To determine which substance needs to be cooled to the lowest temperature before it condenses, we should consider their boiling points and the nature of the bonds within each compound.
### Types of Compounds and Their Properties
1. Ammonia (NH₃)
- Ammonia is a polar covalent compound.
- Polar covalent compounds generally exhibit higher boiling points compared to nonpolar covalent compounds due to the presence of partial charges leading to dipole-dipole interactions.
- Therefore, while ammonia will have a moderate boiling point, it is not the lowest among the listed compounds.
2. Ethane (C₂H₆)
- Ethane is a nonpolar covalent compound.
- Nonpolar covalent compounds have the lowest boiling points due to the weaker Van der Waals forces (dispersion forces) acting between their molecules.
- As a result, ethane will have a much lower boiling point compared to both ammonia and ionic compounds like KBr.
3. Potassium Bromide (KBr)
- Potassium bromide is an ionic compound.
- Ionic compounds possess very high boiling points due to the strong ionic bonds between the positively and negatively charged ions.
- Because of these strong bonds, potassium bromide would have the highest boiling point among the given substances.
### Conclusion
Based on the breakdown of their properties:
- Ammonia (NH₃): Moderate boiling point (polar covalent compound).
- Ethane (C₂H₆): Lowest boiling point (nonpolar covalent compound).
- Potassium bromide (KBr): Highest boiling point (ionic compound).
Thus, to condense ethane (C₂H₆), you would need to cool it to the lowest temperature compared to ammonia (NH₃) and potassium bromide (KBr).
Therefore, you would most likely need to cool ethane (C₂H₆), a nonpolar covalent compound, to the lowest temperature before it condenses.
