Water+and+pH+Lecture+Notes

The Chemistry of Life

Importance of Water

• Water is essential for all life forms.

• It has unique properties due to hydrogen bonds that contribute to its importance.

Unique Properties of Water

Cohesion

• Cohesion refers to the tendency of water molecules to stick to one another.

• This property gives water high surface tension, allowing small objects to float on its surface.

• Example: Raindrops on a leaf.

Adhesion

• Water molecules can form hydrogen bonds with other substances, known as adhesion.

• This property helps water molecules to cling to other surfaces.

• Example: Water traveling up a paper towel, illustrating how adhesion facilitates the movement of water in plants.

Cohesive and Adhesive Properties Combined

• The combination of cohesion and adhesion enables water to move from the roots of trees to their highest leaves through capillary action.

Solvent Properties

Excellent Solvent

• Water is known as a universal solvent due to its ability to dissolve various hydrophilic (water-loving) substances.

• This includes polar solutes and ions.

• The polarity of water molecules is key in dissolving most biologically important molecules.

Dissolving Ions

• Water can dissolve ionic compounds, such as salt (NaCl).

• The slight negative charge of water molecules attracts cations (e.g., Na+), while the slight positive charge attracts anions (e.g., Cl−).

Temperature Regulation

Temperature Resistance

• Water's hydrogen bonds allow it to resist changes in temperature.

• This property results in slow heating and cooling of water, leading to milder temperatures in coastal areas compared to landlocked regions.

• Example: Sweating helps cool the body due to high heat capacity of water.

Ice Density

Expansion Upon Freezing

• When water freezes, hydrogen bonds create a crystalline structure that expands, causing ice to become less dense than liquid water.

• This allows ice to float and provides insulation for aquatic life in winter conditions, preventing entire bodies of water from freezing.

Participation in Chemical Reactions

Role in Reactions

• Water is involved in various chemical reactions, including photosynthesis and cellular respiration, serving as a reactant or solvent in these processes.

Mastering Concepts

Hydrophilic vs. Hydrophobic

• Hydrophilic molecules: Polar substances that readily dissolve in water.

• Hydrophobic molecules: Nonpolar substances that do not dissolve easily