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Why Water Boils - PowerPoint PPT Presentation

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Why Water Boils

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  1. Why Water Boils * Water Boils at 100 oC at STP Jacob Schroeder Chem 292

  2. Purpose • To find out why water boils • To find out what all makes water boil

  3. 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.

  4. Concepts • Intermolecular Attractions • Vapor Pressure • Convection Currents

  5. 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

  6. 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.

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

  8. 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.

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