1. Water and Its Properties Essential to Life

Life’s chemistry is tied to water

Life first evolved in water; all living organisms require water; cells consist of ~75% water :contentReference[oaicite:0]{index=0}

Covalent bond

Two atoms share a pair of electrons (each atom has unpaired electron in outer shell)

Molecule

Two or more atoms held together by covalent bonds

Polar covalent bond

Unequal sharing of electrons due to differences in electronegativity

Why is water polar?

Oxygen is more electronegative than hydrogen, so electrons spend more time near oxygen

Charges in water

Oxygen = slightly negative; Hydrogens = slightly positive

Hydrogen bond

Weak bond between partially positive hydrogen and partially negative atom (like oxygen)

Hydrogen bonds in water

Each H can bond to nearby oxygen of another molecule; each H2O can form up to 4 hydrogen bonds

Why hydrogen bonds are important

Give water high boiling point, surface tension, ability to dissolve substances; stabilize DNA (A-T, G-C); help proteins fold; influence biological structures

Strength of hydrogen bonds

Very weak and easy to break

4 emergent properties of water

Cohesion; ability to moderate temperature; ice floats on water; water as solvent

Cohesion

Attraction between molecules of the same substance

Cohesion in water

Caused by hydrogen bonding between water molecules

Surface tension

Measure of how difficult it is to stretch or break the surface of a liquid

Why water has high surface tension

Water molecules are hydrogen bonded to each other below but not to air above, creating an “invisible film”

Adhesion

Clinging of one substance to another

Water adhesion

Water sticks to other surfaces due to hydrogen bonds

Importance of adhesion

Helps counter gravity in plants (water moving up cell walls)

Examples of adhesion

Water droplets on leaves; plant roots absorbing water

Ability to moderate temperature

Water resists temperature change due to high specific heat

Specific heat capacity

Amount of heat needed to raise temperature; water can absorb/release large heat with little temperature change

Why water stabilizes temperature

Hydrogen bonds absorb heat when breaking and release heat when forming

Heat of vaporization

Amount of heat required to convert liquid to gas

Why water has high heat of vaporization

Hydrogen bonds must be broken to evaporate water

Evaporative cooling

When water evaporates, it removes heat (ex: sweating cools body)

Ice floating on water

Solid water (ice) is less dense than liquid water

Why ice is less dense

Hydrogen bonds form a crystalline structure that spaces molecules farther apart

Importance of ice floating

If ice sank, bodies of water would freeze solid and life would not survive

How ice supports life

Insulates water below and provides habitat (polar bears, seals)

Solution

Uniform mixture of two or more substances

Solvent

Dissolving agent (in aqueous solutions, water is solvent)

Solute

Substance being dissolved

Aqueous solution

Solution where water is the solvent

Why water is a good solvent

Polarity allows it to surround and separate ions (ex: Na+ and Cl-)

Water as solvent of life

Allows chemical reactions necessary for life

Search for extraterrestrial life

Astrobiologists look for water because life depends on it

Evidence of water on Mars

Ancient rivers, lakes, groundwater systems; chemical ingredients for life; warmer/wetter past; sedimentary rocks that preserve biosignatures

Why Mars is important for life search

Most accessible place beyond Earth with evidence of past water and potential life