Water

Formula: Water is chemically represented as H2O (or H-O-H).
Composition: A water molecule is composed of:
- 2 hydrogen atoms (H)
- 1 oxygen atom (O)
Bonding: The atoms are held together by covalent bonds, which involve the sharing of electrons between atoms.
Structure Preference: The notation H-O-H gives a clearer representation of water's chemical structure.

Both hydrogen atoms bind to the central oxygen atom through covalent bonds.
Electron Sharing: The sharing of electrons is not equal:
- Electronegativity: Oxygen, being more electronegative, pulls the shared electrons closer, causing an unequal distribution.
- Oxygen gains a partial negative charge (δ−), while hydrogen atoms acquire partial positive charges (δ+).

The difference in charge distribution leads to the formation of a polar covalent bond.
Definition of Polar Covalent Bond: A bond where electrons are not shared equally, resulting in charged regions within the molecule:
- The Greek letter delta (δ) is used to denote partial charges.

Formation of Hydrogen Bonds: The negatively charged oxygen atom of one water molecule is attracted to the positively charged hydrogen atoms of nearby water molecules.
Nature of Hydrogen Bonds:
- Weak and short-lasting bonds, but their collective presence among numerous water molecules creates significant effects.
- Water molecules exhibit a kind of stickiness reminiscent of "molecular Velcro."

Definition of Specific Heat:
- The amount of heat that needs to be absorbed or lost for 1 gram of a substance to change its temperature by 1°C.
- Serves as a measure of how easily a substance's temperature changes.
Water’s High Specific Heat:
- Water has a notably high specific heat compared to other substances, implying:
- It requires considerable energy to change water's temperature.
- Molecular interactions hinder rapid temperature changes due to hydrogen bonding.
Molecular Movement: When heated, molecules move faster; however, due to hydrogen bonds, water resists temperature increases.

Cooking Example:
- When boiling water in a pot:
- Touching the pot can lead to burns faster than touching the water.
- The pot's lower specific heat allows it to heat and lose heat more rapidly than the water.

Water Content in Organisms: Most living organisms are largely composed of water:
- Humans: ~67% water
- Lettuce: ~94% water
- Apples: ~84% water
- Tomatoes: ~93% water
- Cows: ~80% water
Thermoregulation: High specific heat is critical in stabilizing body temperatures across organisms, preventing drastic temperature fluctuations (e.g., blood boiling on a hot day).

Global Temperature Effects: The specific heat of water significantly moderates temperatures of nearby land:
- Water heats slower than land due to its high specific heat.
- As water absorbs heat, the air above it remains cooler, facilitating temperature moderation on land by:
- Warm, rising air over land being replaced by cooler air from over the water.

Upcoming Discussions: The course will later cover how water can move from the roots of trees to their tops against gravity, highlighting additional fascinating properties of water.