The surface tension in a cup water is caused by
intermolecular forces
The surface tension in a cup of water is caused by the cohesive forces between water molecules. Water molecules are made up of two hydrogen atoms bonded to one oxygen atom, and these molecules are polar, meaning they have a slight positive charge on one side and a slight negative charge on the other. This polarity allows water molecules to form hydrogen bonds with each other.
Surface tension occurs at the interface between water and air. The water molecules at the surface are attracted to each other more strongly than they are attracted to the air molecules above them. This cohesive force between water molecules creates a sort of "skin" on the surface of the water, resulting in surface tension.
To understand why this happens, we can examine the molecular interactions between water molecules. Each water molecule can form hydrogen bonds with up to four neighboring water molecules. Within the bulk of the water, these interactions are balanced in all directions, resulting in a net zero force. However, at the surface, the water molecules do not have neighboring water molecules on one side (as they would within the bulk), so they form stronger hydrogen bonds with the water molecules around them, creating an inward force that minimizes the surface area of the liquid.
This surface tension allows some insects, such as water striders, to walk on water without sinking. It also causes water droplets to form spherical shapes, as this minimizes the surface area for a given volume of water.
In summary, the surface tension in a cup of water is caused by the cohesive forces between water molecules, specifically the hydrogen bonding interactions that result in an inward force at the water-air interface.