b)theoretical
We explain the measured cycle in the following manner:
The fast decay at the beginning of the expansion adiabate is
"normal" adiabatic behaviour of the gas. The increasing pressure
with increasing volume expansion is caused by the evaporation of
the fluid under vacuum situated mainly on the warm wall of the
lid. If the evaporated fluid enters the cylinder it becomes
labile. The plateau region of the pressure and the often following
slow decrease is caused by the dynamic equilibrium between
evaporating fluid and expanding volume. The breakdown of the
pressure at the end point of the expansion is due to the return to
equilibrium.
There are two different ways to explain qualitatively the labile state of the
expansion adiabate.
The first is the hypothesis of Doczekal[4], which says that benzene and water
have different temperatures at the end of the adabatic expansion. For this
interpretation some arguments can be found. The vapour pressure curve of
water-benzene mixtures can be calculated simply, like ideal gases, by adding
the partial pressure of water and benzene respectively [5]. This means that
the gases are independent and do not interact because of the hydrophobicity of
the benzene. The diffusion and the heat exchange between water and benzene is
therefore hindered. Thus, in a fast adiabatic expansion both gases have
different temperatures at the end point.
The second usual way to
