Water and Life - Summary

Water & Life

Importance of Water

• Water is essential for life, covering 3/4 of the Earth.

• Organisms are mostly water (70-95% in cells).

• Water exists in three states: solid, liquid, gas.

Water Molecule

• Water: 2 hydrogens, 1 oxygen, single covalent bonds.

• Oxygen is more electronegative than hydrogen.

• Polar molecule: unequal electron distribution, opposite charges.

Hydrogen Bonds

• Water's ability to form hydrogen bonds is its most outstanding property.

• Hydrogen bond: weak association between partially negative O and partially positive H atoms of separate water molecules.

• Hydrogen bonds constantly break and reform.

Cohesion and Adhesion

• Cohesion: Water molecules stick to each other.

• Adhesion: Water molecules stick to other polar molecules.

Properties of Water

• High specific heat: needs much energy to change temperature.

• High heat of vaporization: evaporation causes cooling.

• Solid water (ice) is less dense than liquid water.

Water as a Solvent

• Water dissolves polar molecules and ions.

• Hydrophilic: water-loving.

• Hydrophobic: water-fearing; aggregate in water.

• Water forms ions: $H_2O \rightarrow OH^- + H^+$.

Acids and Bases

• Acid: increases hydrogen ion concentration.

• Base: reduces hydrogen ion concentration.

• $[H^+]$ and $[OH^-]$ product is constant at 25°C.

• Acidic solutions: higher $[H^+]$ than $[OH^-]$.

• Basic solutions: higher $[OH^-]$ than $[H^+]$.

• Neutral solutions: equal $[H^+]$ and $[OH^-]$.

The pH Scale

• $pH = -log[H^+]$

• Neutral solution: $[H^+] = 10^{-7} M$, pH = 7.

• pH < 7: acidic; pH > 7: basic.

Buffers

• Buffers resist pH changes by releasing or absorbing hydrogen ions.

• Most biological buffers consist of an acid-base pair (e.g., H2CO3$and$HCO_3^-.)

Environmental pH

• Acid precipitation: pH < 5.2.

• Ocean acidification: increased atmospheric CO2$leads to increased$ H2CO_3$$, reducing calcium carbonate for shell formation.