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.