Dissolving Process
Ck12 Science
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Printed: January 5, 2016
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Dissolving Process
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Dissolving Process
• Describe how ionic compounds form solutions.
• Describe how molecular compounds form solutions.
How do you take your coffee?
Many people like to start their day with a cup of coffee (others need coffee all day long in order to get anything
done). The coffee you drink is a type of solution, often a very complex one. The coffee itself has been brewed
so that material from the coffee bean will dissolve in hot water. Some people add sugar, while others add milk or
cream. In some areas of the country, lattés are popular; so one or more special flavors might be added along with
the milk. Whatever the situation, the end-result is an enjoyable solution to drink.
The Dissolving Process
Water typically dissolves many ionic compounds and polar molecules. Nonpolar molecules such as those found in
grease or oil do not dissolve in water. We will first examine the process that occurs when an ionic compound such
as table salt (sodium chloride) dissolves in water.
Water molecules move about continuously due to their kinetic energy. When a crystal of sodium chloride is placed
into water, the water’s molecules collide with the crystal lattice. Recall that the crystal lattice is composed of
alternating positive and negative ions. Water is attracted to the sodium chloride crystal because water is polar and
has both a positive and a negative end. The positively charged sodium ions in the crystal attract the oxygen end of
the water molecules because they are partially negative. The negatively charged chloride ions in the crystal attract
the hydrogen end of the water molecules because they are partially positive. The action of the polar water molecules
takes the crystal lattice apart (see image below).
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After coming apart from the crystal, the individual ions are then surrounded by solvent particles in a process
called solvation. Note that the individual Na+ ions are surrounded by water molecules with the oxygen atom
oriented near the positive ion. Likewise, the chloride ions are surrounded by water molecules with the opposite
orientation. Hydration is the process of solute particles being surrounded by water molecules arranged in a specific
manner. Hydration helps to stabilize aqueous solutions by preventing the positive and negative ions from coming
back together and forming a precipitate.
Table sugar is sucrose (C12 H22 O11 ) and is an example of a molecular compound. Solid sugar consists of individual
sugar molecules held together by intermolecular attractive forces. When water dissolves sugar, it separates the
individual sugar molecules by disrupting the attractive forces, but does not break the covalent bonds between the
carbon, hydrogen, and oxygen atoms. Dissolved sugar molecules are also hydrated, but without as distinct an
orientation to the water molecules as in the case of the ions. The sugar molecule contains many -OH groups that can
form hydrogen bonds with the water molecules, helping form the sucrose solution.
Summary
• Motion of water molecules helps break up interactions between solid ions or molecules.
• Solvation involves surrounding ions with solvent particles.
• Ionic solute molecules are hydrated (surrounded by solvent molecules in a specific orientation).
Practice
Questions
Use the link below to answer the following questions:
http://www.dceverest.com/srhigh/teachers/bheeren/dissolving_process.htm
1. Does the bond-breaking step require energy or release energy?
2. Does the hydration process require energy or release energy?
3. Does the distribution of solvent-solute clusters require energy or release energy?
Review
Questions
1.
2.
3.
4.
How does the motion of water molecules help form a solution?
What is solvation?
What is hydration?
How does sucrose interact with solvent water molecules?
• hydration: The process of solute particles being surrounded by water molecules arranged in a specific manner.
• solvation: Involves surrounding ions with solvent particles.
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Chapter 1. Dissolving Process
References
1. Jon-Isac Lindberg (Wikimedia: Gmurdoc). http://commons.wikimedia.org/wiki/File:White_cup_of_black_co
ffee.jpg .
2. CK-12 Foundation - Christopher Auyeung. .
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