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Exploring Density

Investigation 2 – Lab

Back to Investigation 2

BE PREPARED

Supplies and Equipment:

Class Materials:

5 400 ml beakers
1250 ml water

Group Materials:

1 triple beam balance
3 100 ml beakers
1 100 ml graduated cylinder paper towels
1 calculator

Individual Materials:

1 Student Data Record

Teacher Preparation:

1. Fill five 400 ml beakers with 250 ml of water

2. Organize the required materials at a distribution point.

3. Divide the class into five cooperative groups.

Instruction:

1. Direct each student group to obtain one (1) 400 ml beaker containing 250 ml of water, three (3) 100 ml beakers, one (1) 100 ml graduated cylinder, one (1) triple beam balance, one (1) calculator, and paper towels.

GET FOCUSED

Investigation Two introduces students to density calculations.

INVESTIGATE

Trials 1 through 3 provide students with an opportunity to determine the density of a substance through the use of the formula: density = mass/volume. In these Trials, students will use the graduated cylinder and triple beam balance to determine the mass and volume of three samples of water (30 ml, 50 ml, and 100 ml). Using these measurements, students will then calculate the density of the three samples of water by using the formula for density.

Note to Teacher: Through these trials, students will discover the direct relationship between the mass and volume of a substance. As the volume of a substance increases, so does its mass. As the mass of a substance decreases, so does its volume. In addition, students will discover that although the size of the sample changes, the density of the substance does not. This occurs because the density of a substance is the ratio of its mass to its volume. Therefore, as the volume of the substance increases or decreases, its mass changes by the same relative magnitude. For this reason, density is described as an intensive property of matter. An intensive property of matter is one that is independent of the size of the sample of matter. In addition, by conducting Trials 1 through 3, students will discover that the density of water is approximately 1 g/ml or 1 g/cm3, a value that scientists often use as a standard or reference point for the comparison of the density of other substances. Each trial performed will answer the question:

What is the density of water?

The ability of students to complete Trials 1 through 3 independently may vary. If necessary, perform Trial 1 as a class, encouraging students to work along with you step by step in measuring the volume of the 30 ml sample of water and in determining its mass. After both measurements have been made, walk students through the calculation of density from their measurements. After Trial 1 has been successfully performed, encourage students to complete Trials 2 and 3 on their own. Use the steps below if you choose to perform Trial 1 as a class.

Trial 1

The following procedure for Trial 1 is also the procedure for Trials 2 and 3, except that the volumes of water for Trials 2 and 3 are 50 ml and 100 ml, respectively.

a. Explain to students that they will determine the mass of 30 ml of water using the triple beam balance. They will then use the mass and volume to calculate the density of water.

b. Ask students: Can you determine the mass of the 30 ml of water by pouring the water onto the pan of the balance? Why or why not? Students should indicate that they cannot pour the water onto the pan because the water will run off of the pan. Instead, students should indicate that they need to put the water into some type of container, such as a beaker or weigh dish. Tell students that they need a container to hold the water but remind students that they must measure the mass of the empty container in order to determine the mass of only the water.

c. Instruct students to place an empty 100 ml beaker on the pan of their triple beam balance and obtain its mass. Students should record the mass of the empty beaker in Problem 3d of their Student Data Record.

d. Ask students to use a graduated cylinder to measure 30 ml of water.

f. Instruct students to calculate the mass of the water by subtracting the mass of the empty beaker from the combined mass of the water and the beaker. Encourage students to record the mass of the water in Problem 3d of their Student Data Record.

g. As a class, calculate the density of the 30 ml volume of water by dividing the mass of the water found in Problem 3d of their Student Data Record by the volume. Encourage students to record the density in Problem 3d of their Student Data Record.

Trial 2

In Trial 2, students will predict the density of 50 ml of water. They will then use a graduated cylinder and a triple beam balance to measure the mass of 50 ml of water. Students will use the formula, density = mass/volume, to calculate the density of 50 ml of water.

Encourage students to Predict the density of 50 ml of water. Students should record their predictions in Problem 4a of their Student Data Record.

Trial 3

During this trial, students will predict the density of 100 ml of water. They will then use a graduated cylinder and a triple beam balance to measure the mass of 100 ml of water. Students will use the formula, density = mass/volume, to calculate the density of 100 ml of water.

Encourage students to Predict the density of 100 ml of water. Students should record their predictions in Problem 5a of their Student Data Record.