Question

A student is given four solid samples labeled W, X, Y, and \(Z\). All have a metallic luster. She is told that the solids could be gold, lead sulfide, mica (which is quartz, or \(\mathrm{SiO}_{2}\) ), and iodine. The results of her investigations are: (a) \(\mathrm{W}\) is a good electrical conductor; \(\mathrm{X}, \mathrm{Y},\) and \(\mathrm{Z}\) are poor electrical conductors. (b) When the solids are hit with a hammer, W flattens out, X shatters into many pieces, \(\mathrm{Y}\) is smashed into a powder, and \(\mathrm{Z}\) is not affected. (c) When the solids are heated with a Bunsen burner, Y melts with some sublimation, but \(\mathrm{X}, \mathrm{W}\), and \(Z\) do not melt. (d) In treatment with \(6 \mathrm{M} \mathrm{HNO}_{3}\), \(\mathrm{X}\) dissolves; there is no effect on \(\mathrm{W}, \mathrm{Y},\) or \(\mathrm{Z}\). On the basis of these test results, identify the solids.

Short answer

W is gold, X is lead sulfide, Y is iodine, Z is mica (quartz).

Step-by-step solution

Analyzing Electrical Conductivity

Given information: \( W \) is a good electrical conductor and \( X, Y, Z \) are poor electrical conductors. Among gold, lead sulfide, mica (quartz), and iodine, gold is a metal and thus a good electrical conductor. Hence, \( W \) is gold.

Analyzing Reaction to Hammer Test

Analyzing behavior: When hit, \( W \) flattens (indicative of malleability of metals, consistent with gold), \( X \) shatters, \( Y \) turns to powder, \( Z \) remains unchanged. Lead sulfide and mica (quartz) would likely shatter, while iodine is known to be brittle. Hence, \( X \) and \( Y \) are candidates for lead sulfide and iodine, while \( Z \) is likely mica (quartz).

Analyzing Heating Test Results

\( Y \) melts and sublimates with heat, indicative of iodine's properties, whereas \( X, Z, \) and \( W \) do not melt. Iodine can sublimate at lower temperatures, hinting further that \( Y \) is iodine. \( X \) is then likely to be lead sulfide.

Analyzing Reaction with \(6M \; HNO_3\)

\( X \) dissolves in \(6M \; HNO_3\), lead compounds such as lead sulfide can dissolve in acids, confirming that \( X \) is lead sulfide. This leaves us with \( Z \) being mica (quartz), which is not reactive under these conditions.

Key concepts

Electrical Conductivity

Electrical conductivity is the ability of a material to allow the flow of an electric current through it. Substances like metals are good conductors due to their free-moving electrons. Gold, for example, is an excellent electrical conductor, making it a prime candidate for component W in the exercise. In comparison, materials that lack free-moving electrons, such as lead sulfide, mica (quartz), and iodine, are poor conductors. Thus, substances X, Y, and Z, which are stated to be poor conductors, fall into this category. When identifying unknown substances, testing for electrical conductivity can provide insight into which materials contain free electrons and are likely metals.

Chemical Reactions

Chemical reactions involve breaking and forming bonds to create new substances. In our exercise, treatment with nitric acid (\(6M \; HNO_3\)) is a type of chemical reaction test. Substances interacting differently during this test can indicate their identity. Here, solid X dissolves in the acid, which is characteristic of lead sulfide reacting with strong acids to form a soluble lead compound. Meanwhile, no reaction occurs for W, Y, and Z, suggesting that these substances do not react under the given acid conditions. This further helps in identification by highlighting which substances remain inert and which undergo a chemical transformation.

Sublimation

Sublimation occurs when a solid changes directly into a gas without passing through the liquid phase. This is a relatively rare property observable in iodine. When Y is heated and shows signs of melting and sublimation, this is a strong clue that Y is iodine. The sublimation of iodine at a lower temperature can be visually observed as a vapor, distinguishing it from the other substances. Understanding sublimation helps in determining the unique phase change behaviors of materials, especially in distinguishing them from similar-looking substances under heat.

Malleability

Malleability refers to the ability of a substance to deform under compressive stress, which is a common characteristic of metals. In the exercise, W flattens when hit with a hammer, showcasing its malleability. This indicates that W is likely a metal, such as gold. Malleability is a useful property in identifying metals because it differentiates metallic from non-metallic substances. For instance, while X shatters and Z remains unchanged, highlighting their lack of malleability. This property is essential in determining how a substance behaves under mechanical force, aiding in the categorization of metals versus non-metals.