Class Starter

Write what you think an acid and a base are and why. Guess if thefollowing substances are acids or bases, and give a reason for your choice:

lemon juicewaterhousehold ammoniabloodsoda popmilkantacidshampoo

Can you find any similar characteristics between acids and bases?• Acids are usually sour and corrosive, and bases are usually bitter.

Acid

Neutral

Base

Neutral

Acid

Neutral

Base

Base

Objectives

• What makes water a unique substance?

• How does the presence of substances dissolved in water affect the properties of water?

Section 3.2: Water and Solutions

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.

Properties of Water• Water can absorb a large amount of heat without changing

temperature. This property can help organisms maintain a constant internal temperature.

Cohesion• The attraction of particles of the same substance, such as

water. • keeps water from evaporating easily; thus, water is a liquid

at ordinary temperatures.

Adhesion • The attraction between particles of different substances.• Water molecules also stick to other polar molecules.

COHESION

Water compounds attractTo glass molecules

And form a meniscusWater compounds attract

To one another- causes water to “bead”

ADHESION

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.

Checkpoint• What type of bond forms between water

molecules?

• What is the difference between cohesion and adhesion?

• What is a solution?

• What type of substances can water dissolve?

• What type of substances can’t water dissolve?

Class Starter

• What makes sour candy taste so sour?

Answer

• Sour candy is coated with a mixture of sugar, citric acid and tartaric acid.

– Acids produce a sour taste.

SolutionsAcids and Bases• Some water molecules break apart to form

hydrogen and hydroxide ions.

• In pure water, hydrogen and hydroxide ions are present in equal numbers.

• Acids and bases are compounds that change the balance of these ions.

Acids and Bases

Water can react to form individual ions:

H2O H+ + OH-

• In pure water this occurs naturally but the amount of H+ is always = to the amount of OH- so water remains neutral

SolutionsAcids and BasesAcids• are compounds that form extra hydrogen 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 hydrogen and hydroxide ions react to form water.

SolutionspH 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• Basic solutions have a pH above 7.

pH Scale

SolutionspH 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.

Buffer • a substance that reacts to prevent pH

changes in a solution.

Checkpoint• What is the difference between an acid and a

base?

• What is the pH scale?

• What is the range of pH for:– Acid– Base– Neutral

• What is the function of a buffer?

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.