### E-AIM Tutorial

 Lesson 8: Variation of Deliquescence with Temperature

### 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.

### Conclusion

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.