Discussion
A girl makes a hole near the bottom of a plastic bottle. She puts her thumb over the hole to cover it, fills the bottle with water up to half of its capacity. She then closes the cap of the bottle tightly. What will happen when she removes her thumb from the hole?
Answer: D
The water will not spurt out of the hole because the pressure inside the bottle is less than the outside pressure.
With the cap closed, air is trapped above the water. As soon as a little water starts to leave through the hole, that trapped air expands into the extra space, and its pressure falls below the atmospheric pressure outside. The low air pressure inside, together with the outside air pushing on the hole, holds the water back, so at most a few drops come out and the flow stops.
If the cap is opened, air can enter from the top and the water spurts out. Options A and C say the water spurts, which happens only with the cap open, and option B gives the wrong reason: the pressure inside is lower, not higher.
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