Problem 60 - Chemistry and Chemical Reactivity

Question

You are given a sample of a silvery metal. What information would you seek to prove that the metal is silver?

Step-by-Step Solution

Answer

Check density, melting point, and use XRF for confirmation.

Step 1: Identify Physical Properties

Determine the physical properties of the metal sample. Silver is known to have a bright metallic luster and is typically shiny and silvery-white in color. It is relatively soft and can be slightly scratched with a coin.

Step 2: Check Density

Measure the density of the metal sample. Silver has a density of approximately 10.49 g/cm³. Compare the measured density with that of pure silver.

Step 3: Verify Melting Point

Check the melting point of the metal. Silver melts at 961.8°C. If accessible, verify if the sample melts at or near this temperature.

Step 4: Conduct Chemical Tests

Perform chemical tests such as the nitric acid test. Silver will dissolve in nitric acid, producing silver nitrate and releasing nitrogen dioxide gas, evident by brown fumes.

Step 5: Use X-ray Fluorescence (XRF)

Use X-ray fluorescence spectroscopy to identify the composition of the metal. Silver will show a characteristic spectrum that confirms its presence.

Key Concepts

Physical Properties

Knowing the physical properties of a metal is a crucial step in identifying it. Physical properties are characteristics that can be observed or measured without changing the substance's composition. For a metal, these properties include color, luster, and hardness. Silver is notably known for its bright metallic luster, which gives it a shiny and reflective surface. It typically appears silvery-white and is softer compared to other metals. This softness allows it to be scratched with relative ease, for example, with a coin. Observing these properties will provide initial clues about whether the sample could be silver. Such a visual and tactile inspection can be the fastest way to get starting evidence about the identity of the metal.

Density Measurement

Measuring the density of a metal sample provides further evidence for identification. Density is defined as mass per unit volume, and for silver, it is approximately 10.49 grams per cubic centimeter. To measure the density, you must determine both the mass and volume of the sample. You can find the mass using a balance and the volume by submerging the metal in water and measuring the displacement. Once you have these measurements, calculate the density using the formula: \[\text{Density} = \frac{\text{Mass}}{\text{Volume}}\]Compare your calculated density with the known density of silver. If the values are close, this is good evidence supporting that the metal could be silver. However, always confirm with additional tests for complete certainty.

Melting Point

The melting point of a metal is another distinctive property that can help in its identification. Silver has a precise melting point of 961.8°C. To determine if the metal sample is silver, heat the sample and monitor the temperature at which it melts. If the sample melts at or very near to 961.8°C, it indicates that the sample could be silver. Testing the melting point requires precise equipment and controlled conditions, making it a method that is best performed in laboratory settings. Despite this limitation, it can provide a strong confirmation of the sample's identity when conducted correctly.

Chemical Tests

Chemical tests offer a powerful way to identify metals, including silver. One such test is the reaction with nitric acid. When silver is exposed to nitric acid, a chemical reaction occurs that dissolves the silver and produces silver nitrate, along with the release of brown nitrogen dioxide gas. This reaction is observable and gives a clear indication of silver's presence in the metal sample. The appearance of brown fumes during the reaction serves as a visible clue. It is important to conduct chemical tests with care, using protective equipment and under proper supervision, as they can be hazardous.

X-ray Fluorescence

X-ray fluorescence (XRF) is a sophisticated method used to determine the elemental composition of a sample. It works by exposing the sample to X-rays, which causes the elements within the metal to emit secondary (fluorescent) X-rays. Each element emits X-rays at characteristic energies, allowing for precise identification. For silver, X-ray fluorescence will show a distinct spectrum indicative of its presence. XRF is a non-destructive technique, which means it doesn't alter or damage the sample. This makes it a valuable tool for confirming the presence of silver, particularly when other testing methods are not feasible. The output is rapidly obtained and highly reliable, offering a lot of certainty in the identification process.