Chapter 7
Section 2
Objectives
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Explain how the polarity of water enables it to dissolve many different substances.
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Relate the ability of a solvent to dissolve a solute to the relative strengths of forces
between molecules.
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Describe three ways to increase the rate at which a solute dissolves in a solvent.
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Explain how a solute affects the freezing point and boiling point of a solution.
Water: A Common Solvent
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Many different substances can dissolve in water. For this reason, water is
sometimes called the universal solvent.
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Water can dissolve ionic compounds because of its structure: it is a polar
compound, which is a molecule that has an uneven distribution of electrons.
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Because they are polar, water molecules attract both the positive and
negative ions of an ionic compound.
Polar water molecules pull ionic crystals apart, as shown below.
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The partially negative
oxygen atoms of water
molecules attract the
positively charged sodium
ions.
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The partially positive
hydrogen atoms of water molecules attract the negatively charged chloride
ions.
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Water exhibits hydrogen bonding: the intermolecular force occurring when a
hydrogen atom that is bonded to a highly electronegative atom of one molecule is
attracted to two unshared electrons of another molecule.
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Hydrogen bonding determines many of water’s unique properties.
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Hydrogen bonding enables water to dissolve many molecular compounds, such
as sugar.
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A rule of thumb in chemistry is that like dissolves like.
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This rule means that a solvent will dissolve substances that are like the
solvent in molecular structure.
A nonpolar compound is a compound whose electrons are equally distributed
among its atoms.
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A nonpolar compound usually will not dissolve in water, because its
intermolecular forces do not match with those of water.
The Dissolving Process
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According to the kinetic theory of matter, water molecules in a glass of tea are
always moving.
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When sugar is poured into the tea, water molecules collide with sugar molecules.
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Sugar molecules form a solution with water molecules at the surface of the sugar
crystals.
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As layers of sugar molecules leave the crystal, more layers are uncovered and
dissolve among the solvent (water) molecules in the same way.
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Solutes with a larger surface area dissolve faster.
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Stirring or shaking a solution helps the solute dissolve faster.
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Dissolved solute particles diffuse throughout the solution faster, allowing
more solute particles to dissolve.
Solutes dissolve faster when the solvent is hot.
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More solute particles are exposed to the solvent.
Collisions occur between solute and solvent particles more frequently and
with more energy.
Solutes affect the physical properties of a solution.
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Examples:
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If you dissolve salt in water, it will boil at a higher temperature and
freeze at a lower temperature.