Water Polarity and Hydrogen Bonding

• Water is essential for life due to its unique properties arising from its polarity and hydrogen bonding
• Water polarity enables it to dissolve many substances, making it an excellent solvent

Water Polarity

• Water molecules consist of one oxygen atom and two hydrogen atoms (H2O)
• Oxygen is more electronegative than hydrogen, so it attracts electrons more strongly, resulting in a partial negative charge (δ-) on the oxygen and partial positive charges (δ+) on the hydrogens
• This unequal distribution of charge makes water a polar molecule

Hydrogen Bonding

• The slightly positive hydrogen atoms of one water molecule are attracted to the slightly negative oxygen atoms of another water molecule
• This attraction forms a hydrogen bond
• Each water molecule can form hydrogen bonds with up to four other water molecules
• Hydrogen bonds are relatively weak compared to covalent bonds, but their cumulative effect is significant

Properties of Water

Cohesion

• Cohesion is the attraction between like molecules
• Hydrogen bonds hold water molecules together, contributing to high cohesion
• This allows for surface tension, where water molecules at the surface resist separation

Adhesion

• Adhesion is the attraction between unlike molecules
• Water adheres to other polar molecules via hydrogen bonds
• Capillary action, where water moves up narrow tubes against gravity, results from both cohesion and adhesion

High Specific Heat

• Specific heat is the amount of heat required to raise the temperature of a substance by one degree Celsius
• Water has a high specific heat due to hydrogen bonds, which resist temperature changes by absorbing or releasing heat when they break or form
• This helps stabilize temperatures in organisms and environments

High Heat of Vaporization

• Heat of vaporization is the amount of heat required to convert a liquid to a gas
• Water has a high heat of vaporization because hydrogen bonds must be broken for water to evaporate
• Evaporative cooling occurs as water evaporates, removing heat from the surface

Density Anomaly

• Water is less dense as a solid (ice) than as a liquid
• As water cools, it becomes denser until it reaches 4°C
• Below 4°C, water molecules form a crystalline structure due to hydrogen bonding, which spaces the molecules farther apart
• Ice floats, insulating bodies of water and preventing them from freezing solid

Water as a Solvent

• Water's polarity makes it an excellent solvent for polar and ionic compounds
• Hydration shells form around ions, separating and dispersing them in water

Acids and Bases

• Acids and bases are fundamental concepts related to the concentration of hydrogen ions (H+) in a solution
• Acids increase the H+ concentration, while bases reduce it

Acids

• An acid is a substance that increases the hydrogen ion (H+) concentration in a solution
• Strong acids like hydrochloric acid (HCl) dissociate completely in water, releasing all their H+ ions
• Weak acids like acetic acid (CH3COOH) only partially dissociate, releasing fewer H+ ions

Bases

• A base is a substance that reduces the hydrogen ion (H+) concentration in a solution
• Some bases directly accept H+ ions (e.g., ammonia, NH3), while others release hydroxide ions (OH-) that combine with H+ to form water
• Strong bases like sodium hydroxide (NaOH) dissociate completely, releasing all their OH- ions
• Weak bases like ammonia (NH3) only partially accept H+ ions or release fewer OH- ions

pH Scale

• The pH scale measures the acidity or basicity of a solution
• It ranges from 0 to 14, with 7 being neutral
• pH = -log[H+], where [H+] is the hydrogen ion concentration in moles per liter
• Acidic solutions have pH values less than 7
• Basic (or alkaline) solutions have pH values greater than 7
• Each pH unit represents a tenfold difference in H+ concentration i.e., a solution with a pH of 3 has ten times more H+ ions than a solution with a pH of 4

Buffers

• Buffers are substances that minimize changes in pH by accepting or donating H+ ions when needed
• They typically consist of a weak acid and its conjugate base
• One important buffer system in biological systems is the carbonic acid-bicarbonate buffer:
  • H2CO3 ⇄ HCO3- + H+
  • Carbonic acid (H2CO3) can donate H+ ions, while bicarbonate (HCO3-) can accept H+ ions

Importance of pH

• pH affects biological processes, as enzymes and other biological molecules are sensitive to pH changes.
• Maintaining pH is crucial for homeostasis in living organisms
• Buffers in blood and other bodily fluids help maintain a stable pH for optimal cellular function