1. Effect of Temperature on Solubility
Objectives:
To state the effect of temperature and pressure on the solubility of a gas in a liquid.
To interpret a graph that shows the effect of temperature on the solubility of a solid compound in water.
To interpret a graph of temperature versus solubility and determine whether a solution is saturated, unsaturated, or supersaturated.

2. Solubility vs. Temperature
200
180
160
140
120
100 80 60 40 20 KI
KNO3
Solubility (g solute / 100 g H2O)
NaNO3
The general rule of thumb is that solubility of solids increases with increases in temperature.
Maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility.
– Solubility is expressed as the mass of solute per volume (g/L) or mass of solute per mass of solvent (g/g) or as the moles of solute per volume (mol/L).
– Solubility of a substance depends on energetic factors and on the temperature and, for gases, the pressure.
• A solution that contains the maximum possible amount of solute is saturated.
• If a solution contains less than the maximum amount of solute, it is unsaturated.
When a solution is saturated and excess solute is present, the rate of dissolution is equal to the rate of crystallization.
• Solubility increases with increasing temperature — a saturated solution that was prepared at a higher temperature contains more dissolved solute than it would contain at a lower temperature, when the solution is cooled, it can become supersaturated.
Solubility of a substance generally increases with increasing temperature
No relationship between the structure of a substance and the temperature dependence of its solubility
Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds
This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent
Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution
Solubility of gases in liquids decreases with increasing temperature
Attractive intermolecular interactions in the gas phase are essentially zero for most substances
When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0)
Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas
Na3PO4
NaCl 20 40 60 80
100
Temperature (oC)
Timberlake, Chemistry 7th Edition, page 297

3. Gas Solubility Higher Temperature …Gas is LESS Soluble CH4 O2
2.0 O2
Higher Temperature
…Gas is LESS Soluble CO
Solubility (mM)
1.0
The general rule of thumb is that the solubility of gases decreases when temperature increases.
Solubility of a substance generally increases with increasing temperature
No relationship between the structure of a substance and the temperature dependence of its solubility
Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds
This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent
Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution
Solubility of gases in liquids decreases with increasing temperature
Attractive intermolecular interactions in the gas phase are essentially zero for most substances
When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0)
Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas He 10 20 30 40 50
Temperature (oC)

4. Solubility

5. Solubility Rules General Rules for Solubility of
Ionic Compounds (Salts) in Water at 25 oC
Most nitrate (NO31-) salts are soluble.
Most salts of Na+, K+, and NH4+ are soluble.
Most chloride salts are soluble. Notable exceptions
are AgCl, PbCl2, and Hg2Cl2.
Most sulfate salts are soluble. Notable exceptions
are BaSO4, PbSO4, and CaSO4.
Most hydroxide compounds are only slightly
soluble.* The important exceptions are NaOH and
KOH, Ba(OH)2 and Ca(OH)2 are only moderately
soluble.
Most sulfide (S2-), carbonate (CO32-), and phos-
phate (PO43-) salts are only slightly soluble.
*The terms insoluble and slightly soluble really mean the same
thing. Such a tiny amount dissolves that it is not possible to
detect it with the naked eye.
Ionic substances are most stable in polar solvents, and the higher the lattice energy, the more polar the solvent must be to overcome the lattice energy and dissolve the substance.
• Water is the most common solvent for ionic compounds because of its high polarity.
• Many ionic compounds are soluble in other polar solvents because these solvents consist of molecules that have large dipole moments and can interact favorably with the dissolved ions.
• A more useful measure of the ability of a solvent to dissolve ionic compounds is its dielectric constant (), which is the ability of a bulk substance to decrease the electrostatic forces between two charged particles.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218

6. Solubility of Common Compounds
Soluble compounds
NO3- salts
Na+, K+, NH4+ salts
Except for
those containing
Cl-, Br-, I- salts
Ag+, Hg22+, Pb2+
Except for
those containing
SO42- salts
Ba2+, Pb2+, Ca2+
Insoluble compounds
S2-, CO32-, PO43- salts
Except for
those containing
OH1- salts
Na+, K+, Ca2+
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218

7. Temperature-Solubility Relationships*
190
CsCl
180
NaNO3
170
160
NaC2H3O2
150
140
130
RbCl
120
LiCl
110
Solubility in grams per 100 g of water
100 90 80
NH4Cl 70 60
KCl 50
NaCl 40 30
Li2SO4 20 10
SO2 10 20 30 40 50 60 70 80 90
100
Temperature (oC)
*Values on this graph are valid only when some undissolved solid is in contact with the solution.
For SO2 values are valid when pressure due to SO2 and water vapor is kPa.