Properties of Water

Polarity

• Many of water’s biological functions stem from its chemical structure:– Water is a polar molecule

• It has unequal charge distribution that results in areas of δ- and δ+ charge (δ = partial)

• The oxygen in water is more electronegative than the hydrogen– The oxygen nucleus has a greater attraction for

electrons than the hydrogen nucleus, so the oxygen will have a δ- charge, leaving the hydrogen with a δ+ charge

δδ--

δδ++ δδ++

Solubility of Water

• Since water is polar, it can dissolve polar substances and ionic compounds– Ex: Sugar, salt (NaCl)– Ions play an important role in maintaining

normal body functions• Muscle contractions and nerve impulses

• Water cannot dissolve nonpolar substances– Ex: Oil

Hydrogen Bonding

• The polarity of water causes water molecules to be attracted to one-another

• Hydrogen Bond: the force of attraction between a hydrogen atom with a δ+ and another molecule with a full or partial negative charge– The attraction is strong enough that water will

“cling” to itself and other substances– Accounts for the unique properties of water

• Cohesion, adhesion, temperature moderation, evaporative cooling, density of ice, ability of water to dissolve many substances

Properties of Water

• Cohesion– What is does:• Water molecules are attracted

to other water molecules.– Importance:• Allows water to move up from

the roots of a plant to the leaves • Surface Tension: “skin” that

keeps water in a glass that is overfilled, or droplets on a car after rain. Allows insects to move across the surface of water.

Properties of Water

• Adhesion– What it means:• Attraction between

water and other molecules

– Importance:• Capillary action:

allows water to rise up the stem of a plant, or up a glass tube

Properties of Water

• High Specific Heat– What it means:• Resists temperature changes

– Takes 1 calorie of energy to raise 1 gram of water 1oC» It takes a LARGE heat gain or loss to change the

temperature of water

– Importance:• Keeps cells at an even temperature; stabilizes global

temperatures; moderates coastal areas

Properties of Water

• High Heat of Vaporization– What it means:• Takes a lot of energy to change water from a liquid to a

gas because H-bonds prevent molecules from moving into the atmosphere

– What it does:• Evaporative cooling the “hottest” molecules will

leave first. Cools plants and animals (ex: sweating)

Properties of Water

• Density of Ice– What it does:• H-bonding causes ice to

have a lower density so it floats on liquid water

– Importance:• Ice floats on top of lakes,

insulating them in the winter, which allows the fish and other organisms to survive through the winter