Question

water does not run out of a dropper unless its rubber bulb is pressed. Give reasons

Answer 1

To fill the dropper you have to submerge the end in water and squeeze the bulb. When you release the bulb the dropper fills with water. The science behind this: the dropper is, at first, filled with air. When you squeeze the bulb it pushes air out because the space that was occupied by the air is now occupied by the bulb. So some air leaves the dropper. When you release the dropper the space previously occupied by the bulb is empty, thus creating a vacuum. The vacuum aims to fill the empty space so sucks water into it. The same thing happens when you empty it, except you push water out and the vacuum is filled with air. To get the water out you need to generate a force that is greater than the vacuum by squeezing. This is why you have to press it quite hard.

Answer 2

because the air trapped behind the dropper pushes the water when pressed and the water falls down.