Final answer:
The liquid with the lowest surface tension among the options provided is C₃H₇OH (propyl alcohol) at 75°C, due to its weaker intermolecular forces compared to magnesium bromide and because higher temperatures reduce surface tension. Considering as option number a, b, c and d, option number a is correct.
Step-by-step explanation:
The question seeks to determine the liquid with the lowest surface tension considering the type of substances and the effect of temperature. Surface tension is influenced by intermolecular forces, and different liquids have varying levels of it due to the nature of these forces. Higher temperatures generally reduce surface tension, as they provide energy to overcome these cohesive forces.
In the given options, we have C₃H₇OH (propyl alcohol) at 75°C and 25°C, C₅H₁₁OH (pentyl alcohol) at 25°C, and MgBr₂ (a salt solution) at 25°C. Organic alcohols such as propyl and pentyl alcohol have lower surface tension compared to water due to weaker hydrogen bonding. Magnesium bromide (MgBr₂) in solution would likely have ionic interactions and thus a relatively higher surface tension than alcohols.
Considering both the impacts of the substance and the temperature, C₃H₇OH (propyl alcohol) at 75°C would have the lowest surface tension. This is because temperature has a significant effect on reducing surface tension and propyl alcohol has weaker intermolecular forces compared to MgBr₂. Therefore, given the higher temperature and the nature of propyl alcohol, its surface tension would be the lowest among the options provided.