1. Section 2 How Substances Dissolve
Chapter 7
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 strengths 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.

2. Water: A Common Solvent
Section 2 How Substances Dissolve
Chapter 7
Water: A Common Solvent
Many different substances can dissolve in water. For this reason, water is sometimes called the universal solvent.
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.
Because they are polar, water molecules attract both the positive and negative ions of an ionic compound.

3. Water: A Common Solvent, continued
Section 2 How Substances Dissolve
Chapter 7
Water: A Common Solvent, continued
Polar water molecules pull ionic crystals apart, as shown below.
The partially negative oxygen atoms of water molecules attract the positively charged sodium ions.
The partially positive hydrogen atoms of water molecules attract the negatively charged chloride ions.

4. Water: A Common Solvent, continued
Section 2 How Substances Dissolve
Chapter 7
Water: A Common Solvent, continued
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.
Hydrogen bonding determines many of water’s unique properties.
Hydrogen bonding enables water to dissolve many molecular compounds, such as sugar.

5. Section 2 How Substances Dissolve
Chapter 7
Hydrogen Bonding

6. Water: A Common Solvent, continued
Section 2 How Substances Dissolve
Chapter 7
Water: A Common Solvent, continued
A rule of thumb in chemistry is that like dissolves like.
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.
A nonpolar compound usually will not dissolve in water, because its intermolecular forces do not match with those of water.

7. Section 2 How Substances Dissolve
Chapter 7
Like Dissolves Like

8. The Dissolving Process
Section 2 How Substances Dissolve
Chapter 7
The Dissolving Process
According to the kinetic theory of matter, water molecules in a glass of tea are always moving.
When sugar is poured into the tea, water molecules collide with sugar molecules.
Sugar molecules form a solution with water molecules at the surface of the sugar crystals.
As layers of sugar molecules leave the crystal, more layers are uncovered and dissolve among the solvent (water) molecules in the same way.

9. Section 2 How Substances Dissolve
Chapter 7
Dissolving Process

10. The Dissolving Process, continued
Section 2 How Substances Dissolve
Chapter 7
The Dissolving Process, continued
Solutes with a larger surface area dissolve faster.
More solute particles are exposed to the solvent.
Stirring or shaking a solution helps the solute dissolve faster.
Dissolved solute particles diffuse throughout the solution faster, allowing more solute particles to dissolve.
Solutes dissolve faster when the solvent is hot.
Collisions occur between solute and solvent particles more frequently and with more energy.

11. Section 2 How Substances Dissolve
Chapter 7
Surface Area

12. Factors Affecting the Rate of Dissolution
Section 2 How Substances Dissolve
Chapter 7
Factors Affecting the Rate of Dissolution

13. The Dissolving Process, continued
Section 2 How Substances Dissolve
Chapter 7
The Dissolving Process, continued
Solutes affect the physical properties of a solution.
Examples:
If you dissolve salt in water, it will boil at a higher temperature and freeze at a lower temperature.
The coolant mixture of ethylene glycol (antifreeze) with water keeps a car’s radiator fluid from freezing in winter or boiling in summer.