Activity 1: Density, A Straight Line Function

Introduction

The density of a pure substance does not depend upon the amount of that substance present. Like its temperature or melting point or color, density is an intensive property of a liquid or solid. A ton of lead has the same density as a milligram of lead (11.36 g/cm 3 ). Common units for density are grams per cm 3 for solids, grams per mL for liquids, and grams per L for gases. In this activity we will work with solids.

Purpose

The formula for density will be developed from experimental data by graphical interpretation. To calculate the densities of aluminum metal and iron metal by two methods and compare the methods.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. Use normal safety precautions with this experiment.

Work in pairs to accomplish the following steps:

1. You will be given 6-8 different-sized metallic objects. Tie a thread to each metallic object as indicated by your instructor. This will enable you to hang the object from the balance. Determine the mass of the metallic object as it hangs from the balance, and record the mass in the air to the nearest 0.01g.

2. To find the volume of each object, there are two possible procedures, the volume displacement method and the ArchimedesÕ weight displacement method. If you choose or are instructed to use the volume displacement method, go to Step 3. If the ArchimedesÕ weight displacement method is selected then go to Step 4.

3. Add enough water to the graduated cylinder so that the largest metallic object can be wholly submerged. For each metallic object take an initial water level reading to the nearest 0.2 mL, and record. Carefully lower the metallic object by its thread into the water in the graduated cylinder until it is completely immersed. Read the water level in the graduated cylinder and record to the nearest 0.2 mL. Be sure to take new initial and final readings with each metallic object. Figure 1 is a sample data table to be completed.



Figure 1. Data table for volume displacement method.

4. The ArchimedesÕ weight displacement method is easy to accomplish using a balance equipped with a platform (Figure 2). Add water to a 250-mL beaker and then place the beaker on the platform. Hang the object from the balance by the thread so that it is completely submerged in water. Take care not to let the metallic object touch the sides of the beaker. Read the balance to the nearest 0.01 g and record the data in your data table (Figure 2). Repeat this procedure for each metallic object. STUDENT NOTE: ArchimedesÕ principle states that a body partially or totally submerged in a liquid is buoyed up by a force equal to the weight of the displaced liquid. This accounts for the apparent loss of mass. The density of water is 1.0 g/cm 3 . If an object displaces 25-mL water, the apparent loss of mass is 25 g. Consequently the difference between mass in air and mass in water is equivalent to thevolume of water displaced and the unknown volume of the object itself.

Figure 2. ArchimedesÕ weight displacement apparatus.

Figure 3. Data table for weight displacement method.

7. Make a graph of the mass of the object in air vs. the volume of the object. Record mass along the labelled vertical axis and volume along the labelled horizontal axis. Plot all 6-8 data points on one graph. Give the graph a title; follow accepted graphing procedures.

8. Thoroughly wash your hands before leaving the laboratory.

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