Introduction To Solubility

Let us start with the very basic question ‘what is solubility’? The solubility of a material relates to how much of it can be dissolved in a given amount of solvent at a certain temperature. It is determined by the nature of the solute, temperature and pressure, as well as the solute and solvent. The degree of solubility varies greatly across substances, ranging from endlessly soluble (totally miscible) ethanol in water to slightly soluble silver chloride in water. Particle size has little bearing on solubility; given enough time, even massive particles will dissolve. 

Having understood ‘what is solubility?’ let us consider the impact of these variables in a liquid solution of a solid or a gas.

Solubility of Solids in Liquids

The solubility of solids in liquids is highly dependent on the type of the solid and liquid, temperature, and, to a lesser extent, system pressure. When a solid (solute) is introduced to a solvent, the particles of the solute dissolve. The concentration of the solute in the solution rises as it is dissolved into the liquid. This is referred to as dissolution. 

Some solution solute particles clash with solid solute particles and precipitate out. This is referred to as crystallisation. The dissolving process continues until the solution reaches a predetermined maximum concentration. At the same temperature and pressure, a solution in which no additional solute can be dissolved is referred to as a saturated solution.

An unsaturated solution is a solution in which more solute may be dissolved at the same temperature. At the saturated solution stage, a balance is reached between the two processes: crystallisation and dissolution. The number of solute particles separating out is equal to the number of solute particles separating out, and a condition of dynamic equilibrium is reached.

Solution = Solute + Solvent

The concentration of solute in a saturated solution remains constant under provided conditions, i.e., temperature and pressure are provided conditions. The maximum quantity of solute that a solvent may dissolve at a given temperature is called its solubility.

Factors Affecting Solubility of Solid in Liquids

• Intermolecular Interaction:If the intermolecular interactions between the solute and the solvent are comparable, the solute dissolves in the solvent, i.e., dissolves like. Sodium chloride and sugar, for example, dissolve quickly in water, while naphthalene and anthracene dissolve quickly in benzene.
• Dissolution: When a solid solute is introduced to a solvent, some of it dissolves, and the concentration of the solution increases. This is referred to as dissolution.
• Crystallisation: It occurs when certain solute particles clash with those in the solution and are separated. This is referred to as crystallisation.
• Saturated solution: A saturated solution is one in which no additional solute can be dissolved at the same temperature and pressure.
• Temperature effect: If the dissolving process is endothermic in a nearly saturated solution, the solubility should increase with increasing temperature; if it is exothermic, the solubility should decrease with increasing temperature.
• Pressure Influence: Because solids and liquids are largely incompressible, pressure has little effect on the solubility of solids and liquids.
• Supersaturated solution: A saturated solution becomes supersaturated when more solute can be dissolved at a higher temperature.

Solubility of Gases in Liquids

In liquids, gases dissolve to generate homogenous solutions. The solubility of a gas in a liquid is determined by the following factors: 

1. The nature of the substance (solute) 
2. The type of the solvent
3. The solution’s temperature 
4. Pressure

Nitrogen, hydrogen, oxygen, helium, and other gases dissolve just a little in water, but ammonia, sulphur dioxide, hydrogen chloride, and other gases are extremely soluble. The gases that chemically react with the liquid solvent are the most soluble.

Factors Affecting Solubility of Gases in Liquids

• Effect of temperature: As the temperature rises, the solubility of gas reduces. This is due to the fact that gases dissolve in a liquid when heat is released, which is an exothermic process. As a result, according to Le- Chatelier’s principle, increasing the temperature causes the gas’s solubility to decrease.
• The effect of pressure: As pressure rises, so does the solubility of gases. This conduct is also consistent with Le- Chatelier’s principle. Consider a solution and a gas in dynamic equilibrium. At a pressure of p and a temperature of T, the bottom section represents the solution, while the top part represents the gaseous system. The number of gas molecules entering the solution is equal to the number of dissolved molecules exiting the solution phase because there is a dynamic equilibrium. By compressing the gas to a smaller volume, you may now increase the pressure over the solution phase. The number of gaseous particles per unit volume in the solution will grow as a result of this.

As a result, more molecules will collide with the liquid’s surface, causing more molecules to dissolve and gas solubility to rise until a new equilibrium is established. As a result, increasing the pressure of a gas above a solution improves the gas’s solubility. The dissolved gas concentration is proportional to the pressure of the gas above the solution.

Solubility Product

The solubility product (Ksp) is nothing but the equilibrium constant for a solid substance dissolving in an aqueous solution. It refers to the amount of a material that dissolves in water at a given concentration. The Higher the Ksp value of material means, the more soluble it is. Consider the following (in aqueous solutions) typical dissolving reaction:

aA(s)↽−−⇀cC(aq)+dD(aq)

It is essential to multiply the molarities or concentrations of the products (cC and dD) in order to find the Ksp. If any of the products have coefficients in front of them, they must be raised to that coefficient power (and also multiply the concentration by that coefficient). As an example, consider the following: