NAEST 2020: Boiling Water in a Syringe, Vapour Pressure and

Question: With some water in the syringe the piston is pulled out. Bubbles are seen in the water which continues till the piston is in pulled position. When the piston is released, bubbling stops. Note that the piston comes to the same position where it was before pulling. The main reason for bubbling, among the given options, is

1. Water starts boiling
2. Air leaks from outside and goes through the water
3. Air which was in the syringe, expanded during pulling of the piston and that goes into the water
4. Water got stirred during pulling of the piston and that created bubbles

Question 2: The pressure above the water in the syringe was $P_1$ before pulling the piston and $P_2$ after pulling the piston. Atmospheric pressure is $P_0$. Choose the correct option(s).

1. $P_1 = P_0$ and $P_2 = P_0$
2. $P_1 = P_0$ and $P_2 < P_1$
3. $P_1 = P_0$ and $P_2 < P_0$
4. $P_1 = P_0$ and $P_2 > P_0$
5. $P_1 < P_0$ and $P_2 < P_1$

Solution 1: As the piston is pulled down, the volume of the air inside the syringe increase and its pressure decreases (Boyal's law). When the air pressure in the syringe falls below the water's vapor pressure, the water starts boiling. Thus, option A is correct.

Boiling happens when the vapour pressure of a liquid becomes equal to the air pressure outside. Noramlly, when we heat a liquid its vapour pressure increases gradually and when it becomes equal to the air pressure outside, it starts boiling. In this demo, the air pressure itself is reduced and when it becomes equal to the vapour pressure of water, water in the syringe starts boiling.

Solution 2: Initially, the syringe was open from the top. Thus, air pressure is equal to the atmospheric pressure i.e., $P_1=P_0$. When the piston is pulled down, the air pressure decreases due to Boyal's law i.e., $P_2 < P_1$. Thus, options B and C are correct.

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