1. Section 2: Water and Solutions
Preview
Bellringer
Key Ideas
Properties of Water
Solutions
Summary

2. Bellringer
Write what you think an acid and a base are and why. Guess if the following substances are acids or bases, and give a reason for your choice:
lemon juice, water, household ammonia, blood, soda pop, milk, antacid, shampoo
Can you find any similar characteristics between acids and bases?

3. Key Ideas What makes water a unique substance?
How does the presence of substances dissolved in water affect the properties of water?

4. Properties of Water
Water has many unique properties that make it an important substance for life.
Most of the unique properties of water result because water molecules form hydrogen bonds with each other.
When water freezes, the crystal structure formed due to hydrogen bonding makes ice less dense than liquid water.

5. Properties of Water, continued
Water can absorb a large amount of heat without changing temperature. This property can help organisms maintain a constant internal temperature.
The attraction of particles of the same substance, such as water, is called cohesion. Cohesion keep water from evaporating easily; thus, water is a liquid at ordinary temperatures.
Water molecules also stick to other polar molecules. This attraction between particles of different substances is called adhesion.

6. Visual Concept: Comparing Cohesion and Adhesion

7. Solutions
A solution is a mixture in which ions or molecules of one or more substances are evenly distributed in another substance.
Many substances are transported throughout living things as solutions of water. Dissolved substances can move more easily within and between cells.
Water dissolves many ionic and polar substances but does not dissolve nonpolar substances.

8. Solutions, continued Acids and Bases
Some water molecules break apart to form hydronium and hydroxide ions.
In pure water, hydronium and hydroxide ions are present in equal numbers.
Acids and bases are compounds that change the balance of these ions.

9. Solutions

10. Solutions, continued Acids and Bases
Acids are compounds that form extra hydronium ions when dissolved in water.
Bases are compounds that form extra hydroxide ions when dissolved in water.
When acids and bases are mixed, the extra hydronium and hydroxide ions react to form water.

11. Visual Concept: Acids

12. Visual Concept: Bases

13. Solutions, continued pH and Buffers
pH is a measure of how acidic or basic a solution is.
Each one-point increase in pH represents a 10-fold decrease in hydronium ion concentration.
Pure water has a pH of 7. Acidic solutions have a pH below 7, and basic solutions have a pH above 7.

14. Visual Concept: pH

15. Solutions, continued pH and Buffers
The pH of solutions in living things must be stable.
For a stable pH to be maintained, the solutions in living things contain buffers.
A buffer is a substance that reacts to prevent pH changes in a solution.

16. Summary
The hydrogen bonding between water molecules explains many of the unique properties that make water an important substance for life.
Acids and bases change the concentration of hydronium ions in aqueous solutions. The pH of solutions in living things must be stable.