Introduction
In
Lesson 7 we considered the water uptake behavior of salt and acid particles
as a function of temperature. Here we examine how particle deliquescence,
the phase change from solid to aqueous state, depends on temperature.
Preparation
Before starting, ensure that this browser window occupies only the
left half of your screen. You should leave enough
space for another browser window where you can
enter data into
E-AIM and read the results. If your screen is too small for
two windows, print out this tutorial and use this window to run the
E-AIM model.
In these lessons we assume that you have two browser windows open.
Select this
link to open a second browser window containing the data
input page for "variable relative humidity, or total water" calculations using Model II
(http://www.aim.env.uea.ac.uk/aim/model2/mod2rhw.php).s Arrange
the windows on your screen so that both are visible and the left window
contains this text.
The Lesson
This consists of two sets of calculations to determine the deliquescence
points of electrolytes at different temperatures and then compare the results.
You have completed Lesson 8, and learned that:
- The deliquescence point of NH4NO3,
i.e. the relative humidity at which the solid salt takes up water and
is in equilibrium with its saturated solution, varies strongly with
temperature. Its value at 273.15 K is 76%, corresponding to a saturated
solution molality of 14.6 mol kg-1. At
323.15 K the deliquescence point falls to only 47%, with a much higher
saturated solution molality of 42.7 mol kg-1.
- By contrast, the deliquescence point of (NH4)2SO4
varies little with
temperature. Its value at 273.15 K is about 81.5%, corresponding to a saturated
solution molality of 5.3 mol kg-1. At
323.15 K the deliquescence point falls to 78.5%, with a saturated
solution molality increased only slightly to 6.3 mol kg-1.
- These results show that the variation of the solid/liquid equilibrium
of single salts with temperature – even when they have an ion in common – can vary
greatly. In both cases the factor that controls the deliquescence relative
humidity and its variation with temperature is the solubility of the salt
in water. The solubility of NH4NO3(s) increases
by almost a factor of 3 from 273.15 K to 323.15 K giving a large change
in the deliquescence point; the solubility of (NH4)2SO4(s)
increases by only about 20% and the deliquescence point changes very little
over the 50 K range of temperature.
- For completeness, we note that we have not discussed cases where the
solid phase in equilibrium with its saturated solution of the salt itself changes with
temperature. Aqueous sodium sulphate is an important example: below
305.65 K the precipitating solid phase is Na2SO4 · 10H2O(s),
and at higher temperatures it is Na2SO4(s).
Now proceed to Lesson 9, which examines
how both solid/liquid and gas/particle phase transitions in the same
system change with temperature.