Page 185 - Physical chemistry understanding our chemical world

152 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

Why do we sneeze?

The change in Gibbs function during gas movement: gas
molecules move from high pressure to low

When we sneeze, the gases contained in the lungs are ejected through the throat so
violently that they can move at extraordinary speeds of well over a hundred miles
an hour. One of the obvious reasons for a sneeze is to expel germs, dust, etc. in the
nose – which is why a sneeze can be so messy.
But a sneeze is much more sophisticated than mere germ removal: the pressure of
the expelled gas is quite high because of its speed. Having left the mouth, a partial
vacuum is left near the back of the throat as a result of the Bernoulli effect described
below. Having a partial vacuum at the back of the throat is thermodynamically unsta-
ble, since the pressure in the nose is sure to be higher. As in the example above,
gas from a region of high pressure (the nose) will be sucked into the region of low
pressure (the back of the mouth) to equalize them. The nose is unblocked during this
process of pressure equilibration, so one of the major reasons why we sneeze is to
unblock the nose.
It is relatively easy to unblock a nose by blowing, but a sneeze is a superb means
for unblocking the nose from the opposite direction.

Aside

The Bernoulli effect

Hold two corners of a piece of file paper along its narrow side. It will droop under its
own weight because of the Earth’s gravitational pull acting on it. But the paper will
rise and stand out almost horizontally when we blow gently over its upper surface, as
if by magic (see Figure 4.5). The paper droops before blowing. Blowing induces an
additional force on the paper to counteract the force of gravity.
The air pressure above the upper side of the paper decreases because the air moves
over its surface faster than the air stream running past the paper’s underside. The

Partial vaccum here
causes paper to lift
Blow over face of
paper from here

Paper
held
here
(a) (b)
Figure 4.5 The Bernoulli effect occurs when the flow of fluid over one face of a body is
greater than over another, leading to pressure inequalities. Try it: (a) hold a pace of paper in
both hands, and feel it sag under its own weight. (b) Blow over the paper’s upper surface,
and see it lift