Solubility vs Dissolution
Both these terms go hand in hand and refer to the same chemical scenario with two different standpoints in definition. As a background to the concept, it is important first to understand the three basic components involved here; namely solute, solvent and solution. Solute is the compound which is dissolved in the solvent. Solvent is generally a liquid that is used to dissolve the solute. Solution is referred to as the mixture that results from dissolving the solute in solvent. Solutes can be solids, liquids or gases, and even though solvents are generally liquids there can be solid and gaseous solvents too. E.g. A metal alloy can be considered as a solid solution where a solid solute is mixed with a solid solvent. ‘Solubility’ is a characteristic property of the solute and ‘Dissolution’ is the process in which a solute dissolves in a solvent to result a solution. Therefore by definition, solubility is a thermodynamic factor and dissolution is a kinetic factor.
Solubility
Solubility is a property of a solute which decides how far the solute would dissolve in a solvent to form a particular solution. The chemical and physical characteristics of the solute play a major role in deciding its levels of solubility. When we refer to the concentration of a solution we refer to the level of solubility of a particular solute in the solvent. There’s a limit to the amount of solutes a particular solvent could hold in a solution, in the solution phase. Beyond this limit if solutes were dissolved further it would start to precipitate at the bottom. The dynamic equilibrium between these two states defines the extent of solubility. Therefore, solubility occurs when the rate of dissolution equals the rate of precipitation. Solubility can be quantified and carries the unit mol/kg.
Generally we follow a rule of thumb in solubility known as ‘like dissolves like’. This idea suggests that polar compounds have a greater tendency to dissolve in polar solvents and vice versa. When a solute is completely soluble, we say it’s miscible. This is more often true for the case of two liquids (when a liquid is mixed in another liquid). When the solubility is low, we say that the compound is poorly soluble or insoluble. The solubility of one substance in another depends on the extent of intermolecular forces between the solute and solvent molecules, and various physical and thermodynamic factors affect the extent of solubility. E.g. temperature, pressure, polarity of solvent, the excess or deficiency of a common ion in solution etc. Generally when the temperature is high the solubility of a particular solute is higher than when cooler. At times, dissolution might occur due to a chemical reaction and not due to the pure solubility of the solute. This should not be confused over solubility. When a solute is purely soluble, one should be able to obtain the solute back again after the evaporation of the solvent.
Dissolution
Dissolution is the process where a solute dissolves in a solvent to form a solution. Therefore, this has a kinetic effect. Dissolution can occur at various rates and sometimes for a solute to completely dissolve in a solvent it might require quite a length of time. During the process of dissolution, the structural integrity of the solute is broken down into individual components, molecules or atoms, and the outcome of dissolution is referred to as solubility. Dissolution too is governed by similar physical principles as for solubility, but dissolution itself is a kinetic process. Ionic compounds can be easily dissolved in water and as mentioned above the ‘like dissolves like’ principle can be counted here, as well. The rate of dissolution depends on various factors; mechanical mixing, nature of solvent and solute, mass of dissolved material, temperature etc. Dissolution can be quantified by the unit mol/s.
What is the difference between Solubility and Dissolution?
• Dissolution is the process where a solute dissolves in a solvent to form a solution, whereas solubility is the outcome of dissolution.
• Solubility is a thermodynamic entity whereas dissolution is kinetic.
• Solubility is measured in mol/kg and dissolution is measured in mol/s.