Summary
Highlights
Surface tension is the elastic force on a liquid's surface, requiring energy to stretch or increase its area. Molecules at the surface are pulled inward, creating a 'skin' and shaping the liquid into a sphere to minimize surface area. Liquids with strong intermolecular forces (IMF), like water with its hydrogen bonds, exhibit high surface tension. Surfactants, such as soaps, disrupt this hydrogen bonding, reducing surface tension and increasing the water's ability to wet other substances.
Cohesive forces are attractions between liquid molecules, while adhesive forces are attractions between liquid molecules and another surface. If adhesive forces are stronger (e.g., water on glass), a concave meniscus forms. If cohesive forces are stronger (e.g., mercury), a convex meniscus forms. Capillary action, the rise of liquids in narrow tubes, results from the interplay of these forces, as seen when water rises in capillary tubes, with surface tension pulling the water up.
Viscosity measures a liquid's resistance to flow; higher viscosity means slower flow. It's directly related to the strength of intermolecular forces (IMF) and inversely related to temperature (viscosity increases with stronger IMF and decreasing temperature). For instance, glycerol has a much higher viscosity than water due to its three hydrogen bonding sites, leading to stronger IMF and greater resistance to flow.
Evaporation is the process where liquid molecules escape into the vapor phase. In a closed container, a dynamic equilibrium is reached where the rate of condensation equals the rate of evaporation. The pressure exerted by the gas at this equilibrium is called vapor pressure. Vapor pressure increases with temperature due to more molecules having enough energy to escape. Liquids with high vapor pressure are volatile and have weaker IMF, while those with strong IMF have low vapor pressure. Vapor pressure is inversely proportional to IMF strength.
Molar heat of vaporization is the energy required to vaporize one mole of liquid. Stronger IMF leads to a higher molar heat of vaporization and thus a higher boiling point. The Clausius-Clapeyron equation describes the relationship between temperature and vapor pressure. The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. The normal boiling point occurs at one atmosphere of external pressure. Both molar heat of vaporization and boiling point reflect the strength of intermolecular forces.
Water is an essential and unusual substance due to its unique properties. It is liquid at room temperature despite its low molecular mass, thanks to strong hydrogen bonds. Water is an excellent solvent due to its polarity and hydrogen bonding ability, dissolving many organic substances, salts, ionic compounds, and gases. It also exhibits high surface tension and capillarity. Water has a high specific heat capacity, meaning it can absorb or release significant heat with only slight temperature changes, which moderates Earth's temperature. Finally, solid water (ice) is less dense than liquid water because its hydrogen bonds form an open hexagonal structure, allowing ice to float. This property is crucial for aquatic life, as ice insulates the water below, preventing it from freezing solid and providing a stable environment for organisms.