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Solvation

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Left to right: 1) Iodine dissolved in dichloromethane, 2) Immediately after dissolving triphenylphosphine in it, 3) One minute after the addition of the triphenylphosphine, 4) After adding excess iodine

Solvation is the process of forming a solution by dissolving a solute in a solvent. One of the best known examples of this is the dissolving of sugar cubes (a solute) in tea or coffee (a solvent). Solvation or Dissolution starts with a sample of solvent, either liquid, solid, or gaseous. Then, another substance, the solute, is added and immediately the solvent's molecules begin attacking the solute's. As a result, the solute's molecules disassociate with each other and disperse evenly throughout the solute, resulting in a solution of the two substances, also called a homogeneous mixture.

Dissolution

A 99.95% platinum wire dissolved in a solution of nitric acid and hydrochloric acid.

Dissolution, the main component of the solvation process, occurs as a solution is formed from a solute dissolving in a solvent. [1]

Solute

A solute is a substance that is being dissolved in a solvent.[2] When salt is dissolved in water, salt is the solute. When plastics are dissolved in acetone, the plastics are the solute. Sometimes (as in the case of amalgams), it is difficult to tell the different. The solute is normally the smaller amount, while the solvent is the larger amount.

Solvent

Acetone is a solvent that can dissolve certain nonpolar plastics.

A solvent is the substance that dissolves the solute. The solvent is the medium into which the solutes are mixed or dissolved.[2] A solvent can be in any physical state either as solid, as a liquid or as gas.[2] In the example of salt and water, water is the solvent. In the example of certain plastics and acetone, the acetone is the solvent.

Solvents are in a general way classified into polar and non-polar. The water is an example of polar solvent while CS2 and CDCl3 are examples of non-polar solvents.[3]

Solution

Solutions, also called homogeneous mixtures, are not to be confused with heterogeneous mixtures which contain visibly separate phases of matter. Solutions contain two or more substances that are indistinguishable from each other once mixed together.[4] While most people assume that a solute is always a solid and the solvent is a liquid, solutions can be composed of nearly any combination of states of matter. The table below summarizes all possible combinations of the states of matter as solvents or solutes in a solution.

Solutions
Solvents
gas liquid solid
Solutes gas gas-in-gas gas-in-liquid gas-in-solid
liquid does not exist liquid-in-liquid liquid-in-solid
solid does not exist solid-in-liquid solid-in-solid

Liquids are always the solvent in any solution they are in, and gases are always the solute. In the case of gas-gas, liquid-liquid, or solid-solid solutions, the substance in lesser volume is the solute and the one in greater volume is the solvent. When a solute is introduced into a solvent, the solvent's molecules immediately begin to attack the solute particles, breaking the bonds between solute molecules. The solute molecules, now free of attraction to each other, disperse evenly throughout the solvent because of continuous molecular collisions, as described by the molecular-kinetic theory. Dissolution does not occur at a constant rate for all solvent and solute combinations as many chemical and thermal factors determine this.[5]

Solubility

Creating a salt and water solution by adding amounts of table salt. Water, H20, and table salt, NaCl, are miscible because of their polar molecules

Solubility is the ability of a material (solute) to break down when put into a solvent. Once the material has mixed with the solvent that broke it down, it is referred to as a solution.

As we add more and more of the solute, eventually will realize that the solute added does not dissolve anymore. When this is achieved we say that the solution is a saturated solution.[6] As the solute concentration increases the possibility that particles of the solute re-connect to each other also increases. This process is called crystallization.[7]

As a general rule, solvents that have polar molecules are able to dissolve solutes of polar molecules. Likewise, nonpolar solvents are able to dissolve nonpolar solutes; the general statement, "like dissolves like" summarizes these basic rules. When two substances are able to form a solution together, they are referred to as miscible, or a miscible pair. Solutes and solvents that are unable to form solutions are called insoluble or immiscible.[8] Solubility also relates to the volume of solute a solvent is able to hold. Solubility can be measured in quantifiable amounts by the use of molality. Molality is defined as the number of moles of solute divided by the mass in kilograms of the solvent it is dissolved in.[4]

Molality = mol solute/kg solvent

Rate

The rate at which a solvent dissolves an amount of solute is determined by many contributing factors, whether thermal, chemical, or otherwise. Primarily, solubility determines how quickly a solute dissolves in a particular solvent, as polar, or water-based, solutions dissolve faster than nonpolar, or oil-based solutions. Additionally, temperature heavily influences the rate of solvation because as solvent molecules absorb thermal energy, they begin to move faster. This increase in kinetic energy causes the solvent molecules to collide more times per second, including collisions with solute particles, thereby increasing the rate at which the solute molecules disassociate with each other. Additionally, the increase in movement in the solute molecules allows them to more easily break free of intermolecular forces between them.[9] Once free, the faster moving molecules more quickly disperse throughout the solvent, expediting the rate of dispersion. Additionally, a sample of solute with a larger surface area will dissolve faster because more solvent particles are in contact with each solute molecule on more sides. A larger surface area is produced as the size of the solute particles decreases because the ratio of surface area is inversely proportional to volume.[10]

Rate of Solvation = SA:V =(SA) / (V) = x^2/x^3 = x^(-1)

Where SA stands for surface area and V for volume. Because surface area only involves two dimensions, it is represented with an x^2. Volume is represented with an x^3 because it represents three dimensions.

Saturation

A given amount of solvent can only hold a certain amount of solute in it, varying based on the solvent's temperature. As solute dissolves and disperses throughout a solvent, the solvent is said to become more saturated. When the maximum amount of solute is dissolved in a given solvent, the solution is said to be saturated; while more solute can be held, the solvent is considered unsaturated. Once a solution reaches the point of saturation, any excess solute will simply "fall out" of the solvent, unable to stay suspended. For example, when a sample of water becomes saturated with sugar, additional sugar will fall to the bottom of the container as the water molecules' collisions cannot suspend any further solute.[8]

However, the solvent's temperature and other various factors can influence the amount of solute it can hold. When a solvent's temperature is raised, the molecules in it have more energy and move more quickly. This increase in particle motion and collisions enables the solvent to keep more solute particles suspended in it without falling out. When a solvent contains more solute than normal conditions allow, it is said to be supersaturated. When a supersaturated solution is brought back down to regular temperatures, the excess solute does not fall out of the solution. Rather, the solute stays suspended within the solvent despite the cooler temperature, but when the solution is disturbed by sudden movement or introduction of a crystal, the solute almost immediately crystallizes inside the solvent. [11]

Candy like these conversation hearts are formed by the creation of supersaturated solutions of sugar and water

Supersaturated solutions are vitally important to the production and manufacturing of many products, especially in the food industry. Almost all candy is made from a supersaturated solution of sugar and water, with hard candy containing up to ninety percent sugar to create its dense crystalline structure. Water is heated to a certain temperatures to allow additional sugar to be held within it. After the proper amount of sugar is added, the solution is allowed to cool before cutting and packaging the product. Very specific percentages of sugar correspond to a variety of consistencies in candy, resulting in the textures seen in candies such as taffy, candy corn, and crystalline confections, like peppermints.

Video

A brief video demonstrating how composition and particle size influence the rate of solvation.

References

  1. Difference Between Solubility and Dissolution Difference Between. Web. Last accessed May 5, 2015.
  2. 2.0 2.1 2.2 Brady, James E.; Holum, John R (1996). Chemistry: The Study of Matter and its Changes (2nd ed.). New York: John Wiley & Sons. p. 128. ISBN 0-471-10042-0. 
  3. Sinnott, Michael L (2007). Carbohydrate Chemistry and Biochemistry:Structure and Mechanism. Cambridge: RSC Publishing. p. 41; 59. ISBN 978-0-85404-256-2. 
  4. 4.0 4.1 Mixtures University of Memphis: Department of Chemistry. Web. Last accessed May 5, 2015.
  5. Ball, David, John Hill, and Rhonda Scott. The Dissolution Process Flat World Knowledge. Web. Last accessed May 5, 2015.
  6. Brown, Lawrence S.; Holme, Thomas A (2011). Chemistry for Engineering Students (2nd ed.). Belmont, CA: Brooks/Cole. p. 75. ISBN 978-1-4390-4791-0. 
  7. Brown, Theodore L.; LeMay Jr., H. Eugene; Bursten, Bruce E.; Murphy, Catherine J.; Woodward, Parrick (2009). Chemistry: The Central Science (11th ed.). Upper Saddle River, NJ: Pearson Education, Inc.. p. 534-535. ISBN 978-0-13-600617-6. 
  8. 8.0 8.1 The Secrets of Solvation Prevor. Web. Last accessed May 5, 2015.
  9. Factors Affecting the Rate of Dissolution Weber School District. Web. Last accessed May 5, 2015.
  10. Rate of Dissolving McAdam High. Web. Last accessed May 5, 2015.
  11. Chemistry Review: Dissolving ACS. Web. Last accessed May 5, 2015.