Why Water Boils * Water Boils at 100 o C at STP Jacob Schroeder Chem 292

Purpose n To find out why water boils n To find out what all makes water boil

Demonstration n Put a random amount of water into good sized beaker (400 or 600 mL) n Add heat by a Bunsen burner or a hot plate n To see the convection currents, add an insoluble liquid to the water.

Concepts n Intermolecular Attractions n Vapor Pressure n Convection Currents

Intermolecular Attractions n The stronger the attractive forces, the higher the temperature at which the liquid boils. n Water contains all three types of Intermolecular forces, (Van der Waals, dipole-dipole, and Hydrogen bonding). n Figure of the Dipole

Vapor Pressure n Vapor Pressure - pressure exerted by vapor when liquid and vapor are in equilibrium. n Water boils when its vapor pressure equals the external pressure acting on the surface. n If the external pressure increases, so does the vapor pressure. n Water’s vapor pressure is 760 torr when water boils at at a temp of 100 o C.

Convection Currents n When water is heated it becomes less dense and rises. n This circulation accounts for the uniform heating of water.

Conclusions n If the external air pressure would be higher, water would boil at a higher temperature. n If water was not polar, it would boil at a lower temperature. n Convection currents circulate the water to allow for uniform heating.

Comments n What makes water boil on the molecular level. n How convection currents aid the boiling process. n The variation of boiling points at different pressures.