The activity coefficient (𝛶) is used to show the deviation from ideal solution. when two similar components are mixed to form solution then the behaviour of the solution is close to ideal solution because the forces between like molecules will be similar. but when we mix different component (different in size, nature, properties) the forces between like molecules and unlike molecules will be different and leads to deviation from ideal solution or pure species solution.
When the interaction between different molecules is stronger than like molecules, the molecules like to stay in liquid phase and it shows negative deviation that means the partial pressure will be less than that calculated from ideal solution, the value of activity coefficient (𝛶) will be less than 1 and if the interaction between different molecules is weaker than like molecules, the molecules like to go in vapour phase and it shows positive deviation that means the partial pressure will be more than that calculated from ideal solution, the value of activity coefficient (𝛶) will be greater than 1. most of the solution shows positive deviation.
When the interaction between different molecules is stronger than like molecules, the molecules like to stay in liquid phase and it shows negative deviation that means the partial pressure will be less than that calculated from ideal solution, the value of activity coefficient (𝛶) will be less than 1 and if the interaction between different molecules is weaker than like molecules, the molecules like to go in vapour phase and it shows positive deviation that means the partial pressure will be more than that calculated from ideal solution, the value of activity coefficient (𝛶) will be greater than 1. most of the solution shows positive deviation.
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