What happens to the surface tension of the water when a detergent is added to it?

Identify the true statements about surface tension.

a. Molecules along the surface of a liquid behave differently than those in the bulk liquid.

b. Mercury has a lower surface tension than water.

c. Cohesive forces attract the molecules of the liquid to one another.

d. Surface tension increases as the temperature of the liquid rise.

e. Water forming a droplet as it falls from a faucet is a primary example of surface tension.

The weight of a rectangular block of low-density material is 15.0 N. With a thin string, the center of the horizontal bottom face is tied to the bottom of a beaker partly filled with water. When 25.0% of the block's volume is submerged, the tension in the string is 10.0 N. 

(a) Find the buoyant force on the block, 

(b) Oil with a density of 800 kg/cu. m is now steadily added to the beaker, forming a layer above the water surrounding the block. The oil exerts forces on each of the four sidewalls of the block that the oil touches. What are the direction of these forces?

(c) What happens to the string tension as the oil is added? Explain how the oil has this effect on the string tension. 

(d) The string breaks when the tension reaches 60.0 N. At this moment, 25.0% of the block's volume is still below the waterline. What additional fraction of the block's volume is below the top surface of the oil?