Supersaturated Solution: Explanation, Characteristics and Examples

James Jerden, Amanda Robb
  • Author
    James Jerden

    James holds a doctorate in geochemistry from Virginia Tech and a master’s degree in geology from Boston College. He has worked as a research scientist for over 20 years and has developed and taught college-level geoscience courses throughout his career.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.

How is a supersaturated solution formed? Explore the process of how a supersaturated solution is made and identify some of its unique characteristics. See examples. Updated: 10/14/2021

Table of Contents


What is a Solution?

A solution is a homogeneous mixture of two or more substances (solutes) dissolved in another substance (solvent). For example, salt in water, carbon dioxide in carbonated beverages, water vapor in air, and carbon in steel are all homogeneous mixtures and thus are solutions. In a homogeneous mixture, the dissolved substances (solutes) are mixed with the solvent at an atomic or molecular scale. Another way of saying this is that a solution (i.e., a homogeneous mixture) is a single-phase (solid, liquid, or gas) with a uniform composition (i.e., the chemical composition does not vary with location within the mixture).

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What is a Supersaturated Solution?

What is a supersaturated solution? As the name implies, a supersaturated solution is beyond saturated. A solution is saturated when its solute is in dynamic equilibrium with the undissolved form of the solute. This is equivalent to saying that a solution is saturated when its solute is present at its maximum theoretical concentration. Where concentration refers to the mass of solute (grams) dissolved per volume of solution (liters). The maximum theoretical concentration is also referred to as the solubility of the solute. So, a supersaturated solution has a solute concentration greater than its maximum theoretical concentration. That is, the solute is not in equilibrium with its undissolved form. To further clarify the supersaturated solution definition, we will now explore the concept of dynamic equilibrium.

Bubbles released from sparkling wine supersaturated in carbon dioxide.

Supersaturated solution example: bubbles released from sparkling wine supersaturated in carbon dioxide.

What is dynamic equilibrium?

Dynamic equilibrium is one of the most important concepts in the study of solution chemistry. When two opposing chemical reactions (e.g., dissolution and crystallization of a solute) are operating simultaneously at equal rates, the overall process is said to be in dynamic equilibrium. This process can be summarized for a liquid solution by the following reaction:

{eq}\text{Solute}\;+\text{Solvent}\; \underset{crystallization}{\stackrel{dissolution}{\rightleftarrows}} \;\text{Solution}\; {/eq}

For a gas phase solution such as water vapor in air, the reaction will involve condensation rather than crystallization:

{eq}\text{Solute}\;+\text{Solvent}\; \underset{condensation}{\stackrel{dissolution}{\rightleftarrows}} \;\text{Solution}\; {/eq}

When the processes of crystallization, condensation, or bubble formation do not occur for some reason, the solute concentration can exceed its solubility. That is, the solute can be present at levels that exceed the saturation point of the solution. Phenomena that block crystallization and condensation are called a kinetic barrier to nucleation. Nucleation is the process of solute coming out of solution to form either a solid crystal (for liquid solution) or a droplet of liquid (for the gas phase solution). The nucleation of crystals is often a kinetic barrier to the crystallization process because the formation of the atomic structure of a crystal requires energy (heat). Under some conditions, that energy is not available. A saturated salt solution offers a real-world example of a reaction in dynamic equilibrium:

{eq}\text{NaCl}(solid)\;{\rightleftarrows}\;\text{Na}^{+}(dissolved)\;+\;\text{Cl}^{-}(dissolved) {/eq}

This reaction indicates that when the salt dissolves, it breaks into its constituent atoms. Because of chemical interactions in the solvent, the sodium and chlorine atoms become electrically charged and are referred to as ions in solution. Sodium becomes a positively charged ion, and chlorine becomes a negatively charged ion. In the absence of a kinetic barrier to nucleation, the ions in solution can recombine through the crystallization process to form a crystal of solid sodium chloride.

How Does a Solution Become Supersaturated?

A solution becomes supersaturated when the amount of solute per volume of solution exceeds its maximum theoretical concentration. That is when the solute concentration exceeds its solubility. As discussed above, this occurs when there is a kinetic barrier to crystallization or condensation processes. A typical way to overcome a kinetic barrier to the nucleation is to add what is called a seed crystal or condensate seed. The seed added to a supersaturated solution eliminates the need for nucleation and thus eliminates the kinetic barrier to crystallization or condensation. A simple experiment involving sugar dissolved in water can be used to demonstrate this process.

Why Do Sugar Crystals Form In A Supersaturated Solution?

Like most substances, the solubility of sugar increases with increasing temperature. So, according to the solubility table below, we can make a saturated solution by adding 487 grams of sugar (sucrose) to 100 milliliters of water that has been heated to 100 degrees Celsius (C). Now, what happens if we let the sugar water cool down to 25C? The solubility table indicates that if the sugar does not crystallize (i.e., if there is a kinetic barrier to nucleation), you would end up with a supersaturated solution. You can calculate the amount of supersaturation by subtracting the solubility of sugar at 25C (211 g per 100 mL) from the amount of sugar we added to the solution when it was at 100C (487 g per 100 mL). So, at 25C, our sugar solution is supersaturated by 276 grams per 100 milliliters of solution.

Temperature (degrees Celsius) Solubility (grams of sugar dissolved in 100 milliliters of water)
0 179
25 211
75 340
100 487

So, why do sugar crystals form in a supersaturated solution? The answer has to do with the concept of seed crystals mentioned above. Seed crystals eliminate the kinetic barrier to crystallization. The surface of the seed attracts the sugar molecules as they come out of solution because its surface represents a low-energy nucleation site. That is, a site where sugar crystals can form without any kinetic barriers to nucleation. Once all of the excess sugar has crystallized onto the seed, the dynamic equilibrium of the solution is reestablished, and you now have a saturated sugar solution. Seeds do not always have to be crystals. For example, just placing a string or wooden stick in a supersaturated sugar solution is enough to overcome the kinetic barrier to nucleation. The excess sugar crystallizes onto the string or stick to form rock candy.

Making A Supersaturated Solution Through Evaporation

Supersaturated solutions can also be created by evaporating a saturated salt solution. Evaporation removes solvent from the solution, thus increasing the concentration of solutes. If enough solvent is removed, the solute concentration can exceed its solubility, thus creating a supersaturated solution. An important example of this process involves evaporating salty water from natural environments to generate salt deposits that can be used for industrial or culinary purposes. Seawater is typically undersaturated with sodium chloride. However, in salt fields, large enclosed regions of seawater are allowed to evaporate until they become supersaturated. Crystallization occurs and forms large deposits of salt that can be harvested and sold.

The dawn salt harvest at Hon Khoi Salt Fields, Vietnam.

Supersaturated solution example: the dawn salt harvest at Hon Khoi Salt Fields, Vietnam.

Characteristics of Supersaturated Solutions

Supersaturated solutions have three defining characteristics: (1) the solute concentration exceeds its solubility, (2) the solution is metastable (i.e., not in equilibrium), and (3) the processes of crystallization or condensation are kinetically inhibited.

Characteristic 1: Solute Concentration Exceeds Its Solubility

As discussed above, the solubility of a solute typically increases with increasing temperature. Therefore, if a saturated solution formed at a high temperature is cooled, it will become a supersaturated solution as long as the processes of crystallization or condensation are blocked from occurring (e.g., by a kinetic barrier to crystal nucleation). Supersaturated solutions formed in this way include:

  • Solids dissolved in liquids (e.g., sugar in water).
  • Gases dissolved in gases (e.g., water vapor in air).
  • Gases dissolved in liquids (e.g., oxygen dissolved in water).

Characteristic 2: the Solution is Metastable

The term metastable describes a system that is essentially stuck in an energy state higher than its stable condition. In the case of a supersaturated solution, the excess energy of the system is represented by the excess solute. Another way of saying this is that: a supersaturated solution is metastable because it is not in dynamic equilibrium. A saturated solution, on the other hand, is in dynamic equilibrium and is thus stable. A supersaturated solution can be stabilized (i.e., brought into dynamic equilibrium) by the crystallization or condensation of the excess solute present in the solution.

Characteristic 3: the Processes of Crystallization or Condensation are Kinetically Inhibited

As discussed above, supersaturated solutions form because the crystallization and condensation processes that would otherwise bring the solutions into dynamic equilibrium are kinetically blocked. These kinetic limitations happen because the formation of solute crystals or condensate droplets require a certain amount of energy to form (i.e., nucleate). In some chemical scenarios, such as cooling a supersaturated solution, the energy needed to create these crystals and droplets is not present. In such cases, the only way to overcome the kinetic limitations is to add seed crystals that remove the need for nucleation and thus facilitate the crystallization or condensation of the excess solute. This leads to the formation of a saturated solution (i.e., a solution in dynamic equilibrium).

Examples of Supersaturated Solutions

Saturated solutions occur in several different environments. Some are involved in food preparation, some occur in natural environments, and some are used for industrial purposes. Several common examples of saturated solutions are described in the table below.

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Frequently Asked Questions

What are some examples of supersaturated solutions?

There is a diverse range of natural and anthropogenic examples of supersaturated solutions. Some of the most common and most important include: (1) sugar solutions prepared at high temperature to make rock candy, (2) carbonated beverages before they are opened (popping the cork releases the supersaturated carbon dioxide), (3) cave waters supersaturated in calcium carbonate that form stalactites and stalagmites, (4) air masses that dissolve water vapor at high temperature but are then transported to cooler parts of the atmosphere, supersaturated drug delivery systems, (5) the blood of deep-sea divers, which can supersaturate with nitrogen due to high water pressures, (6) sodium acetate hand warmers.

What is supersaturated solution?

A supersaturated solution is a homogeneous mixture in which the solute is not in equilibrium with its undissolved form. This is equivalent to saying that a supersaturated solution has a solute concentration greater than its maximum theoretical concentration (i.e., greater than its solubility).

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