AP Biology Properties of Water

Biology – the study of life

Water and it’s life giving properties

How many drops of water can you fit on a penny?

How does water do to ink?

What does a drop of water look like?

Can you float a paperclip on water?

Observations

What observations did you make about water?

What do you already know?

The Chemical Properties of Water

Your Objective

Be able to state at least 2 properties of water that help

support life

AND

Be able to explain how the chemical nature of water gives

rise to that property

Water and Life

Life on Earth began in water and evolved there for 3 billion years.

Modern life still remains tied to water

Cells are composed of 70%-95% water

•Water is found

as a liquid over

71% of the

earth•The abundance

of water is a

major reason

Earth is

habitable

Studied in isolation, the water molecule is deceptively simple Its two hydrogen atoms are joined to one

oxygen atom by single covalent bonds

The structure of water

H

O

H

But the electrons of the covalent bonds are not shared equally between oxygen and hydrogen

This unequal sharing makes water a polar moleculeOxygen is more electronegative than hydrogen, so it has a greater pull on the electrons

() ()

() ()

The polarity of water results in weak electrical attractions between neighboring water molecules

These interactions are called hydrogen bonds

(b)

()

Hydrogen bond()

()()

()

()

()()

Polar Structure

Electronegativity of H20

Quick Think Why is a molecule

of water said to have polar covalent bonds?

What kind of bonds hold individual water molecules together?

Water’s Life Supporting Properties

The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties Water’s cohesive nature Water’s ability to moderate temperature Floating ice Versatility of water as a solvent

Water molecules stick together as a result of hydrogen bonding

This is called cohesion

The Cohesion of Water

Cohesion is vital for water transport in plants

Microscopic tubes

Which of our mini-lab demos seems to illustrate cohesion?

Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid

Hydrogen bonds give water an unusually high surface tension Cohesion between water molecules form a skin-like surface Can support animals like “water

striders” in ponds

Surface Tension

Water drops are round because all the molecules on the edge are pulled to the middle.

Adhesion

Water will also adhere to other polar substances

This is called adhesion

It is due to the polar nature of the water molecule

Caused by adhesion the water runs along the glass and does not fall straight.

Capillary Action

Glass has polar molecules.

Glass can hydrogen bond.

Attracts the water molecules.

Some of the pull is up.

Meniscus

Water curves up along the side.

This makes the meniscus.

Quick Think

How are adhesion and cohesion similar and different?

Give an example of how these properties help support life.

8-15-14 Warm up

Use the clay and toothpicks to make a model of a water molecule

Working with someone near you, use your models to demonstrate cohesion.

HW:1.) Read section 3-1 and 3-2 in your textbook2.) Watch CrashCourse Biology – Water on YouTube3.) water worksheet

Objective: Explain and diagram the transpiration-cohesion mechanism responsible for water transport in plants

Water moderates temperature

Because of hydrogen bonding, water has a strong resistance to temperature change

Water moderates temperature

Heat and temperature are related, but different Heat is a measure of the amount of kinetic

energy in the atoms and molecules in something

Temperature measures the intensity of the heat

Whenever 2 objects meet, the cooler object absorbs heat from the warmer object until they are the same temperature

Water moderates temperature

Water has a high specific heat Specific heat = the amount of heat that must

be absorbed or lost to change the temperature of 1g of the substance 1°C

Water moderates temperature much of the absorbed heat is used to

break hydrogen bonds, not increase the kinetic energy of the molecules

Water moderates temperature

Earth’s giant water supply causes temperatures to stay within limits that permit life

How water moderates temperature

Water also has:

High heat of fusion• The temp at which liquid turns solid

High heat of vaporization• The temp at which liquid turns to gas

Quick Think

Why does it take so much energy to change the temperature of water?

When water molecules get cold, they move apart, forming ice

The Biological Significance of Ice Floating

A chunk of ice has fewer molecules than an equal volume of liquid water

IceLiquid water

The density of ice is lower than liquid water This is why ice floats

Figure 2.15

Hydrogen bond

Liquid water

Hydrogen bondsconstantly break and re-form

Ice

Stable hydrogen bonds

Change of State

Dipole Structure

Ice floats in water because all ice molecules are held in hexagons

Center is open space, making ice 8% less dense than water.

Since ice floats, ponds, lakes, and even the oceans do not freeze solid

Marine life could not survive if bodies of water froze solid

Floating ice insulates water below, preventing freezing: critical for ocean animals

Maximum density: 3.98oC

Below this temp, form hexagonal polymers and decrease density

Above this, molecules are energetic, water behaves like other liquids - expanding when warm and contracting when cool

Quick Think

Of all the properties we’ve learned so far, which ones could you competently write about in your essay?

A solution is a liquid consisting of two or more substances evenly mixed

Water as the Solvent of Life

The dissolving agent is called the solventThe dissolved substance is called the solute

Ion in solutionSalt crystal

Water is a good solvent because it is polar

Ionic (salts) and polar (sugars) compounds dissolve readily in water

Solvent Properties Water dissolves salts by surrounding the atoms in the

salt molecule and neutralizing the ionic bond holding the molecule together

Acids, Bases &

Buffers

Acid

Acids, Bases, and pH

A chemical compound that donates H+ ions to solutionsBase

A compound that accepts H+ ions and removes them from solution or a compound that dissociates in water to form hydroxide ions

Basicsolution

Neutralsolution

Acidicsolution

Oven cleaner

Household bleach

Household ammonia

Milk of magnesia

Seawater

Human bloodPure water

Urine

Tomato juice

Grapefruit juice

Lemon juice;gastric juice

pH scale

To describe the acidity of a solution, we use the pH scale

pH scale (log scale)

Each number on the scale is 10x difference from the number next to it

• pH 1 is 10x more acidic than pH 2, 100x more acidic than pH 3, 1000x more acidic that pH 4, and so on

pH - Percent Hydronium

A measure of the percent of hydronium ions in the solution

The greater the percent hydronium ions, the more acidic the solution is

H2CO3 -------------> H+ + HCO3 -

CARBONIC ACID HYDRONIUM BICARBONATE

ION ION

Measuring Acidity

Buffers are substances that resist pH change

They accept H+ ions when they are in excessThey donate H+ ions when they are depleted

Buffering is not foolproof

Example: acid precipitation

normal rain water (pH of 5-6) pH of acid rain is between 3-4

Buffers

Dissolved CO2 in water acts as a buffer, a substance that prevents large shifts in pH.

Buffers help keep pool and spa water clean

Buffer Systems

CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3

-2

H2CO3 is carbonic acid,

H+ is the hydronium ion

HCO3- is the bicarbonate ion

CO3-2 is the carbonate ion

Buffer Systems

CO2 + H2O H2CO3 H+ + HCO3-H+ + CO3

-2

Adding CO2 shifts the reaction to the right and produces more H+ ions making the water more acid.

Buffer Systems

CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3

-2

Removing CO2 shifts the reaction to the left, combining H+ ions with carbonate and bicarbonate ions reducing the acidity.

Ocean Buffers