Unique properties of wter水独特的性质.pptVIP

Liquids and Solids Use the Force Luke! Learning objectives Describe origins of surface tension and meniscus Describe different types of cohesive force Identify type of cohesive force based on molecular formula Describe origin of hydrogen bonding Explain unique properties of water Determine concentrations of solutions using the common concentration scales Liquids and solids exist because of forces Cohesive forces are attractive forces between molecules of the same substance Adhesive forces are attractive forces between molecules of different substances Surface tension results from cohesive forces Molecules on surface are drawn inwards Tendency to form sphere Surface behaves like a shrink-wrap film Liquids form into spherical drops Denser objects can “float” on the surface tension Consequences of surface tension: What will these hands ne’er be clean? Cleaning requires wetting Surface tension prevents wetting Beading on a waxed car Detergent acts as go-between Cohesive forces and meniscus Adhesive forces pull H2O molecules to maximize coverage Cohesive forces between H2O molecules drag liquid up Gravity pushes liquid down Solid: strong interactions Fixed shape Not compressible Rigid Dense Solids Atoms in solids rotate and vibrate but do not translate Melting occurs when the translational energy of the atoms is sufficient to break free of the lattice Usually this is a very well defined point With amorphous solids it can be smeared out – softening of fats Liquid: medium interactions Liquid Not rigid Assumes shape of container Not compressible Dense Vapour pressure and boiling Molecules do not all have the same energy Evaporation: High energy molecules escape the liquid – vapour pressure When vapour pressure = atmospheric pressure boiling occurs – all liquid becomes gas Sublimation is direct transition of solid to gas (dry ice) The Four Forces of the Apocalypse Dispersion force Arises from fluctuations in electron clouds in atoms Only force present in nonpolar molecules