Question

At higher altitudes air becomes rarer amd amount of oxygen per litre of air decreases

Answer 1

We all live underneath an ocean of air which is several miles deep known as the atmosphere. Air is compressible that's the air above compresses the molecules of air around us making it denser. What happens when we move up the altitude is that the atmosphere becomes thinner because of less molecules present there. Though the percentage of oxygen is still 21% but because the atmosphere is thinner so the molecules also decreases including those of oxygen so it becomes difficult to breathe at higher altitude.