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