Problem 49 - Chemistry The Molecular Science

Question

Assume you have an unlabeled bottle containing a white, crystalline powder. The powder melts at \(310^{\circ} \mathrm{C}\). You are told that it could be \(\mathrm{NH}_{3}, \mathrm{NO}_{2}\), or \(\mathrm{NaNO}_{3}\). What do you think it is and why?

Step-by-Step Solution

Answer

The powder is likely sodium nitrate ( \(\mathrm{NaNO}_3\) ) due to its melting point close to \(310^{\circ} \text{C}\) .

Step 1: Analyze the compounds

Ammonia (\(\mathrm{NH}_3\)) is a gas at room temperature, hence it cannot be a white crystalline powder. Nitrogen dioxide (\(\mathrm{NO}_2\)) is also not a solid at room temperature; it exists as a reddish-brown gas. Sodium nitrate (\(\mathrm{NaNO}_3\)) is a white crystalline solid.

Step 2: Research the melting points

Check the melting points of the compounds:- \(\mathrm{NH}_3\) is a gas; thus, it does not have a melting point relevant here.- \(\mathrm{NO}_2\) is a gas under normal conditions, so its melting point is irrelevant.- \(\mathrm{NaNO}_3\) has a melting point of approximately \(306.8^{\circ} \text{C}\).

Step 3: Compare the data

Compare the melting point of the given crystalline substance (\(310^{\circ} \text{C}\)) with the compounds:- \(\mathrm{NaNO}_3\) has a melting point very close to \(310^{\circ} \text{C}\), specifically \(306.8^{\circ} \text{C}\). This is consistent with the compound being sodium nitrate.

Step 4: Conclusion

Given that the substance is a white crystalline powder and has a melting point near \(310^{\circ} \text{C}\), it is consistent with sodium nitrate (\(\mathrm{NaNO}_3\)).

Key Concepts

Melting Point Analysis

To identify a substance, a chemist often begins with a melting point analysis. This powerful technique is valuable for recognizing crystalline solids. By determining at what temperature a substance transitions from solid to liquid, one can compare this data to known values for potential substances. Sodium nitrate (\(\mathrm{NaNO}_3\)) has a precise melting point of about \(306.8^{\circ} \mathrm{C}\), which helps us distinguish it from other substances.
It's crucial to note that the melting point is a fundamental property that remains consistent for pure compounds. It allows scientists to match the result to a database of known materials. This technique is quick and reliable, making it widely used in various chemical analyses.

Properties of Gases and Solids

Gases and solids have distinct characteristics that help in their identification. At room temperature, ammonia (\(\mathrm{NH}_3\)) and nitrogen dioxide (\(\mathrm{NO}_2\)) are gases and do not appear as solids.
Gases like these are less dense, often colorless, or in the case of nitrogen dioxide, reddish-brown. Solids, on the other hand, are dense, with a fixed volume and shape, often appearing in a crystalline form.

• Ammonia (\(\mathrm{NH}_3\)): Gas at room temperature, colorless.
• Nitrogen Dioxide (\(\mathrm{NO}_2\)): Reddish-brown gas.
• Sodium Nitrate (\(\mathrm{NaNO}_3\)): White crystalline solid.

These attributes lead us to correctly eliminate ammonia and nitrogen dioxide when faced with a white crystalline substance.

Chemical Identification Techniques

Chemical identification techniques encompass a range of methods for determining the composition of a substance. Melting point analysis is one method, but others include spectroscopy, chromatography, and qualitative tests.
These techniques aid in confirming the identity of a substance by evaluating various chemical and physical properties.

• Spectroscopy: Uses light to identify components based on absorption or emission spectra.
• Chromatography: Separates compounds based on their movement through a medium.
• Qualitative Tests: Simple chemical reactions that provide quick identifications.

Combining these methods can give a clear, conclusive identification, ensuring accurate results in determining a substance like sodium nitrate.