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AP Bio



Unit 1

1.1 Structure of Water and Hydrogen Bonding

2 min readnovember 6, 2020


Danna Esther Gelfand

Water Molecules

Water is a polar molecule, meaning the ends have opposite partial charges (unequal distribution of charge), Hydrogen being a partial positive and oxygen being a partial negative. A hydrophilic substance attracts water (has an affinity for it). A hydrophobic substance is one that avoids water (does not have an affinity for water). “Like attracts like.” Polar attracts polar. Nonpolar attracts nonpolar.
Example: Lipids are hydrophobic because they have relatively nonpolar bonds while water is polar. Water is the most common molecule in living organisms.

Hydrogen Bonding

  • Intermolecular bonds: Between water molecules (Hydrogen bonds); the ability of hydrogen to interact with Fluorine, Nitrogen, and Oxygen (WEAK ATTRACTION). It is important to note that Hydrogen bonds are NOT covalent bonds.
  • Hydrogen bonds between water molecules give water its properties of cohesion, adhesion, and surface tension, specific heat, and evaporative cooling.

Key Terms

Cohesion is the attraction of water molecules. Strong cohesive forces are present because they form hydrogen bonds with each other.
Example: Cohesion due to hydrogen bonding contributes to the transport of water and nutrients against gravity in plants. Transpiration is the loss of water from a plant in the form of water vapor.

Image courtesy of ABCteach.

Adhesion - One substance is attracted to another. Water adheres to other molecules. Example: water on glass.
Surface Tension - Difficulty to break the surface of the water because of cohesive forces. Example: water strider insect relies on surface tension to stay afloat on the surface of the water.
Specific Heat - The amount of heat energy it takes to raise or lower the temperature of one gram of a substance 1 degree Celsius. Water has a high specific heat, so it can absorb or release a large amount of heat with only a slight change in its own temperature so large bodies of water take a while to evaporate.
Evaporative Cooling - Water has a high heat of vaporization, so the water can absorb a lot of heat and leave the surface cooler. Example: perspiration/sweat.
Dissociation of water - Hydrogen shifts from one water molecule to another. When a molecule is increasing hydronium concentration by releasing hydrogen ions into solutions it is an acid. (HCl ➡️ H+ + Cl-) When a molecule is increasing hydroxide concentration by absorbing or accepting Hydrogen ions it is a base. (NaOH ➡️ Na+ + OH-) 
  • The counterbalance of hydronium and Hydroxide results in water having a PH level of 7.0 (neutral)
  • PH scale range (0-14) acidic (<7) neutral (7), alkaline/basic (>7). Each 1 level on the PH scale is a ten-fold change. Most biological fluids are in the range 6-8. pH = −log [H+]
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