Question

Addition of KCI to phenol-water system (a) raises the CST (b) lower the CST (c) has no effect on CST (d) none of the above

Short answer

The correct answer is (b) lower the CST, as adding KCl to the phenol-water system would enhance the solubility of the solute, thereby requiring a lower temperature for complete miscibility, which signifies lowering of CST.

Step-by-step solution

Understanding Salts effect on Solubility

Salts like KCl usually increase the solubility of organic compounds in water. This is due to 'salting in' effect. So, the first step is recognizing that the addition of KCl can alter the solubilities.

Comparing Solubility with Critical Solution Temperature

The CST is the temperature limit above which the two immiscible liquids (phenol and water here) become miscible with each other. If 'salting in' causes an increase in solubility, it impacts the CST. Hence, adding KCl would influence the temperature at which phenol and water mix freely.

Analyzing the Effect of KCl on CST

Given that the addition of KCl to the phenol-water system enhances solubility, it would consequently lower the CST, since a lower temperature would now be required to facilitate complete miscibility.

Key concepts

Phenol-Water System

The phenol-water system is a mixture of two distinct liquids: phenol and water. Phenol is an organic compound that, at certain temperatures and concentrations, does not mix well with water. This system is intriguing because it displays unique phase behavior. As the temperature increases, phenol becomes more soluble in water, reaching a point known as the Critical Solution Temperature (CST), beyond which the two liquids are completely miscible.
This phenomenon is important in understanding how temperature influences the interaction of organic compounds with water. The CST is critical in applications involving solute segregation or integration, making comprehension of this concept essential for students exploring chemical solutions.

KCl Effect on Solubility

Potassium chloride (KCl) is a type of salt that, when added to a solution, can affect the solubility of various compounds. When introduced to a phenol-water system, KCl plays a role by altering the solvation dynamics. Typically, the presence of KCl can enhance the solubility of phenol in water, a phenomenon known as 'salting in'.
This behavior is due to the ions from KCl interacting with water molecules, effectively changing the solution's structure and influencing solubility. As these ions interact, they can disrupt water's hydrogen bonding network, providing increased opportunity for phenol to dissolve. This effect is significant because it can modify the behavior of solutions, particularly in industrial or laboratory settings, where precise control of solubility is crucial.

Salting In Effect

'Salting in' is a term used to describe the phenomenon where the addition of salts, like KCl, increases the solubility of a solute in a solvent. The effect occurs because the salt ions disrupt the structure of water. This disruption allows more room for solute molecules (like phenol) to dissolve.
In the context of the phenol-water system, 'salting in' is essential because it demonstrates how ionic compounds can manipulate physical properties of solutions, such as the CST. As KCl is added, the temperature at which phenol and water mix is lowered. This means that by altering ionic concentrations, one can influence solution behaviors, opening pathways for new applications in the formulation of pharmaceuticals, chemical reactions, and beyond.