To determine which substance Lars observed, we need to follow a detailed, step-by-step approach considering the given properties:
1. Density Criterion:
- The substance floats in water, meaning its density must be less than 1 g/cm³.
Let's examine the densities of the substances:
- Carbon tetrachloride: 1.59 g/cm³ (denser than water, sinks)
- Cetyl alcohol: 0.811 g/cm³ (less dense than water, floats)
- Dichlorobenzene: 1.25 g/cm³ (denser than water, sinks)
- Sulfur hexafluoride: 0.00617 g/cm³ (less dense than water, floats)
Therefore, the possible substances based on the density criterion are:
- Cetyl alcohol
- Sulfur hexafluoride
2. State at Room Temperature:
- The observed substance is a solid at room temperature [tex]\( (23^{\circ}C) \)[/tex].
Let's check the melting points:
- Cetyl alcohol: 49.3 [tex]\(^{\circ}C\)[/tex] (melting point is above room temperature, so it is solid at room temperature)
- Sulfur hexafluoride: -64 [tex]\(^{\circ}C\)[/tex] (melting point is far below room temperature, so it is a gas at room temperature)
Therefore, based on both the criteria (density less than water and being solid at room temperature), cetyl alcohol is the only suitable substance.
Examining the available options:
- A. Carbon tetrachloride (not selected, it doesn’t float in water and isn’t solid at room temperature)
- B. Cetyl alcohol (selected, it floats in water and is solid at room temperature)
- C. Dichlorobenzene (not selected, it doesn’t float in water)
- D. Sulfur hexafluoride (not selected, it isn’t solid at room temperature)
Thus, Lars's observation most likely refers to B. Cetyl alcohol.
