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Solubility Equilibria

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  • Last Updated : 04 Apr, 2022
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The word “solubility product” refers to inexpensively soluble salts. It is the greatest product of the molar concentration of the ions (raised to their appropriate powers) produced by compound dissociation. The solubility product is constant at any given temperature. The lower the solubility product’s value, the lower the solubility, and the higher the solubility product’s value, the higher the solubility.

Solubility Product Constant

The solubility product constant is the equilibrium constant for the dissolving of a solid substance into an aqueous solution. It’s represented by the symbol Ksp. The solubility product is a type of equilibrium constant whose value varies with temperature. Ksp typically rises as temperature rises due to increased solubility. 

Solubility is defined as the ability of a substance known as a solute to dissolve in a solvent and create a solution. The solubility of ionic substances in water (which dissociate to form cations and anions) varies greatly. 

Some substances are highly soluble and may even absorb moisture from the environment, whilst others are quite insoluble.

Significance of Solubility Product

  • The lattice enthalpy of the salt and the solvation enthalpy of the ions in the solution are the most critical elements in determining solubility.
  • When a salt is dissolved in a solvent, the strong forces of attraction of the solute must be countered by interactions between ions and the solvent (lattice enthalpy of its ions).
  • Ion solvation enthalpy is always negative, indicating that energy is released during the process.
  • The solvation enthalpy, or the amount of energy released during solvation, is determined by the solvent’s composition.
  • Because non-polar solvents have a low solvation enthalpy, this energy is insufficient to overcome the lattice enthalpy.
  • As a result, the salts cannot be dissolved in non-polar solvents. As a result, in order for salt to dissolve in a solvent, its solvation enthalpy must be greater than its lattice enthalpy.
  • Effect of Temperature on Solubility: We can increase a solute’s solubility by increasing its temperature. Water generally dissolves solutes at 20° C or 100° C. By increasing the temperature, soluble solid or liquid substances can be entirely liquefied. However, in the case of gaseous substances, the temperature has an inverse effect on solubility, which means that as the temperature rises, gases expand and escape from their solvent.

Solubility of Liquids In Liquids

  • Except for a few exceptions, water is known as a universal solvent because it dissolves practically every solute. A substance’s solubility can be affected by a variety of factors. Solubility is the production of new bonds between solute molecules and solvent molecules. 
  • In terms of amount, solubility is the greatest concentration of solute that dissolves in a known concentration of solvent at a given temperature. Based on the concentration at which they dissolve in a solvent, solutes are classed as highly soluble, sparingly soluble, or insoluble. 
  • A solute is said to be soluble if it can be dissolved in a 100ml solvent at a concentration of 0.1 g or higher. While a concentration of less than 0.1 g in the solvent is known to be sparingly soluble. As a result, it is well understood that solubility is a quantitative statement articulated by the unit gram/litre (g/L).
  • Solubility can be used to create a variety of solutions. A saturated solution is one in which a particular amount of solute is completely soluble in a solvent at a given temperature. A supersaturated solution, on the other hand, is one in which a certain quantity of solute is dissolved at the same temperature and the solute begins to salt out or precipitate.

Sample Questions

Question 1: What is the difference between solubility and solubility product constant?


A substance’s solubility in a solvent is the total amount of solute that can be dissolved in the solvent at equilibrium. The solubility product constant, on the other hand, is an equilibrium constant that offers information on the equilibrium between the solid solute and its constituent ions that are dissociated across the solution.

Question 2: What is the solubility product constant for calcium chloride?


CaCl2 is the chemical formula for calcium chloride. One calcium chloride molecule dissociates into one calcium cation and two chloride anions when dissolved in polar liquids. This equilibrium reaction is illustrated below.

CaCl2 ⇌ Ca2+ + 2Cl

As a result, the solubility product constant can be written as:

Ksp = [Ca2+][Cl]2

Question 3: What factors affect the value of Ksp?


Some key parameters influencing the solubility product constant are:

  1. The effect of common ions (the presence of a common ion lowers the value of Ksp).
  2. The ion-diversity effect (if the ions of the solutes are uncommon, the value of Ksp will be high).
  3. Ion-pairs are present.

Question 4: How is Solubility Related to Concentration Why is it Important to Know the Solubility of a Substance?


The amount of solute that can dissolve in a given amount of solvent at a given temperature is referred to as solubility. A solution’s concentration is the amount of solute in a given amount of solution. A dilute solution has a low solute concentration. Solubility determines how chemicals dissolve into one another. The solute is the material that dissolves into the other, and the solvent is the substance that is dissolved into. These factors can either speed up or slow down the rate of solubility of certain compounds.

Question 5: Do gases also dissolve in water and can we also call them Solute?


Water is an excellent solvent that is abundant on Earth. It is soluble in a wide range of substances. Everyone is familiar with solid-state compounds that can dissolve in water. Surprisingly, several chemicals that naturally exist in a gaseous state are also dissolved in water. Canon dioxide is a well-known gas for this purpose, and it is widely utilised in the manufacturing of soda water or soft drinks. Oxygen is also soluble in water in small levels but is critical to the living system that all bodies of water support. Another known gas that dissolves in water is ammonia.

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