To identify the substance observed by Lars, we need to analyze the properties of each given substance and determine whether it meets the necessary criteria of being a solid and floating in water at room temperature (23°C).
### Step-by-Step Analysis:
1. Determine which substances are solids at room temperature:
- Carbon tetrachloride: Melting point is -22.9°C. Since room temperature (23°C) is higher than -22.9°C, carbon tetrachloride is a liquid at room temperature.
- Cetyl alcohol: Melting point is 49.3°C. Since room temperature (23°C) is lower than 49.3°C, cetyl alcohol is a solid at room temperature.
- Dichlorobenzene: Melting point is 53.5°C. Since room temperature (23°C) is lower than 53.5°C, dichlorobenzene is a solid at room temperature.
- Sulfur hexafluoride: Melting point is -64°C. Since room temperature (23°C) is higher than -64°C, sulfur hexafluoride is a gas at room temperature.
Based on the melting points, the substances that are solids at room temperature are cetyl alcohol and dichlorobenzene.
2. Determine which of these solid substances float in water:
- The density of water at room temperature is approximately 1.0 g/cm³.
- Cetyl alcohol: Density is 0.811 g/cm³. Since 0.811 g/cm³ is less than 1.0 g/cm³, cetyl alcohol would float in water.
- Dichlorobenzene: Density is 1.25 g/cm³. Since 1.25 g/cm³ is greater than 1.0 g/cm³, dichlorobenzene would sink in water.
Therefore, among the solid substances, cetyl alcohol is the one that floats in water.
### Conclusion:
The most likely identity of Lars's sample, which is a solid and floats in water at room temperature, is cetyl alcohol.
The correct answer is:
B. cetyl alcohol
