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 δ-δ-δ-δ- δ+δ+δ+δ+ δ+δ+δ+δ+

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 1 o C » 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