Water : A Molecule Essential for Life

Thoughts to Ponder…… Why can’t we survive more than a week without water? How does water rise up 300 feet from the roots of a redwood tree without a pump? Why does water take so long to boil? Can it really boil faster if salt is added? Why do oil and water not mix?

3 And how does this Spider Walk on Water? Source:

Life is dependent on Water Source: Why?

Main Idea: Water’s molecular structure gives it unique properties that support life

Water is a covalently bonded molecule – Hydrogen and Oxygen share electrons

The electrons are not shared equally – the Oxygen atom pulls the shared electrons closer to itself. This “pulling force” is called electronegativity.

The shared electrons stay closer to the Oxygen atom because oxygen has a greater electronegativity than hydrogen.

Because the e- are closer to the oxygen atom, it gets a slight/partial negative charge

Because the e- are away from the hydrogen atom, it gets a slight/partial positive charge

A molecule that has different charges on opposite sides is called a Polar Molecule Water is a polar molecule. Negative Pole/ Side Positive Pole/Side

What else do you know of that has opposite poles or sides?

A polar molecule behaves a like a magnet due to having different charges on either side

Water’s polarity gives it the ability to form hydrogen bonds with other charged molecules or ions. A hydrogen bond is an attractive force between a positively charged hydrogen atom in one molecule and an negatively charged atom or ion in a different molecule.

Hydrogen bond strength One hydrogen bond is not very strong - it doesn’t require much energy to break it. But when there are many hydrogen bonds working together A LOT of energy is needed to break the bonds.

Strength in Numbers! Source: bonddiagram.gif. Let’s be water molecules and try this out!

This is why it takes so much energy to melt and evaporate water – billions of hydrogen bonds between individual water molecules must first be broken before the individual molecules can be separated !

Hydrogen Bond Formation Intermolecular force Between molecules Intramolecular force Within molecule

Intramolecular bonds – form between atoms within the same molecule Ex. Covalent and Ionic bonds Intermolecular bonds – form between atoms in different molecules Ex. Hydrogen bonds

Oxygen’s electronegativity -> Water’s polar structure -> Hydrogen bonding These characteristics of water molecules give rise to the properties that support life. It is an important example of how a molecule’s structure determines how it functions /what it can do. Remember a common theme is biology: Structure and Function are related.

Water’s ability to Hydrogen Bond gives it Properties which Support Life Cohesion / Surface Tension Adhesion High heat capacity Dissolves polar and ionic compounds Hydrophobic effect on nonpolar substances Expands and less dense as it freezes

1. Because water is a polar molecule it is attracted to itself Cohesion – the attractive force between Water molecules Water molecules “stick” together Creates surface tension

2. Water molecules are attracted to Polar molecules and Ions which also have a charge.

3. Capillarity – ability of water to move upwards in small tubes due to its attraction for each other (cohesion) AND the solid surface it’s in contact with (adhesion).

Ex.’s of Capillarity

4. Water exists in all three states (liquid, solid, gas) over a wide range of temperatures 32 – 212 F 0 – 100 C

5.. Water can absorb and release large quantities of heat energy without changing its temperature. Ex. It takes a long time for water to boil and to cool down.

When’s the water the warmest at the beach?

High Heat Holding Capacity Cities located near large bodies of water benefit from this property The slow release of heat the water absorbed in the summer warms the air in the winter and the absorption of heat energy cools the air in the summer. Water helps to maintain stable temperatures Our bodies are made of 70% water which helps keep our temp. stable

6. Large amounts of heat E are needed to break the hydrogen bonds between liquid water molecules so they can evaporate into gas. Sweating is a homeostatic mechanism designed to cool your body down as heat from your body is used to evaporate the sweat

7. Water’s ability to hydrogen bond lets it dissolve many polar and ionic compounds which also have a charge.

2 Parts of a Solution Solvent 1. Substance that’s present in the greater amount 2. Dissolves another substance Solute 1. Substance that’s present in lesser amount 2. Gets dissolved

Most chemical reactions must occur in water Water dissolves many ions and molecules that can be carried throughout the body in blood and urine (both of which are mostly water) Water absorbs the heat released during cellular chemical reactions.

STAY HYDRATED – YOUR HEALTH DEPENDS ON IT!

8. Water cannot dissolve nonpolar substances like fats, waxes and oils – they have No Charge to attract water with. “Oil and Water Don’t Mix” As Water molecules hydrogen bond with each other they exclude nonpolar substances - Hydrophobic effect Causes molecules like DNA and proteins to fold into their specific shapes

Why Water and Oil Don’t Mix Water is NOT able to dissolve nonpolar molecules (molecules that don’t have a charge) Examples of nonpolar molecules: *Fats *Waxes *Oils

When fat molecules and water molecules are mixed, water molecules are attracted to each other by hydrogen bond and push the fat molecules away because they can’t hydrogen bond with them YouTube: Why Oil and Water Don't Mix

9.9. The space between water molecules expands as water freezes making ice less dense than liquid water - ice floats Water bodies freeze from the top down with the ice layer insulating the water and organisms below.

Biology Crash Course Videos Water - Liquid Awesome HGg Polar and Nonpolar molecules nc