Problem 46 - Chemistry and Chemical Reactivity

Question

You are given a sample of a silvery metal. What information could you use to prove the metal is silver?

Step-by-Step Solution

Answer

Check visual appearance, calculate density, test conductivity, perform chemical tests, and use XRF analysis to verify if the metal is silver.

Step 1: Visual Inspection

Examine the physical appearance of the metal. Silver has a distinctive, shiny, silvery-white color. Check for signs of tarnish, which can indicate oxidation, as silver tends to tarnish over time when exposed to air.

Step 2: Density Check

Measure the mass and the volume of the metal sample to calculate its density using the formula \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \). The density of silver is approximately 10.49 g/cm³. Compare the calculated density to this value to see if they match.

Step 3: Conductivity Test

Test the electrical conductivity of the metal. Pure silver is the best conductor of electricity among metals. Use an ohmmeter to measure the resistance across the metal; lower resistance indicates higher conductivity consistent with silver.

Step 4: Chemical Test

Perform a chemical test using nitric acid and potassium dichromate. Silver will dissolve in nitric acid, forming a clear silver nitrate solution. Adding potassium dichromate to this solution will cause a red solid (silver chromate) to precipitate if the metal is silver.

Step 5: Use X-ray Fluorescence (XRF)

Use X-ray fluorescence (XRF) analysis to determine the composition of the metal. Silver will show strong peaks at specific wavelengths characteristic of silver.

Key Concepts

Density Measurement

To identify a silver metal, one common method involves measuring its density. Density is defined as mass per unit volume, expressed using the formula: \[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]Silver has a known density of about 10.49 g/cm³. To determine if your sample is silver, follow these steps:

• Measure the mass of the metal using a precise scale.
• Determine the volume by displacement of water or by using geometrical calculations, if the sample shape allows.
• Calculate the density using the measured mass and volume.

Compare your calculated density to 10.49 g/cm³. A close match suggests the sample could indeed be silver.

Electrical Conductivity Test

Electrical conductivity tests can help confirm if a metal is silver. Silver is renowned for its excellent conductivity, surpassing all other metals. To perform this test, you will need an ohmmeter:

• Connect the ohmmeter to the sample to measure resistance.
• Note that silver will show extremely low resistance values.

The lower the resistance, the higher the conductivity. A resistance close to that of silver indicates that the sample conducts electricity well, consistent with silver's properties. This characteristic is a significant indicator that your metal sample is likely silver.

Chemical Tests with Silver

Chemical testing reveals unique reactions that can further confirm the identity of silver. One such test involves using nitric acid (HNO₃) and potassium dichromate (K₂Cr₂O₇). Here's how you can perform a chemical test:

1. Dissolve a small piece of the metal in concentrated nitric acid to form a clear solution of silver nitrate if silver is present.
2. Add potassium dichromate to the solution. If the metal is silver, a reaction should occur, producing a characteristic red solid known as silver chromate.

This red precipitate is a strong indicator of silver, allowing for a straightforward confirmation when visual assessments are not definitive.

X-ray Fluorescence Analysis

X-ray fluorescence (XRF) analysis offers a non-destructive way to determine the elemental composition of a metal sample. Here’s how it works:

• The sample is exposed to X-rays.
• The energy from the X-rays causes atoms in the sample to emit secondary X-rays, or fluorescence, at specific wavelengths.
• For silver, these fluorescence peaks will appear at known characteristic wavelengths.

Using XRF can confirm the presence of silver by identifying these signature spectral peaks. It's a precise method to corroborate findings from previous tests such as density and conductivity measurements, providing a comprehensive analysis of the material's composition.