Why Water Boils * Water Boils at 100 oC at STP Jacob Schroeder Chem 292
Purpose • To find out why water boils • To find out what all makes water boil
Demonstration • Put a random amount of water into good sized beaker (400 or 600 mL) • Add heat by a Bunsen burner or a hot plate • To see the convection currents, add an insoluble liquid to the water.
Concepts • Intermolecular Attractions • Vapor Pressure • Convection Currents
Intermolecular Attractions • The stronger the attractive forces, the higher the temperature at which the liquid boils. • Water contains all three types of Intermolecular forces, (Van der Waals, dipole-dipole, and Hydrogen bonding). • Figure of the Dipole
Vapor Pressure • Vapor Pressure - pressure exerted by vapor when liquid and vapor are in equilibrium. • Water boils when its vapor pressure equals the external pressure acting on the surface. • If the external pressure increases, so does the vapor pressure. • Water’s vapor pressure is 760 torr when water boils at at a temp of 100 oC.
Convection Currents • When water is heated it becomes less dense and rises. • This circulation accounts for the uniform heating of water.
Conclusions • If the external air pressure would be higher, water would boil at a higher temperature. • If water was not polar, it would boil at a lower temperature. • Convection currents circulate the water to allow for uniform heating.
Comments • What makes water boil on the molecular level. • How convection currents aid the boiling process. • The variation of boiling points at different pressures.