Chapter 7.2
HOW SUBSTANCES DISSOLVE
Objectives
 Explain how the polarity of water enables it to
dissolve many different substances.
 Relate the ability of a solvent to dissolve a solute to
the relative strength of forces between molecules.
 Describe three ways to increase the rate at which a
solute dissolves in a solvent.
 Explain how a solute affects the freezing point and
boiling point of a solution.
Water: A Common Solvent
 Water is often referred to as the universal solvent because it dissolves so
many different substances.
 Water can dissolve most ionic compounds because of its polar structure.
Water has a positive and negative pole which makes it natural at dissolving
ionic compounds.
 Polar compound – a molecule that has an uneven distribution of
electrons.
 Water molecules attract both the positive and negative ions of most ionic
compounds, eventually pulling the ionic solid apart.
 The force of attraction between the water and the ions is greater than the
attraction between the positive and negative ions (at least in most cases.)
Water: A Common Solvent
 Not all ionic compounds will dissolve in water; some ions have a
greater attraction than the dissolving force of the water. In those cases,
the ionic compound will not dissolve.
 Dissolves  water attraction > ion attraction
 Does not dissolve  water attraction < ion attraction
Water: A Common Solvent
 Water can also dissolve many molecular compounds.
 The hydrogen bonds that are found in water contribute to the ability of
water to dissolve these molecular substances.
 Examine Figure 13 on page 234.
 Essentially, water forms hydrogen bonds with parts of the molecular
compound, which eventually pull the compound away from the solid –
causing it to dissolve.
“Like Dissolves Like”
 A general rule of thumb in chemistry:
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Polar substances will dissolve polar substances.
Non-polar substances will dissolve non-polar substances.
 Generally speaking, non-polar compounds will not dissolve in water
because it is a polar compound.
The Dissolving Process
 The Kinetic Theory contributes to the dissolving process – remember
all matter is made of tiny particles that are constantly in motion. These
particles constantly collide and transfer energy in the process – aiding
the dissolving process.
 There are several factors that increase the rate of dissolve:
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Increasing the surface area – break the solute into several pieces.
Stirring or shaking – increases the number of collisions.
Increase the temperature of the solvent – speeds up particle
collisions.
Solutes & Physical Properties
 Physical properties, such as, boiling point and freezing point
can be greatly affected by adding a solute.
 Many solutes increase the boiling point of a pure solvent, and
also decrease the freezing point of that solvent.
 For example, pure water normally boils at 100 degrees
Celsius; however, when you add sodium chloride to it, the
boiling point increases to 102 degrees Celsius. Water typically
freezes at 0 degrees Celsius, but after adding sodium chloride,
the freezing point becomes -7 degrees Celsius. That’s why we
spread salt on ice, it lowers the freezing point, which makes it
harder to freeze. Thus, it melts.