and transferring the salt into the beaker.

3. Divide the class into five cooperative groups.

Instruction:

1. Direct each student group to obtain the following necessary materials from the distribution center: one (1) 100 ml graduated cylinder, three (3) empty 400 ml beakers, one (1) 400 ml beaker containing 20 g of salt, one (1) 400 ml beaker containing 45 g of salt, one (1) liter pitcher, one (1) triple beam balance, one (1) glass stir rod, one (1) acrylic cylinder, one (1) calculator and three (3) paper towels. Instruct one student from each group to fill their liter pitcher with 300 ml of water.

GET FOCUSED

Investigation Five gives students an opportunity to determine the density of various concentrations of saltwater.

INVESTIGATE

1. In this Investigation, students will examine the density of a salt solution. During Trial 1, students will observe the difference between the densities of an acrylic cylinder and a 0% salt solution (pure water). In Trials 2 and 3, students will test a 20% salt solution and 45% salt solution, respectively.

They will then compare the density of solutions to the density of an acrylic cylinder. As students perform the following Trials, they should discover that different amounts of a solute can be dissolved in a solvent. This will produce solutions that differ in density. As students conduct these three trials, they will focus on the following questions:

What happens to the density of water when a solute is added?

How can you determine the density of a solution?

How does the density of a solution compare with the density of a solid?

Trial 1

2. Trial 1: In this trial, students will calculate the average density of water. Some students may use a triple beam balance and graduated cylinder to measure the mass and volume of the water. They may then use these values to calculate density. Other students may simply refer back to the values that were calculated in Investigation Two. Students will then compare the density of water (0% salt solution) to the density of an acrylic cylinder. As students complete this trial, they will consider the following questions:

What is the density of water?

What happens when you place an acrylic cylinder into a 0% salt solution (pure water)?

Ask students to make a Prediction regarding whether or not the acrylic cylinder will float in the water.

Trial 2

3. Trial 2: During this Trial, students will calculate the density of a 20% salt solution. Students will use the triple beam balance and graduated cylinder to measure the mass and volume of thesolution. Density can be calculated from these values using the equation, density = mass ÷ volume.

Students will then place an acrylic cylinder into the 20% salt solution to compare the densities of the solution and acrylic cylinder. Students should discern that the density of the 20% salt solution is greater than the density of water. Students should also observe that the density of the solution is less than the density of the acrylic cylinder. As students complete this Trial, they should consider the following:

What happens to the density of water when solute is added?

What happens when you place an acrylic cylinder into a 20% salt solution?

Ask students to make a Prediction regarding whether or not the acrylic cylinder will float in the 20% salt solution.

Trial 3

4. Trial 3: During this Trial, students will calculate the density of a 45% salt solution. Students will use the triple beam balance and graduated cylinder to measure the mass and volume of the solution. Density can be calculated from these values using the equation, density = mass ÷ volume.

Students will then place an acrylic cylinder into the 45% salt solution to compare the densities of the solution and acrylic cylinder. By performing this trial, students should discover that the density of the 45% salt solution is greater than the density of water. Students should also discern that the density of the solution is greater than the density of the acrylic cylinder. That is, the acrylic cylinder will float in the 45% salt solution. As students complete this Trial, they should consider the following:

What happens to the density of water when solute is added?

What happens when you place an acrylic cylinder into a 45% salt solution?

Ask students to make a Prediction regarding whether or not the acrylic cylinder will float in the 45% salt solution.