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6.2: Solution Formation

  • Page ID
    50501
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    Skills to Develop

    • Explain why solutions form.
    • Explain the significance of the statement "like dissolves like".
    • Discuss the idea of water as the "universal solvent".
    • Explain how water molecules attract ionic solids when they dissolve in water.

    We have learned that solutions can be formed in a variety of combinations using solids, liquids, and gases. We also know that solutions have constant composition and we can also vary this composition up to a point to maintain the homogeneous nature of the solution. But how exactly do solutions form? Why is it that oil and water will not form a solution and yet vinegar and water will? Why could we dissolve table salt in water but not in vegetable oil? The reasons why solutions will form will be explored in this section, along with a discussion of why water is used most frequently to dissolve substances of various types.

    Ionic Compounds in Solution

    Recall that metals form positive ions by losing electrons and nonmetals form negative ions by gaining electrons. In ionic compounds, the ions in the solid are held together by the attraction of these oppositely charged particles. Since ionic compounds can dissolve in polar solutions, specifically water, we can extend this concept to say that ions themselves are attracted to the water molecules because the ions of the ionic solid are attracted to the polar water molecule. When you dissolve table salt in a cup of water, the table salt dissociates into sodium ions and chloride ions:

    \[\ce{NaCl} \left( s \right) \rightarrow \ce{Na^+} \left( aq \right) + \ce{Cl^-} \left( aq \right)\]

    How does salt dissolve, though? Dissolving is based on electrostatic attraction, that is, the attraction between positive and negative charges. The sodium ions get attracted to the partially negative ends of the water molecules and the chloride ions get attracted to the partially positive ends of the water molecules.

    CK12 Screenshot 6-2-1.png

    CK12 Screenshot 6-2-2.pngTo understand why salt will dissolve in water, we first must remember what it means for water to be polar. The more electronegative oxygen atom pulls the shared electrons away from the hydrogen atoms in a water molecule causing an unequal distribution of electrons. The hydrogen end of the water molecule will be slightly positive and the oxygen end of the water molecule will be slightly negative. These partial charges allow water to be attracted to the various ions in salt, which pulls the salt crystal apart.

    The same process is true for any ionic compound dissolving in water. The ionic compound will separate into the positive and negative ions and the positive ion will be attracted to the partially negative ends of the water molecules (oxygen) while the negative ion will be attracted to the partially positive ends of the water molecules (hydrogen).

    Covalent Compounds in Solution

    Some other covalent compounds, aside from water, are also polar. Having these partial positive and negative charges within the molecule gives polar compounds the ability to be attracted to water as well. Because of these partial charges, polar molecules are able to dissolve in other polar compounds.

    If you mix a nonpolar compound with a polar compound, they will not form an even mixture. The polar compound is more attracted to the other molecules of the same compound than they are attracted to the nonpolar compound. If you have tried to mix oil and water together you may have witnessed this. Water is much more polar than oil, so the oil does not dissolve in the water. Instead, you will see two different layers form.

    However, when a nonpolar compound is mixed with another nonpolar compound, neither of them have partial charges to be attracted to. They are instead attracted by London dispersion forces and are able to dissolve together, forming a solution. The similarity in type and strength of intermolecular forces allows two nonpolar compounds such as \(\ce{CO_2}\) and benzene, \(\ce{C_6H_6}\), to form solutions.

    When we studied how ionic solids dissolve, we said that as they dissolve in solutions, these solids separate into ions. More specifically, ionic solids separate into their positive ions and negative ions in solution. This is not true for molecular compounds. Molecular compounds are held together with covalent bonds meaning they share electrons. When they share electrons, their bonds do not easily break apart, thus the molecules stay together even in solution. For example, when you dissolve a spoonful of sugar into a glass of water, the intermolecular forces are broken but not the bonds. You can write the following equation for the dissolution of sugar in water.

    \[\ce{C_{12}H_{22}O_{11}} \left( s \right) \rightarrow \ce{C_{12}H_{22}O_{11}} \left( aq \right)\]

    Notice how the molecules of sugar are now separated by water molecules (\(aq\)). In other words, sugar molecules are separated from neighboring sugar molecules due to attraction for the water, but the molecules themselves have not. The bonds within the molecules have not broken.

    Example \(\PageIndex{1}\)

    Which compounds will dissolve in solution to separate into ions?

    1. \(\ce{LiF}\)
    2. \(\ce{P_2F_5}\)
    3. \(\ce{C_2H_5OH}\)

    Solution:

    \(\ce{LiF}\) will separate into ions when dissolved in solution, because it is an ionic compound. \(\ce{P_2F_5}\) and \(\ce{C_2H_5OH}\) are both covalent and will stay as molecules in a solution.

    A simple way to predict which compounds will dissolve in other compounds is the phrase "like dissolves like". What this means is that polar compounds dissolve polar compounds, nonpolar compounds dissolve nonpolar compounds, but polar and nonpolar do not dissolve in each other.

    Even some nonpolar substances dissolve in water but only to a limited degree. Have you ever wondered why fish are able to breather? Oxygen gas, an nonpolar molecules, does dissolve in water and it is this oxygen that the fish take in through their gills. Or, one more example of a nonpolar compound that dissolves in water is the reason we can enjoy carbonated sodas. Pepsi-cola and all the other sodas have carbon dioxide gas, \(\ce{CO_2}\), a nonpolar compound, dissolved in a sugar-water solution. In this case, to keep as much gas in solution as possible, the sodas are kept under pressure.

    This general trend of "like dissolves like" is summarized in the following table:

    CK12 Screenshot 6-2-3.png

    Note that every time charged particles (ionic compounds or polar substances) are mixed, a solution is formed. When particles with no charges (nonpolar compounds) are mixed, they will form a solution. However, if substances with charges are mixed with other substances without charges a solution does not form.

    Summary

    • Whether or not solutions are formed depends on the similarity of polarity or the "like dissolves like" rule.
    • Polar molecules dissolve in polar solvents, nonpolar molecules dissolve in nonpolar solvents.
    • Ionic compounds dissolve in polar solvents, especially water. This occurs when the positive cation from the ionic solid is attracted to the negative end of the water molecule (oxygen) and the negative anion of the ionic solid is attracted to the positive end of the water molecule (hydrogen).
    • Water is considered as the the universal solvent since it can dissolve both ionic and polar solutes, as well as some nonpolar solutes (in very limited amounts).

    Vocabulary

    • Miscible: Liquids that have the ability to dissolve in each other.
    • Immiscible: Liquids that do not have the ability to dissolve in each other.
    • Electrostatic attraction: The attraction of oppositely charged particles.

    Contributors

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