Hydrogen bonding in water | Water, acids, and bases | Biology | Khan Academy

Hydrogen Bonding in Water

Hydrogen Bonds:

  • Hydrogen bonds are weak, electrostatic attractions that occur between a hydrogen atom covalently bonded to a more electronegative atom (like oxygen) and another electronegative atom.

  • In water (H₂O), each water molecule can form hydrogen bonds with several neighboring water molecules due to the polarity of the molecule.

Water's Unique Properties Due to Hydrogen Bonding:

  • Cohesion: Water molecules are attracted to each other, leading to surface tension. This allows small objects, like a needle, to float on water if placed carefully.

  • Adhesion: Water molecules can also stick to other substances. This property is important in the movement of water in plants through adhesion to the xylem.

  • High Specific Heat: Water can absorb or release large amounts of energy with minimal change in temperature. This helps regulate temperature in organisms and environments.

  • High Heat of Vaporization: A lot of energy is required to convert water from a liquid to a gas, which helps cool organisms through processes like sweating.

Water as a Solvent:

  • Water is known as the "universal solvent" due to its ability to dissolve a wide range of substances. This is crucial in biological systems, as it allows for the transportation of nutrients and waste in organisms.

  • Ionization of Water: Water can also dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻), playing a key role in acid-base chemistry.

Acids and Bases:

  • Acids: Substances that increase the concentration of hydrogen ions (H⁺) in a solution. Common examples include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

  • Bases: Substances that increase the concentration of hydroxide ions (OH⁻) in a solution or decrease H⁺ concentration. Examples include sodium hydroxide (NaOH) and ammonia (NH₃).

  • The pH scale measures the acidity or basicity of a solution, with lower pH values representing acidic solutions and higher pH values representing basic solutions.

Relevance in Biology:

  • Understanding hydrogen bonding is essential for appreciating how water interacts with various biological molecules, such as proteins and nucleic acids, influencing their structure and function.

Summary:

  • Hydrogen bonding in water is responsible for its unique properties, including cohesion and adhesion, high specific heat, and its effectiveness as a solvent, playing a crucial role in biological processes, especially regarding acids and bases.