Understanding Solubility | Chemistry for EmSAT Achieve PDF Download

What is Solubility?

Solubility refers to the maximum quantity of solute that can dissolve in a specified volume of solvent at a particular temperature.
A solution denotes a uniform blend of one or more solutes within a solvent. A typical illustration of a solution is adding sugar cubes to a cup of tea or coffee. Solubility pertains to the property enabling sugar molecules to dissolve, characterized as the ability of a substance (solute) to dissolve in a particular solvent. A solute encompasses any substance, whether solid, liquid, or gas, that dissolves in a solvent.

Solubility Product

The concept of solubility product typically pertains to salts that exhibit limited solubility. It represents the maximum product of the molar concentrations of ions (raised to their respective powers) resulting from the dissociation of the compound.
At a specified temperature, the solubility product remains constant. A lower solubility product value signifies reduced solubility, whereas a higher value indicates greater solubility.

On the basis of solubility, the factors affecting solubility vary on the state of the solute:

1. Solubility of Liquids In Liquids

Water earns the moniker "universal solvent" due to its ability to dissolve nearly all solutes, with only a few exceptions. Various factors can influence the solubility of a substance.
Solubility refers to the establishment of new bonds between solute and solvent molecules. Quantitatively, it represents the maximum concentration of solute that can dissolve in a known volume of solvent at a specific temperature. Solutes are classified as highly soluble, sparingly soluble, or insoluble based on their concentration in the solvent. Solubility is quantitatively expressed in grams per liter (g/L).
Different types of solutions can be derived based on solubility. A saturated solution occurs when a specified amount of solute fully dissolves in a solvent at a given temperature. Conversely, supersaturated solutions are those in which solute begins to precipitate out after a certain concentration is dissolved at the same temperature.

Factors Affecting Solubility

The solubility of a substance is contingent upon its inherent physical and chemical properties, alongside several modifiable conditions. Among these conditions, temperature, pressure, and the nature of bonding and intermolecular forces between particles play pivotal roles.

• Temperature: Temperature adjustments can augment the solubility of a solute. Typically, water dissolves solutes at either 20°C or 100°C. Elevated temperatures can completely dissolve sparingly soluble solids or liquids. Conversely, for gaseous substances, solubility is inversely affected by temperature; as temperature rises, gases tend to expand and escape from their solvents.
• Intermolecular Forces and Bonds: The principle "like dissolves like" underscores the significance of similar intermolecular forces and bonds in promoting solubility between substances. The compatibility of dissimilar substances for solubility is more challenging. For instance, water, a polar solvent, readily dissolves polar solutes like ethanol.
• Pressure: Pressure exerts a greater influence on gaseous substances compared to solids and liquids. Increasing the partial pressure of a gas enhances its solubility. A notable example is the carbonation process in a soda bottle, where CO2 is contained under high pressure.

2. Solubility of Solids In Liquids

• It has been noted that the solubility of a solid is influenced by both the characteristics of the solute and the solvent. For instance, substances like sugar and common salt (NaCl) readily dissolve in water, while substances like naphthalene do not dissolve in water. Through various observations and experimental findings, it has been established that polar solvents tend to dissolve polar solutes, whereas non-polar solvents dissolve non-polar solutes. 
• Consequently, the nature of the solvent emerges as a significant factor affecting solubility. This observation gave rise to the adage "like dissolves like," signifying that polar solvents dissolve polar solutes and non-polar solvents dissolve non-polar solutes.
• Now, let's delve into the process by which a solid dissolves in a solvent. When a solid solute is introduced to a solvent, the solute particles dissolve into the solvent, a phenomenon known as dissolution. Within the solution, solute particles collide with one another, and some of these particles separate from the solution, a process termed crystallization.
• A state of dynamic equilibrium ensues between these two processes, wherein the number of solute molecules entering the solution equates to the number of particles departing from the solution. Consequently, the solute concentration in the solution remains constant at a given temperature and pressure.
• A solution in which no additional solute can dissolve in the solvent at a specific temperature and pressure is referred to as a saturated solution, as it contains the maximum amount of solute. The concentration of solute in such a solution is termed its solubility at that temperature and pressure. Conversely, if more solute can be added to a solution, it is classified as an unsaturated solution.

Factors Affecting Solubility

• Effect of Temperature: Apart from the nature of solute and solvent, temperature also affects solid solubility considerably. If the dissolution process is endothermic then the solubility should increase with an increase in temperature in accordance with Le Chateliers Principle. If the dissolution process is exothermic the solid solubility should decrease.
• Effect of Pressure: Solid solubility hardly gets affected by changes in pressure. This is due to the fact that solids and liquids are highly incompressible and practically do not get affected by changes in pressure.

3. Solubility of Gases In Liquids

The concept of gas solubility in liquids pertains to the dissolution of gases in a solvent. Solubility, in general, refers to the maximum quantity of solute that a given solvent can dissolve at a specific temperature. Gas solubility in liquids is profoundly influenced by temperature and pressure, alongside the characteristics of both the solute and the solvent.

There are many gases that readily dissolve in water, while there are gases that do not dissolve in water under normal conditions. Oxygen is only sparingly soluble in water while HCl or ammonia readily dissolves in water.

The impact of pressure

• Studies have revealed that the solubility of gases in liquids rises with an increase in pressure. To gain a deeper insight into this phenomenon, let's examine a scenario involving a gas dissolved in a solvent within a closed container, reaching a state of dynamic equilibrium. At this equilibrium, the rate of gaseous molecules entering the solution equals the rate of those leaving it.
• Now, let's consider raising the pressure of the system by compressing the gas molecules within the solution. This increase in pressure causes the gas molecules to become more concentrated in a smaller volume. Consequently, there's a greater number of gas molecules per unit volume above the solution. 
• With more gas molecules above the solution, the rate at which they enter the solution also increases. Consequently, the number of gas molecules in the solution rises until a new equilibrium is reached. Hence, the solubility of gases increases with an increase in the pressure of the gas above the solution.

The solubility of gases in liquids

• Henry’s Law gives a quantitative relation between pressure and gas solubility in a liquid. It states that:
  The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution.
• The most general way of using Henry’s Law is that the partial pressure of a gas above a solution is proportional to the mole fraction of the gas in the solution.
  P = K_Hx
  Where, p = partial pressure of the gas
  x = mole fraction of the gas in solution
  K_H = Henry’s law constant

Effect of Temperature

• Gas solubility in liquids is found to decrease with increase in temperature. The gas molecules in a liquid are dissolved by the process of dissolution. During this process, heat is evolved. According to Le Chatelier’s Principle which states that when the equilibrium of a system is disturbed, the system readjusts itself in such a way that the effect that has caused the change in equilibrium is countered.  
• So, as we know that dissolution is an exothermic process, the solubility should decrease with an increase in temperature to validate Le Chatelier’s Principle.