Identifying Unknown Metals

Purpose Your objective is t o determine the identity of unknown metals by calculating their densities.

BackgroundThe density (D) of a substance is the ratio of its mass (M) to its volume (V). The formula for density is:

density= massvolume

∨D=MV

You will be given samples of two or more metals and asked to identify the metals by calculating their densities and comparing the experimental values to a list of accepted values.

ProcedureDesign an experiment to determine the density and identity of the unknown metals as accurately as possible.

Safety Information

After reviewing your procedure, the instructor will discuss any safety precautions that are specific to your experiment.

Table of Accepted ValuesMetal Density (g/cm3) Metal Density (g/cm3)

Aluminum 2.7 Iron 7.89Zirconium 6.4 Nickel 8.9Zinc 7.14 Copper 8.92Chromium 7.19 Lead 11.34Tin 7.31 Platinum 21.45

Lab ReportA lab report should include the following sections:

I. TitleII. PurposeIII. MaterialsIV. ProcedureV. DataVI. CalculationsVII. Conclusions (including a discussion of sources of error

VIII. Answers to the Questions

Questions for Further Thought1. Is density an intensive or extensive property? Is density a chemical or physical property?2. Are the mass and volume of a given substance directly or indirectly proportional?3. Sketch a graph of mass versus volume for a given substance.4. How could density be used to determine the concentration of antifreeze in the cooling system of

an automobile?

Identifying Unknown Metals

Teacher NotesThe objectives of this lab are to introduce some of the characteristics of a good experiment, such as performing multiple trials, organizing and labeling data and calculations correctly, identifying sources of error, recording measurements correctly, the proper use of significant figures in calculations, and writing an organized lab report.

1. Encourage students to before they begin to design the best experiment possible. Explain that one of the goals of the lab is to develop an understanding of what constitutes a good experiment and that this is one of the aspects that you will be evaluating.

2. Have beakers of the unknown metals prepared and labeled for the students, identified as unknowns A, B, C, and so on. Tell the students they can use all or part of the sample.

3. Discuss with the students ahead of time using the water displacement method of to measure the volume of an irregular solid.

4. Have students pour used metal shot and water into a funnel with filter paper to avoid dumping them into the sink.

5. Students may initially be uncomfortable with designing their own experiment and seek constant approval and assistance. Be supportive and encouraging. Students will become more confident and independent with experience.

6. Students may not consider the following when creating their procedure:a. Doing several trials with each metal to minimize error.b. Trying more than one method to determine volume of the metal such as V = L x

W x H for a rectangular solid.

c. Discussing what sample size to use in order to increase accuracy due to significant figures. For example, using a sample size of 0.6 mL will limit the resulting density value to one significant figure.

TimeTime needed to design the experiment should be around 15 minutes if the students are familiar with the water displacement technique. Time needed to run the experiment should be 30 minutes.

Materials

Require the use of goggles, gloves, and aprons/lab coats. The following supplies should be readily available:

Metals (3 or 4 of the following metals): Zinc (strips, mossy zinc, 1” rectangles, etc.) Copper (shot, wire, strips, etc.) Aluminum (shot, wire, strips, etc.) Lead shot (see safety precautions) Iron nails

100-mL graduated cylinder Balance Weighing paper or dishes Funnel and filter paper Beakers

Common Misconceptions, Procedural and Calculation Errors1. Students might measure volume before measuring mass. This will affect the mass

measurement since the metal will be wet.2. Students might think that they have to use the same amount of metal, for example, the

same number of BBs, in each trial. This shows that students do not understand the proportional relationship between mass and volume for a given substance.

3. Students might use a sample size that is too small, for example one BB, causing the volume change to be too small to be accurate.

4. Students may measure volume by pouring a sample of dry BBs into a beaker or cylinder. This doesn’t account for the volume occupied by the air found in between the BBs.

5. Students may not realize that 1 mL = 1 cm2 and therefore will not know how to compare their calculated density to those given in the table of accepted values.

6. Students may determine the volume of the metal by performing water displacement in a beaker rather than a graduated cylinder.

7. Students might only perform one trial with each metal.

Lab Report

A lab report should include the following sections:I. TitleII. PurposeIII. MaterialsIV. ProcedureV. DataVI. CalculationsVII. Conclusions (including a discussion of sources of errorVIII. Answers to the Questions

Answers to Questions1. Density is an intensive property as well as a physical property.2. The mass and volume of a given substance are directly proportional. 3. A graph of mass versus volume would be a straight line that passes through the origin.4. Because the density of water and ethylene glycol (antifreeze) are different, the density of

the solution in the cooling system can be used to determine the concentration of the mixture and therefore how well it will protect against freezing. A hydrometer is used to determine the density of antifreeze solution in the radiator.