What is Solubility?

The maximum amount of a solute or any substance that can be dissolved in 100 grams of a solvent at a given temperature is called the solubility of that solute or substance in that solvent. So, the amount of a solute dissolved in a saturated solution at a given temperature is the solubility of that solute. Besides this, solubility is measured and expressed in grams/100 grams of a solvent.

For example, 30 gm is the maximum amount of a solute that can be dissolved in 100 gm of a solvent at a 20 degree centigrade to form a saturated solution. So, in this case, the solubility of the solute is 30 gm/100gm of the solvent at 20 degree centigrade.

Furthermore, we can say that solubility is a property of a substance that allows it to dissolve in another substance that acts as a solvent. The substance which is capable of dissolving in another substance is known as a soluble substance. For example, when we dissolve sugar in water, we can say that sugar is soluble in water and solubility is the property of sugar.

Based on the solubility of a solute, it can be of two types highly soluble, sparingly soluble or insoluble. For example, a solute is called soluble if we can dissolve 0.1 gm or more amount of it in 100 ml of a solvent. However, if less than 0.1 gm of a solute dissolves in 100ml of a solvent, it is called a sparingly soluble or insoluble solute.

Effect of temperature and pressure on the solubility

There are different types of solutions based on the physical state of the solute and solvent. Let us see the effect of temperature and pressure on liquid solutions!

Liquid solutions are of three 3 types

i) Solid in liquid: A solid solute is dissolved in a liquid solvent.

• Effect of temperature: The solubility of a solid solute in a liquid solvent increases with an increase in temperature. Let us understand how?
  The intermolecular spaces in solids are very less due to strong forces of attraction between their constituting molecules or particles. Whereas, in liquids, the intermolecular space is more as compared to solids as in liquids the force of attraction is less than solids. Now, when the temperature increases the interparticle space between liquid molecules increases further and solid particles get more space to get dissolved in a liquid solvent and thus solubility increases.
  However, solubility not always increases on increasing the temperature. For example, if the dissolution of solute is an endothermic process (takes or absorbs energy), the solubility increases with an increase in temperature. Whereas, if the dissolution is an exothermic reaction (heat is released), the solubility decreases on increasing temperature.
• Effect of pressure: Solids are non-compressible, and liquids are also nearly non-compressible. So pressure does not affect much the solubility of solids in liquids.
• Nature of solute and solvent:
  The nature of solute or solvent also affects solubility. For example, polar solutes are more soluble in polar solvents but insoluble in non-polar solvents, 'like dissolves likes'. Such as the polar ionic compound like salt easily dissolves in polar solvents such as water.
• Surface area: It affects the solubility in a different way, if a solute gets more surface area of the solvent, it will tend to dissolve quickly. On the other hand, if the surface area is less, the solute will take more time to dissolve completely.

ii) Gases in liquids: It contains gas as a solute in a liquid solvent.

• Effect of temperature: The solubility of gases decreases in liquid solvents on increasing temperature. When the temperate increases, the heat energy or kinetic energy of gas particles increases and they tend to move out of the liquid due to the increased kinetic energy and thus their solubility decreases. On the other hand, when the temperature is reduced, the gas particles' kinetic energy decreases and their tendency to leave the solvent also decreases and thus their solubility increases when the temperature is less.
• Effect of pressure: The solubility of gases in liquids increases with an increase in pressure and decreases if the pressure is decreased. When the pressure is applied, gas particles come closer, their kinetic energy and space between them decreases and thus more particles enter into the solvent, which increases their solubility. When the pressure is reduced, the gas particles return to their original state, their kinetic energy and space between them increase and they tend to move out of the liquid solvent and thus solubility decreases. For example, in aerated beverages, carbon dioxide is dissolved in a liquid solvent by applying pressure.

iii) Liquids in Liquids: A liquid solute is dissolved in a liquid solvent.

• Effect of pressure: It does not affect the solubility of liquids in liquids.
• Nature of solute and solvent: Polar liquids are soluble in polar solvents and non-polar solvents and soluble in non-polar solvents. For example, ethanol mixes with water as both are polar liquids. Whereas, oil does not mix with water as oil is non-polar and water is polar.

What is miscibility?

Miscibility is the ability or property of a liquid to completely dissolve in another liquid in all proportions.

Types of liquid solutions based on Miscibility

Based on miscibility, s solution can be of two types; completely miscible and partial miscible solution as described below;

• Completely miscible solutions: One liquid or solute is completely miscible in another liquid or solvent in all proportions, e.g. ethyl alcohol in water; glycerine and alcohol; and acetone and alcohol form completely miscible solutions as they can mix completely in all proportions. Although polar and semi-polar liquids form completely miscible solutions, some non-polar solvents also mix completely to form completely miscible solutions such as a solution of benzene and CCl₄ (carbon tetrachloride).
• Partial miscible solutions: The liquid solutions in which one liquid is not completely miscible in another liquid are called partial miscible solutions. For example, phenol and water; ether and water form partial miscible solutions. Such liquids when mixed form two layers, each layer contains a certain amount of the other liquid.

However, the miscibility of partial miscible solutions is affected by temperature. The temperature at which they become completely miscible at all proportions is called the upper critical temperature or upper consolute temperature.

Furthermore, the liquids that are not miscible with each other and thus do not form a solution and known as completely immiscible liquids, such as water and mercury.

Difference between Solubility and Miscibility

The term 'Solubility' is generally refers to the ability of a solid solute to dissolve in a liquid solvent. Whereas, the term 'Miscibility' specifically used for the ability of a liquid solute to dissolve in another liquid solvent in all proportions. Whereas, in Solubility, the solute is not soluble in solvent in all proportions, it has a limit beyond which it may not dissolve.

Examples of Miscibility or Miscible Solutions

• Orange juice and carbonated water, which are mixed together to prepare a delicious orange juice mix well due to miscibility.
• Gasoline which is used as fuel for vehicles is also a miscible liquid as it is made of various organic solvents like xylene, toluene and benzene.
• Water and organic compounds like alcohols, aldehydes and ketones also form miscible solutions.