Explain the following: A balloon collapses when air is removed from it.

Hint: Pressure is defined as the continuous physical force which is exerted on or against an object by something which is in contact with it. The force applied will be perpendicular to the surface of the objects.

Complete solution:
When the air is removed from a balloon, the balloon collapses because air exerts pressure on the walls of the balloon. Whenever air is filled into the balloon, it bulges out and the air inside the balloon occupies space and thus possesses mass and volume.
We know that the gases move from a higher pressure region to a lower pressure region and thus when the thread tied to the neck of the balloon is removed, the air which is inside the balloon will go out. When gases escape, the density of the air inside the balloon reduces and it becomes almost like a vacuum.
Also, when the air is removed from the balloon, the pressure inside the balloon will be much less than the atmospheric pressure outside and thus the balloon collapses.

Note: 1. The atmospheric pressure which is also known as the barometric pressure is defined as the pressure within the atmosphere of the earth. At the sea level, the atmospheric pressure will have an average value of one atmosphere and as the altitude increases, it gradually decreases.
2. Pressure can also be defined as the force exerts a unit area whereas the atmospheric pressure is the equivalent to the weight of the air within its atmosphere.
3. Unit of pressure is pascal in SI.