What are the Properties of Water

• Polarity • Attraction- cohesion,

adhesion, surface tension, & capillary action

• High specific heat • Density – greatest at

4oC • Universal solvent of

life

What is Polarity of Water

• A water molecule with an uneven distribution of charges in different regions of the molecule

• Water has a variety of unusual properties because of attractions between these polar molecules.

– The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.1

What is a Hydrogen Bond?

Hydrogen bond is a weak

attraction between a

hydrogen atom and

other atom

Properties of Water

Covalent bonding vs. Hydrogen bonding

Covalent Bond

Hydrogen Bond

What is Cohesion

• cohesion = water attracted to other water molecules because of polar properties

What is Cohesion?

• Cohesion is responsible for the transport of the water column in plants

• Cohesion among water molecules plays a key role in the transport of water against gravity in plants

Hydrogen bonds hold the

substance together, a

phenomenon called

COHESION

What is Adhesion?

•Water attracted to other materials

What is surface tension?

Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.

Surface tension

– Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.

– Water behaves as if covered by an invisible film.

– Some animals can stand, walk, or run on water without breaking the surface.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.3

What is capillary action?

It is the tendency of water to rise in a thin tube.

Capillary Action

Because water has both

adhesive and cohesive

properties, capillary action is

present.

Capillary Action = water’s adhesive property is the cause of capillary action. Water is attracted to some other material and then through cohesion, other water molecules move too as a result of the original adhesion.

Ex: Think water in a straw

Ex: Water moves through trees this way

What is specific heat?

Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1oC.

Moderates Temperatures on Earth

• What is kinetic energy?

• Heat?

• Temperature?

• Calorie?

• What is the difference in cal and Cal?

• What is specific heat?

Celsius Scale at Sea Level

100oC

Water boils

37oC

Human body

temperature

23oC

Room temperature

0oC

Water freezes

Water stabilizes air temperatures by absorbing heat from

warmer air and releasing heat to cooler air.

Water can absorb or release relatively large amounts of heat

with only a slight change in its own temperature.

Three-fourths of the earth is covered

by water. The water serves as a

large heat sink responsible for:

1. Prevention of temperature

fluctuations that are outside the

range suitable for life.

2. Coastal areas having a mild

climate

3. A stable marine environment

Specific heat

What is Density It when hydrogen bonds in water expand

as it warms and contracts as it cools. The

hydrogen bonds keeps the molecules far

enough a part to make ice have fewer

molecules. Solid – water molecules are bonded together – space between fixed

Liquid – water molecules are constantly bonding and rebonding – space is always changing

Density of Water

• Most dense at 4oC

• Contracts until 4oC

• Expands from 4oC to 0oC

The density of water:

Mass-to-volume ratio of a substance; measure of how tightly matter

is packed

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface first—the freezing of the water

releases heat to the water below creating insulation.

3. Makes transition between season less abrupt.

– When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.

– As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.

– Ice is about 10% less dense than water at 4oC.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.5

What is Solvent for Life

• Solution – Solute

– solvent

• Aqueous solution

• Hydrophilic – Ionic compounds

dissolve in water

– Polar molecules (generally) are water soluble

• Hydrophobic – Nonpolar compounds

Water and Solutions Copyright © McGraw-Hill Education

Mixtures with Water

.

• A solution is another name for a homogeneous

mixture.

• In a solution, there are two components:

• A solvent is a substance in which another

substance is dissolved.

• A solute is the substance that is dissolved in the

solvent.

Water and Solutions Copyright © McGraw-Hill Education

Mixtures with Water

Acids and bases

• Many solutes readily dissolve in water because of

water’s polarity.

• Substances that release H+ when they are dissolved

in water are called acids.

• Substances that release hydroxide ions (OH−) when

they are dissolved in water are called bases.

Water and Solutions Copyright © McGraw-Hill Education

Mixtures with Water

pH and buffers

• The amount of hydrogen or hydroxide ions in a

solution determines the strength of an acid or base.

• The measure of concentration of H+ in a solution is

called pH.

• Acids have pH values < 7; bases have pH values

> 7

• Buffers are mixtures that can react with acids or

bases to keep the pH within a particular range.