Figure 1. Helium Filled Balloons
Why do Helium Balloons Float in the Air?
Have you ever wondered why helium filled balloons sold in stores ﬂoat in the air, but the ones you blow air into do not, even
though they are the same size? The reason is that the same volume of helium weighs less than air. According to Avogadro's
Law, balloons of similar volume contain similar number of moles of substance. However, helium is lighter than air because its
molar mass is lower than oxygen's and nitrogen's, the two major components of air.
Gases are different from liquids and solids in that they have neither a fixed volume nor a fixed shape. The behavior of gases
depends on four variables:
(number of moles,
). There are three key
laws that express the relationship between these variables:
Law. These laws express the
effect of one variable on another, while the other two variables remain constant.
The Italian chemist and physicist, Amedeo Avogadro (1776 – 1856), established Avogadro's Law in 1811. This gas law states
that equal volumes of gases at fixed temperature and pressure contain the same number of particles. This means that a
sample of one gas under specific conditions of pressure and temperature will have the same number of moles as another
gas, as long as the pressure and temperature are kept constant.