Final answer:
Tap water has greater cohesion than salt water because the addition of salt disrupts the hydrogen bonding between water molecules, leading to reduced surface tension and cohesive properties.
Step-by-step explanation:
Tap water has more cohesion than salt water because salt water disrupts hydrogen bonding in water molecules. This disruption is caused by the dissolution of salt, or sodium chloride (NaCl), which separates into positively-charged sodium ions (Na+) and negatively-charged chloride ions (Cl−) in water. These ions attract the polar water molecules, forming spheres of hydration around them and weakening the hydrogen bonds between the water molecules themselves. As a result, the cohesion of the water diminishes due to the interference of ionic interactions, reducing the water's surface tension and its ability to form cohesive dome-like shapes or to support objects through surface tension, such as a floating needle.
Hydrogen bonding is a key component of water's cohesive properties. In pure water, these bonds create a strong network that gives rise to high surface tension, allowing phenomena like water striding insects and droplets that hold together strongly. By contrast, when salt is added, the ionic interactions take precedence, and the cohesive hydrogen bonding network becomes less effective, thus leading to lower surface tension and different behavior on a macroscopic level. The correct answer to the student's question is (d) Salt water disrupts hydrogen bonding in water molecules.