Surface tension in water is caused primarily by:

Surface tension in water is primarily caused by the cohesion of water molecules at the surface. Water molecules are polar, meaning they have a positive and negative end. This polarity causes water molecules to attract one another, creating cohesive forces. At the surface of the water, molecules experience a different environment than those in the bulk of the liquid, as they are only surrounded by other water molecules on one side. This results in stronger attractions for the molecules at the surface, creating a 'skin-like' effect.

The cohesion of these water molecules leads to surface tension, allowing small objects, like a needle, to float even if they are denser than water, as long as they don't break the surface. This phenomenon is crucial in various aquatic environments, affecting the behavior of organisms that rely on surface tension to move or rest on the water's surface.

While factors like temperature and the presence of pollutants can influence the surface tension, they do not define the primary cause. Likewise, depth can affect other properties of water but does not directly impact the cohesion of water molecules that leads to surface tension.