Supersaturated Solution

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What is a Solution?

A solution is defined as a homogeneous mixture of either two or more pure substances on a molecular level, whose composition may vary within particular limits. A solution which is having two components (otherwise substances) is called the solute and the solvent.


What is Supersaturated Solution?

A supersaturated solution definition is given as the one, which contains more dissolved solute than needed for preparing a saturated solution and is prepared by heating a saturated solution, adding excess solute, and then by gently cooling it. And, the excess dissolved solute will crystallize by seeding the supersaturated solution with fewer solute crystals.


For example, the pan boiling object is the production of a fine even crop of sugar crystals. A major condition for the attainment of this end is given as the maintenance of control over the crystallization rate during the growth. Generally, a crystal surface, which is maintained in a solution will only grow when the concentration of the solution is maintained at a greater level compared to the saturation concentration. Such a kind of solution is said to be supersaturated.


Supersaturation in Phase Change (Crystallization and Condensation)

  • Physical and chemical processes in the vapour melt or the solution phase of each system occur through the formation of three-dimensional nuclei of a new phase and take place only when the medium gets supersaturated.

  • The production of the nuclei is associated with a change in the system’s free energy. In the homogeneous system, the nuclei of the new phase are not produced as soon as the system becomes supersaturated though thermodynamically, such a situation becomes possible.

  • The system is known to be in a metastable equilibrium state, and it can remain in the same state without attaining the less or minimum free energy corresponding to the equilibrium state.

  • In other terms, in such cases, the nucleation of new phase sets in after some time period, where the value depends on such factors as the pressure and temperature of the system, the presence of chemical phases varies from the increased supersaturation level, and nucleating phase facilitates the nucleation process of the new phase.

  • However, there is a supersaturation level always when a new phase instantaneously nucleates. That is called the new phase precipitates.

  • This type of supersaturation level corresponds to the upper limit of the metastable equilibrium state and defines the metastable width.

Applications of Supersaturated Solution

When a solid solute dissolved solution in a liquid solvent is saturated, it is in the thermodynamic equilibrium. In order for the crystallization to take place, the system state must be shifted to the nonequilibrium state, where the concentration of the solute present in the solution exceeds its equilibrium concentration at given solution conditions. And, the solutions present in the nonequilibrium state are known to be supersaturated. The easiest method to create a supersaturated solution is achieved by cooling.


Initially, a solution can be prepared at the point A. If the prepared solution is cooled, it will be saturated when it intersects the line of saturation. If it is cooled previously, the saturation line to point B, it will be supersaturated. Only because this solution is supersaturated, but, it does not mean that it will crystallize immediately. Supersaturated solutions are said to be metastable. It means there is a free energy barrier which should be overcome for the phase transition, which is to be overcome.


The most common and simplest method of making a supersaturated solution is given by cooling. However, this is not the only available method to be used. There are several other methods such as temperature change, change in pH, solvent evaporation, chemical reaction & alteration in solvent composition.


Supersaturated Solution Examples

Let us look at the saturated solution examples.

The supersaturated solution holds more dissolved substances compared to a saturated solution. For example, 40g NaCl present in 100ml H2O. The additional 4.0g NaCl remains to be undissolved.


Solved Examples

1. Give the Mass Percent of Sodium Hydroxide Present in a Solution, Made by Dissolving 8.00g NaOH in 50.0g H2O?


Solution

It is given that 8.00g NaOH.

50.0gms H2O

Solving for mass per cent

= 8.00g NaOH/8.00gms NaOH + 50.0g H2O

= 13.8% NaOH solution.


2. Will a Solution Prepared by Adding 2.5g of CuSO4 to 10gms of H2O be Saturated for the Unsaturated Solution at 20oC?


Solution

First, we need to know the CuSO4 solution at 20oC. From the above-given figure, we can see that the CuSO4 solubility at 20oC is about 21gms per 100g of H2O. This particular amount is equivalent to 2.1gms of CuSO4 per 10gms of H2O.

Since 2.5gms per 10gms of H2O is greater than the 2.1gms per 10gms of H2O, the solution will become saturated, and 0.4gms of CuSO4 will be maintained.

FAQ (Frequently Asked Questions)

Q1. Give One Example of a Supersaturated Solution?

Answer: A supersaturated solution is defined as the excess solute solution that can be dissolved by the solvent. If we are not aware of what a solute or solvent is, the material that can be dissolved in the solution, like salts but not restricted to salts, is known as a solution. The most popular example can be given as sodium acetate, which is supersaturated.

Q2. Explain What Causes a Supersaturated Solution?

Answer: A supersaturated solution is described as a solution which contains more than the average solvent, which can be dissolved at a given specific temperature. And, the recrystallization of the excess dissolved solvent present in a super-saturated solution is started by inserting a small solute crystal, which is known as a seed crystal.

Q3. Explain about a Supersaturated Sugar Solution?

Answer: According to the substance’s solubility, a “supersaturated” solution will form more dissolved content than it should be limited to. Whereas, in the case of sugar, whose chemical name is given as “sucrose,” nearly 211 grams of water can dissolve in 100 millilitres. The solubility is dependent on temperature; more sugar dissolves in hot water than cold water.

Q4. Why are Supersaturated Solutions Known to be Unstable?

Answer: Under suitable conditions, solutions can often be formulated which contain an excessive amount of solvent than the one required to produce a saturated solution. Owing to the presence of a solute, which is present in a supersaturated solution of a concentration greater than the equilibrium concentration, the super-saturated solutions are unstable.

Q5. Discuss the Result When a Supersaturated Solution Gets Cooled?

Answer: The solid crystals present in the hydrated crystals will dissolve into the bath, by producing a supersaturated solution. And, when sodium thiosulfate solution is gradually cooled, the super-saturated solution will remain as liquid form. Placing a tiny crystal in the over-saturated solution would turn the liquid into solid.