Test: Surface Tension & Capillarity - NEET MCQ

The angle of contact for liquid on a solid surface is the angle between the tangent to the liquid surface at the point of contact and the solid surface. This is the definition of angle of contact.

When impurities are mixed in liquid, the surface tension of liquid decreases. The soluble substances when dissolved in water, decreases the surface tension of water.

If drops and bubbles do not collapse under the effect of gravity, it indicates that the pressure inside the drop is greater than the pressure outside. The greater inner pressure prevents the drop from collapsing.

Plants use capillary action to bring water from the soil up through capillaries, small tubes in the plants, to the rest of the plant. Due to gravity, capillary action is not strong enough for the water to travel to the top of the plant. Another process, transpiration pull does the rest of the work.

As we know,

S=rhdg/2cosθ

⇒ Pressure difference =hdg=2Scosθ/r

Surface tension is defined as the ratio of surface force applied on a liquid to the length along which the force acts. so it is newton/meter

Excess pressure inside a cavity or air bubble in liquid formula
P[excess] = P[inside] - P [outside]
P [outside]  =P[atm]
P[inside]= P[atm] +hpg [here p represents density] +2T/R
P[excess] = P[atm] +hpg [here p represents density] +2T/R-P[atm]
P[excess] = hpg+2T/R
Now substitute value for T
P[excess] = hpg+2S/R
Hence C is correct.

Excess pressure in water bubble = 2T/r (due to 1 surface only)
Excess pressure in a soap bubble = 4T/r (due to 2 surfaces)