Question

What happens when a detergent is added to water?
A. Surface tension increases.
B. Surface tension decreases.
C. Viscosity increases.
D. Viscosity decreases.

Answer 1

Hint: Surface tension is defined as the attractive force which is applied to the molecules available on the surface of the liquid by the molecules underneath. A detergent is known as a surfactant or a mixture of surfactants with cleansing properties in dilute solutions . Viscosity is defined as a fluid’s resistance to flow.

Complete answer:
When detergent is added to water, the detergent molecules tend to break down the cohesive attractive forces of the water molecules at the surface.
When detergent is added to water, it lowers the water's surface tension. Surfactants are compounds that lower the surface tension of water by separating the water molecules from one another.
As a result of the attractive forces between water molecules and detergents, the surface tension is broken and viscosity increases.
Factors that affect surface tension are:
Surfactant
Chemical additions
Temperature
Oxidation

Hence, the correct options are:
B. Surface tension decreases.
C. Viscosity increases.

Additional information:
 Some examples of surface tension in real life are:
• The needle floating on the surface of water.
• Soap and detergent cleansing action on garments
• There are water droplets on some of the leaves' surfaces. Droplets resemble tiny bubbles.

Note:
Detergents are usually alkylbenzene sulfonates, a family of compounds that are similar to soap but are more soluble in hard water, because the polar sulfonate (of detergents) is less likely than the polar carboxylate (of soap) to bind to calcium and other ions found in hard water.