Properties of Water

Essential Standard 2.2Understand chemical bonding and

chemical interactions.

Learning Objective 2.2.2

Infer the type of chemical bond that

occurs, whether covalent, ionic, or

metallic, in a given substance.

I Can StatementsAt the end of this lesson, you should be

able to say, with confidence:

• I can explain how and why hydrogen bonds

form between water molecules

• I can how the polar nature of water and its

hydrogen bonds help maintain life on Earth

Water, H2O consists of 2 hydrogen atoms

covalently bonded to 1 oxygen atom.

The water molecule

is often compared to

Mickey Mouse.

Water Molecule

Lewis StructureThe reason why the hydrogen atoms are positioned near

each other rather than on opposite sides of the oxygen

atom can be explained with water’s Lewis structure.

After the single covalent

bonds are formed with

each hydrogen atom,

there are two lone pairs of

electrons left on the

oxygen atom.

The two lone pairs of electrons force the hydrogen

atoms together, creating what is called a “bent” shape

for the water molecule.

Polar Covalent MoleculeWater is a polar covalent molecule, meaning that

it will have opposite charges at each end.

Polar Covalent ∆E 0.5 – 1.7

H2O

∆E.N. = EN(O) 3.5 – EN(H) 2.1 = 1.4

Determining Charges

Oxygen’s Electronegative value = 3.5

Hydrogen’s Electronegative value = 2.1

In a polar molecule, the atom with the higher

electronegativity develops the partial negative

charge, denoted as δ-.

Determining ChargesSince the oxygen atom has a higher

electronegative value, it attracts the electrons

more, resulting in a slight negative charge (δ-).

The hydrogen atoms have

a lower electronegative

value, so they don’t attract

the electrons as much,

resulting in a slight

positive charge (δ+).

Hydrogen Bonds When water molecules come in contact, the

negative oxygen atom from one water molecule

forms an attraction bond to the positive

hydrogen atom from another water molecule.

These attraction bonds

are called hydrogen

bonds.

Unique Water Properties The polar nature of water molecules and

the hydrogen bonds they form give water

many of its unique properties that helps to

enable life on Earth.

CohesionCohesion refers to water’s

ability to stick to itself.

The hydrogen bonds hold water

molecules together, allowing it to

form water droplets.

AdhesionAdhesion refers to

water’s ability to stick to

other things.

While water molecules will form hydrogen bonds with

each other, they actually prefer to form bonds with

other polar substances.

Notice that the water in this

graduated cylinder extends higher

where the water is in contact with

the glass and dips in the middle.

Meniscus

Capillary Action The combination of cohesion and adhesion forces in

water result in capillary action which allows water to flow

upwards in a plant without the use of energy.

Capillary Action in

the xylem tubes

in the stem

Water enters the roots of plants

through osmosis. (No energy required = passive transport)

Water sticks to the sides of the xylem

tubes inside the plant stem (adhesion)

and to each other (cohesion).

As water evaporates out of the stoma

openings in the leaves, cohesive forces

pull the rest of the water upward.

Surface tension refers to

water’s ability to resist

rupture when placed under

stress or tension.

Surface Tension

Because there are no water

molecules above the surface,

the water molecules, at the

surface, form stronger cohesive

bonds to each other.

Heat breaks the surface tension which is

why we wash things in hot water.

When water freezes, the hydrogen bonds hold

the water molecules further apart than they are

in a liquid state, making ice less dense so it

floats on liquid water.

Lower Density of Ice

Lower Density of IceWhen a body of water is covered with ice, the

water underneath is insulated from further

freezing temperatures.

Since the water

beneath the ice

remains in the

liquid state,

aquatic organisms

can still live under

the ice.

Specific Heat Specific heat refers the amount energy it takes to raise

the temperature of a substance by 1 degree Celsius.

When heat is applied to

water the thermal

energy begins to break

the hydrogen bonds.

The water molecules can only begin to vibrate faster,

resulting in a rise in temperature, only after the

hydrogen bonds are broken.

Specific Heat

Boiling Point

Melting Point

Before water can

change states, hydrogen

bonds between the

molecules must be

broken or reformed.

The two flat points on

the phase change graph

show that thermal

energy is being used to

break bonds instead of

raising the temperature.

Specific Heat Due to the hydrogen bonds between water molecules,

water has a much higher specific heat than other

substances, such as sand.

Specific Heat The difference in the specific heat of water

compared to sand, is why water remains cool

while the sand heats up quickly.

Sea Breezes Because the sand heats up faster, the air above land

becomes hot, less dense, and rises.

Cooler, denser air,

from over the

ocean flows onto

land to replace the

rising hot air.

The flow of cool air from the sea to land is called a sea

breeze and is responsible for cooling off coastal areas.

Evaporative CoolingWhen we sweat we release water onto our skin.

Thermal energy from

our bodies is then

used to break the

hydrogen bonds

between the water

molecules in order for

the water to evaporate.

The use of thermal energy from our bodies results in a

decrease in body temperature.

The End