Notes on Water Properties: Cohesion, Adhesion, Solubility, Hydrophilic/Hydrophobic, and Amphipathic Molecules

Water properties arise from hydrogen bonds between water molecules.
Cohesion: attraction of identical molecules to each other, driven by hydrogen bonding.
Surface tension: a result of cohesion at the air–water interface, making the surface behave like a stretched skin.
Example: a water skipper (water strider) can walk on water due to surface tension.
Water molecules prefer to stay connected to each other and resist breaking hydrogen bonds.

Adhesion: attraction between different molecules.
Because water is a polar molecule, it can adhere to surfaces with positive or negative charges.
Importance in plants: adhesion helps transport water through the plant system to various tissues.
Both adhesion and cohesion are strong in water due to hydrogen bonding and the dipole nature of water.

Water dissolves a large number of substances because it is polar.
Not every substance dissolves, but water is especially good at dissolving many solutes.
Mechanism: the partial charges on water molecules enable interaction with other substances.

Ionic bonds: compounds like salt consist of positive and negative ions bonded together.
In water, water molecules (with partial charges) interact with these ions and help separate them.
This interaction can replace the electrostatic attraction between ions, effectively dissolving the compound.
Hydration: water molecules surround dissolved ions, forming an aqueous solution (e.g., aqueous salt).
Hydrophilic substances: polar or ionic molecules that readily dissolve in water.

Hydrophobic (water-fearing) substances are nonpolar and do not dissolve well in water.
Example: oil.
These substances do not interact strongly with the polar water molecules, so they tend to separate rather than dissolve.

Amphipathic molecules have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
One part may be polar and attracted to water; the other part nonpolar and not attracted to water.
They can orient themselves at interfaces between water and nonpolar substances (e.g., at the water–oil interface).
Real-world relevance: many biological molecules and detergents exhibit amphipathic properties.

Hydrogen bonding underpins cohesion, adhesion, surface tension, and solubility behaviors of water.
Water’s polarity enables it to act as a solvent for many substances, especially ionic and other polar compounds.
The interplay of cohesive and adhesive forces explains everyday observations (e.g., water striders; capillary action) and biological processes (plant water transport).
Hydrophilic vs hydrophobic tendencies determine solubility outcomes for different substances.
Amphipathic molecules bridge polar and nonpolar environments, explaining their behavior at interfaces.

Biological systems rely on water’s solvent properties for metabolism, transport, and biochemical reactions.
Environmental relevance: solubility and transport of substances in water affect ecosystem health and pollutant dispersion.
Engineering and chemistry: solubility principles inform solutions, formulations, and material interfaces.

Cohesion: attraction between identical molecules.
Adhesion: attraction between different molecules.
Surface Tension: cohesive forces at the surface of a liquid.
Polar Molecule: molecule with an uneven distribution of charge.
Hydrophilic: water-loving; polar or ionic substances that dissolve in water.
Hydrophobic: water-fearing; nonpolar substances that do not dissolve well in water.
Amphipathic: molecules with both hydrophilic and hydrophobic regions.
Aqueous Solution: a solution where water is the solvent.