Select the correct answer.

Lars observes a substance to be a solid and to float in water at room temperature [tex]\left(23^{\circ} C \right)[/tex]. Based on the given properties, which substance is the most likely identity of this sample?

\begin{tabular}{|l|c|c|c|}
\hline & \begin{tabular}{c}
Melting \\
Point \\
[tex]$\left({ }^{\circ} C \right)$[/tex]
\end{tabular} & \begin{tabular}{c}
Boiling \\
Point \\
[tex]$\left({ }^{\circ} C \right)$[/tex]
\end{tabular} & \begin{tabular}{c}
Density at \\
Room \\
Temperature \\
[tex]$\left( g / cm ^3\right)$[/tex]
\end{tabular} \\
\hline \begin{tabular}{l}
carbon \\
tetrachloride
\end{tabular} & -22.9 & 76.7 & 1.59 \\
\hline cetyl alcohol & 49.3 & 344 & 0.811 \\
\hline dichlorobenzene & 53.5 & 174 & 1.25 \\
\hline \begin{tabular}{l}
sulfur \\
hexafluoride
\end{tabular} & -64 & -50.8 & 0.00617 \\
\hline
\end{tabular}

A. carbon tetrachloride
B. cetyl alcohol
C. dichlorobenzene
D. sulfur hexafluoride



Answer :

To determine which substance matches Lars's observations of being solid and floating in water at room temperature [tex]\( \left(23^{\circ}C\right) \)[/tex], let's evaluate each property given:

1. Carbon tetrachloride
- Melting Point: -22.9°C
- Boiling Point: 76.7°C
- Density: 1.59 g/cm³

At 23°C:
- It is in the liquid state because 23°C falls between its melting and boiling point.
- Hence, Carbon tetrachloride does not match the observation of being a solid.

2. Cetyl alcohol
- Melting Point: 49.3°C
- Boiling Point: 344°C
- Density: 0.811 g/cm³

At 23°C:
- It is in the solid state because 23°C is below its melting point.
- Its density is 0.811 g/cm³, which is less than water's density (1 g/cm³), so it floats in water.

Cetyl alcohol matches both conditions: being solid and having a density less than water.

3. Dichlorobenzene
- Melting Point: 53.5°C
- Boiling Point: 174°C
- Density: 1.25 g/cm³

At 23°C:
- It is in the solid state because 23°C is below its melting point.
- However, its density is 1.25 g/cm³, which is greater than water's density, so it does not float in water.

4. Sulfur hexafluoride
- Melting Point: -64°C
- Boiling Point: -50.8°C
- Density: 0.00617 g/cm³

At 23°C:
- It is in the gaseous state because its boiling point is far below 23°C.
- Hence, Sulfur hexafluoride does not match the observation of being a solid.

After evaluating all options:

- Carbon tetrachloride: Liquid at 23°C (does not match)
- Cetyl alcohol: Solid at 23°C (matches) and floats in water.
- Dichlorobenzene: Solid at 23°C but sinks in water (does not match)
- Sulfur hexafluoride: Gas at 23°C (does not match)

Therefore, the substance that Lars observed is:

B. cetyl alcohol