Heat and Thermodynamics part-(2)

41. The atmosphere exerts enormous pressure on us. But, we do not feel it because: [U.P.P.C.S.(pre) 2010]

(a) Our blood exerts a pressure slightly more than that of the atmosphere.
(b) We are used to it.
(c) Our bones are very strong and can withstand the pressure.
(d) The surface area of our head is very small.

Correct Answer: (a) Our blood exerts a pressure slightly more than that of the atmosphere.

Solution:The atmosphere exerts enormous pressure on us but we do not feel it, because our blood exerts pressure slightly more than that of the atmosphere. We feel this pressure in water because the pressure under water is greater than our normal blood pressure.

42. Which of the following statement is not true : [I.A.S. (PRE) 2002]

(a) boiling point of any water solution is higher than the pure water.
(b) additional of solute to a solution increase its water potential.
(c) water vapour pressure of any solution is lower than the vapour pressure of pure water.
(d) when a solution is separated with the help of semipermeable membrane then water flow can be stopped by giving pressure to the solution.

Correct Answer: (b) additional of solute to a solution increase its water potential.

Solution:Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure or matrix effects such as capillary action (which is caused by surface tension).

The addition of solutes in water, lowers its water potential by which it becomes negatively charged. Adding solutes makes the water molecules less concentrated and this reduces the chemical potential of water in the system.

43. Why does water boil at a temperature below 100°C at higher altitudes? [U.P.P.C.S. (Pre) 1994]

(a) The atmospheric pressure decreases and hence boiling point is lowered.
(b) The gravitational attraction is less.
(c) Because of heavy winds in mountains.
(d) None of the above is correct.

Correct Answer: (a) The atmospheric pressure decreases and hence boiling point is lowered.

Solution:At higher altitudes of a mountain, the atmospheric pressure decreases due to lack of air. Therefore, the boiling water pressure decreases and water boils below 100°C due to lowering of boiling point.

The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid in a partial vacuum has a lower boiling point than that of liquid at atmospheric pressure. A liquid at high pressure has a higher boiling point than when that liquid is at atmospheric pressure.

44. Assertion (A): The boiling point of water decreases as the altitude increases. Reason (R): The atmospheric pressure increases with altitude. [I.A.S.(PRE) 2001, U.P.R.O/A.R.O. (PRE) 2023]

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true but (R) is false.
(d) (A) is false but (R) is true.

Correct Answer: (c) (A) is true but (R) is false.

Solution:The atmospheric pressure decreases as one climbs higher in the atmosphere and increases closer to the Earth's surface. The boiling point of water decreases as the altitude increases. Thus, (A) is true but (R) is false.