7 junio, 2024

Saturated solution: what it is, factors, solubility curves, examples

What is a saturated solution?

A saturated solution is a chemical solution that contains the maximum concentration of solute dissolved in a solvent. It is considered a state of dynamic equilibrium where the rates at which the solvent dissolves the solute and the rate of recrystallization are equal.

The extra solute will not dissolve in a saturated solution and will appear in a distinct phase, either a precipitate if a solid in a liquid, or an effervescence if a gas in a liquid.

Examples of solutions are shown in the image. As the precipitate darkens, the saturation point begins. The point of maximum concentration of a solute in a solvent is known as the saturation point.

Factors Affecting Saturation

The amount of solute that can be dissolved in a solvent will depend on different factors, among them, the most important are:

Temperature

The solubility increases with temperature. For example, more salt can be dissolved in hot water than in cold water.

However, there may be exceptions, for example, the solubility of gases in water decreases with increasing temperature. In this case, the solute molecules receive kinetic energy when heated, which facilitates their escape.

Pressure

The increase in pressure can force the solute to dissolve. This is commonly used to dissolve gases in liquids.

Chemical composition

The nature of the solute and the solvent, and the presence of other chemical compounds in the solution, affect solubility. For example, a greater amount of sugar can be dissolved in water than salt can be in water. In this case, the sugar is said to be more soluble.

Ethanol in water are completely soluble with each other. In this particular case, the solvent will be the compound that is in the greatest amount.

mechanical factors

In contrast to the rate of dissolution, which depends mainly on temperature, the rate of recrystallization depends on the concentration of solute at the surface of the crystal lattice, a case that is favored when a solution is immobile.

Therefore, stirring the solution prevents this accumulation, maximizing dissolution.

Saturation and solubility curves

Solubility curves are a graphical database where the amount of solute that dissolves in an amount of solvent is compared at a given temperature.

Solubility curves are commonly plotted for an amount of solute, either solid or gas, in 100 grams of water.

On the coordinate axis is the temperature in degrees Celsius, and on the abscissa axis is the solute concentration expressed in grams of solute per 100 grams of water.

The curve indicates the saturation point at a given temperature. The area below the curve indicates that you have an unsaturated solution, and therefore you can add more solute.

The area above the curve has a supersaturated solution.

Taking sodium chloride (NaCl) as an example, at 25 degrees Celsius approximately 35 grams of NaCl can be dissolved in 100 grams of water to obtain a saturated solution.

Examples of saturated solutions

Saturated solutions can be found everyday. The solvent does not necessarily have to be water. Below are everyday examples of saturated solutions:

Soda and soft drinks in general are saturated solutions of carbon dioxide in water. That is why, when the pressure is released, bubbles of carbon dioxide are formed. Earthen soils are saturated with nitrogen. Sugar or salt can be added to the vinegar to form a saturated solution. Add powdered chocolate to milk until it dissolves, forming a saturated solution. Milk can be saturated with flour to such an extent that no more flour can be added to the milk. Melted butter can be saturated with salt, when the salt no longer dissolves.

What is a supersaturated solution?

The definition of a supersaturated solution is one that contains more dissolved solute than could normally be dissolved in the solvent. This is usually done by increasing the temperature of the solution.

A slight disturbance of the solution or introduction of a «seed» or tiny crystal of solute will force the excess solute to crystallize. If there is no nucleation point for crystal formation, excess solute can remain in solution.

Another form of supersaturation can occur when a saturated solution is carefully cooled. This change in conditions means that the concentration is actually greater than the saturation point, and the solution has become supersaturated.

This can be used in the recrystallization process to purify a chemical: it dissolves to saturation point in hot solvent, then as the solvent cools and solubility decreases, excess solute precipitates.

The impurities, which are present in a much lower concentration, do not saturate the solvent and thus remain dissolved in the liquid.

References

Anne Marie Helmenstine, P. Saturated Solution Definition and Examples. Retrieved from about.com. Examples of Saturated Solution. Retrieved from examples.yourdictionary.com. Saturated Solution: Definition & Examples. Retrieved from study.com. Solubility Curves. Retrieved from kentchemistry.com. Types of saturation. Retrieved from chem.libretexts.org.

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