Question

When we press the bulb of a dropper, with its nozzle kept under water, air in the dropper is seen to escape in the form of bubbles. If we then release the pressure on the bulb, water gets filled in the dropper. The rise in water in the dropper is due to
A. Liquid pressure
B. Weight of the bulb
C. Gravity of the earth
D. atmospheric pressure

Answer 1

Hint: Think of the basic mechanism that happens when a dropper is pressed and released. Remember that the water moves from higher pressure to lower pressure, so there must be a pressure difference. Think of the possible source of pressure acting on the water to result in the dropper getting filled up.

Complete answer:
So in the question we are given a dropper which you may be very familiar with from using eye drops and ear drops in daily life to taking chemicals in chemistry labs by using droppers.
From your prior experience using droppers, you know that when the bulb of the dropper is pressed, keeping it under water or any other liquid, the air in the dropper is seen to escape in the form of bubbles. And when we release the bulb of the dropper, the water or whatsoever liquid that the dropper is dipped in fills into it. We are asked to find the reason behind this rise in water.
So what you think is the reason behind this rise? Is it the liquid pressure, or the weight of the bulb, or the gravity of earth or the atmospheric pressure?
Basically what happens when you press a dropper is that you create a vacuum inside the dropper by expelling all little droplets and air present inside as bubbles which further results in a decrease in pressure inside the dropper. But there exists a pressure exerted on the surface of the liquid by the atmosphere. So, when the bulb of the dropper is released, the pressure difference between the inside and the outside of the dropper forces the water to move from higher pressure region to lower pressure region (Inside of dropper) and hence water gets filled in the dropper.
Clearly, atmospheric pressure is responsible for the dropper being filled by water.

So, the correct answer is “Option D”.

Note:
All the options in the question appear correct at first glance. All you have to do is look for the physics behind what is happening. Hence, you will get better clarity and hence better justification to support your answer.