Importance of Water

• Water is essential to life and is crucial for all significant biological processes.
• Biological processes occur mainly in water; cytoplasm in cells is predominantly water.
• Humans are composed of 60% to 70% water.
• The search for extraterrestrial life focuses on finding water, as it is crucial for life as we know it.

Structure of Water

• Water (H2O) comprises one oxygen atom bonded to two hydrogen atoms through covalent bonds.
• Covalent bonds involve shared pairs of electrons between atoms.
• Oxygen has two lone electron pairs that repel each other, influencing the shape of the water molecule.
• In three dimensions, water has a tetrahedral shape, with hydrogens at one end and lone pairs of electrons at the other.

Behavior of Electrons

• Electrons are not static; they orbit in a cloud of probability around atoms.
• Understanding the behavior of electrons is critical to comprehending water's properties.

Electronegativity of Oxygen

• Oxygen is highly electronegative, meaning it has a strong tendency to attract electrons to itself.
• Oxygen holds electrons closer than hydrogen does, creating a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms.
• The Greek letter delta (Δ) denotes partial charges.

Hydrogen Bonds

• Water molecules interact through hydrogen bonds, formed between the positively charged hydrogen of one molecule and the negatively charged oxygen of another.
• Hydrogen bonds are weaker than covalent bonds but are crucial for many of water's unique properties.
• These bonds allow water to have a fluid nature, exhibiting both cohesion (molecules sticking together) and the ability to flow past one another.

Unique Properties of Water

• Hydrogen bonds are responsible for water's:
• Ability to regulate temperature.
• Low density as a solid (therefore, ice floats while liquid water remains below).
• High heat capacity, which pertains to how much heat water can absorb before its temperature changes significantly.
• Evaporative cooling and surface tension.
• Cohesion and adhesion, which are essential for transport in biological systems.
• Crucially, the polarity of water molecules makes it an excellent solvent, allowing polar substances to dissolve easily in it.