Importance of Water

• The molecule that supports all of life

• Water is the biological medium here on Earth

• All living organisms require water more than any other substance

Water Facts

• ¾ of the Earth’s surface is submerged in water

• The abundance of water is the main reason the Earth is habitable

• The polarity of water molecules results in hydrogen bonding

• Water is a polar molecule

Water’s Polarity

• The polarity of water molecules

  • Allows them to form hydrogen bonds with each other

  • Contributes to the various properties' water exhibits

Properties of water

• Cohesion/adhesion

  • Is the bonding of a high percentage of the molecules to neighboring water molecules

  • Is due to hydrogen bonding

  • Like molecules bonding to each other

    • Water attracting other water molecules

  • Helps pull water up though the microscopic vessels of plants

    • capillarity

• Surface tension

  • It is a measure of how hard it is to break the surface of a liquid

  • Is related to cohesion

• Temperature moderation

  • Water moderates air temperature

  • By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler

  • Kinetic energy

    • Energy of motion

  • Heat

    • Measure of the total amount of kinetic energy due to molecular motion

  • Temperature

    • Measures intensity of heat

• High specific heat

  • The amount of heat that must be absorbed or lost for 1 gram of the substance to change its temperature by 1℃

  • Water has a high specific heat, which allows it to minimize temperature fluctuation to within limits that permit life

    • Heat is absorbed when hydrogen bonds break

    • Heat is released when hydrogen bonds form

• Evaporative cooling

  • Evaporation

    • The transformation of a substance from a liquid to a gas

    • Requires energy

  • Heat of vaporization

    • The quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas

    • 100℃ steam has more heat than 100℃ boiling water

  • Due to the water’s high heat of vaporization

  • Allows water to cool a surface

  • Sweating cools the body as heat energy from the body changes sweat into gas

  • Insulation of Bodies of Water by Floating Ice

    • The hydrogen bonds in the ice are more “ordered: than in liquid water, making the ice less dense

    • Since ice floats in water, life can exist under the frozen surfaces of lakes and polar seas

• Universal solvent

  • Water is a versatile solvent due to its polarity

  • It can form aqueous solutions

  • Called the universal solvent because so many substances dissolve in water

  • The different regions of the polar water molecules can interact with ionic compounds called solutes and dissolve them

  • Water can also interact with polar molecules such as proteins

• Hydrophilic and Hydrophobic Substances

  • Hydrophobic

    • Does not have an affinity for water

    • Nonpolar

    • Ex: lipids

  • Hydrophilic

    • Has an affinity for water

    • Polar or ionic

    • Ex: carbohydrates, salts

Solute Concentration in Aqueous Solutions

• Since most biochemical reactions occur in water inside cells, it is important to learn to calculate the concentration of solutes in an aqueous solution

Moles and Molarity

• A mole

  • Represents the exact number of molecules of a substance in a given mass

• Molarity

  • Number of moles of solutes per liter of solution

Acids and Bases

• Acid

  • Any substance that increases the hydrogen ion concentration of a solution

• Base

  • Any substance that reduced the hydrogen ion concentration of a solution

• Dissociation of water molecules leads to acidic and basic conditions that affect living organisms

• Organisms must maintain homeostasis in the pH of their internal and external environments

The pH Scale

• Scale goes from 1-14, where 7 is neutral

• The pH of a solution is determined by the relative concentration of hydrogen ions

• Difference of 10x in hydrogen ion concentration between any two pH values

• Acids have a higher number of H+ ions than a base

• Acids produce H+ ion in a solution

• Bases produce OH- in a solution

Effects of Changes in pH

• Water can disassociate into hydronium ions (H+ or H3O+) and hydroxide (OH-) ions

• Changes in the concentration of these ions can have a great effect on the pH in living organisms

Buffers

• Substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution

• Consist of an acid-base pair that reversibly combines with hydrogen ions

• Made by organisms

The Threat of Acid Precipitation

• Refers to rain, snow, or fog with a pH lower than pH 5.6

• It is caused primarily by the mixing of different pollutants with water in the air

• Can damage life in Earth’s ecosystems