Why Do Things Dissolve

A substance will dissolve, quite simply, if it is energetically favourable for it to do so. If the sum of the energies required to break apart those forces holding the potential solute together and to separate the solvent molecules from one another is outweighed by the energy released on solvation, then the substance will dissolve. However, dissolution is a kinetic as well as a thermodynamic process, and solutes that dissolve slowly may be accelerated greatly by the employment of heat or ultrasound. 

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Although solvents are used as dispersing agents and in the formation of emulsions, they are generally used to dissolve materials. Whether this is to clean a surface or a reaction vessel, or to act as a heat transfer medium in a reaction, we need to consider the question— Why do things dissolve Generally, the reasons are thermodynamic, in that if the dissolution process is energetically favourable it will occur. However, kinetics can also play a role and solutes that are poorly soluble at room temperature can be heated to increase solubility, a technique that is widely employed in recrystallizations. 

Chemistry makes connections among the composition, structure, and behavior of matter. As you study chemistry, you will learn how the chemical and physical properties of matter are important clues to its submicro-scopic structure and behavior. You U see how knowing the structure of an atom of an element can enable you to predict the chemical behavior of that element. You U learn how the state of a substance at room temperature provides clues to the way its atoms are arranged. You ll find out why some things dissolve in water but others do not, how metals corrode, how batteries work, why compounds containing carbon are important to life, and how a nuclear reactor works. 

Why do some things dissolve in one solvent and not another For example, oil and water will not mix to form a solution, but oil will dissolve in gasoline. There s a general rule of solubility that says like-dissolves-like in regards to polarity of both the solvent and solutes. Water, for example, is a polar material it s composed of polar covalent bonds with a positive and negative end of the molecule. (For a rousing discussion of water and its polar covalent bonds, see Chapter 7.) Water will dissolve polar solutes, such as salts and alcohols. Oil, however, is composed of largely nonpolar bonds. So water will not act as a suitable solvent for oil. 

Why do some things dissolve in one solvent and not another For example, oil and water don t mix to form a solution, but oil does dissolve in gasoline. 

In the last section, we looked at an electrolytic cell containing molten NaCl. Very high temperatures are needed to melt sodium chloride, so you might ask, Why can t we just dissolve it in water and do the same thing Before answering that question, you may wish to consider a procedure that you may have seen demonstrated for you (or even performed) in an... 

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