solubility 1

Solubility of Gas in Liquid
The solubility of gas in liquids depends on
1- Mass of gas molecules.
2- Pressure
3- Temperature
1-The Mass of gas molecules.
Solubility of gas molecules was increased by increasing mass of gas molecules because the larger mass of gas molecules causes stronger London and Debye forces between gas and solvent molecules.
2- pressure :- the solubility of gas was increased by increasing of pressure due to increase collision of the gas molecules with the surface of the solvent and hence greater solubility.
3-Temperature :- Gases decrease in solubility with an increase in temperature because increasing temperature causes an increase in kinetic energy of gas molecules which lead breakdown of intermolecular bond and gas escaping from solution,e.g. when temperature was increased CO2 gases escape from carbonate drink.
Determining Thermodynamic and “Kinetic” Solubility
The Phase Rule and Solubility
Solubility can be described in a concise manner by the use of the Gibbs phase rule.

where F is the number of degrees of freedom, that is, the number of independent variables (usually temperature, pressure, and concentration) that must be fixed to completely determine the system
C is the smallest number of components that are adequate to describe the chemical composition of each phase.
P is the number of phases.
The Phase Rule can be used to determine the thermodynamic solubility of a drug substance. This method is based on the thermodynamic principles of heterogeneous equilibria that are among the soundest theoretical concepts in chemistry. drug is added in a specific amount of solvent. After equilibrium is achieved, excess drug is removed (usually by filtering) and then the concentration of the dissolved drug is measured using standard analysis techniques such as high-performance liquid chromatography.
Figure 9-6 show Phase-solubility diagram for a pure drug substance.
? The line A–B represents one phase since the conc. of drug substance is below the saturation concentration. Since the temperature, pressure, and solution concentration are constant at drug concentrations above the saturation concentration.
? Line B–C represents two-phase of a pure solid in a saturated solution at equilibrium.
Figure.9-7 show Phase-solubility curve when the drug substance contains one impurity. At point B, the solution becomes saturated with component 1 (the drug).
? B–C represents two phases—a solution phase saturated with the drug and some of the impurity and a solid phase of the drug. At C the solution is saturated with drug, while the impurity does not reach saturation until it reaches point C on the line.