Boiling Liquids at Reduced Pressure

1. Boiling Liquids at Reduced Pressure Acetone is boiled without heating.

2. Purpose • To boil acetone using an alternative method. • To boil acetone below its normal boiling point.

3. Demonstration • Acetone will be made to boil by reducing the vapor pressure.

4. Concepts • Reduced Pressure • Boiling Point • Entropy

5. Reduced Pressure • External pressure above the liquid is reduced. • Reducing external pressure, reduces the vapor pressure needed to induce boiling .

6. Boiling Point • Point at which vapor pressure of liquid slightly exceeds the external pressure above liquid. • The reduction in vapor pressure allows acetone to boil below it’s normal boiling point.

7. Entropy • A measure of disorder or randomness in a closed system. • Boiling causes molecules of the liquid to be converted from an ordered liquid state to a disordered gas state, increasing the entropy of the system.

8. Conclusions • By reducing external pressure above the liquid, the vapor pressure is also reduced. • At boiling point, the vapor pressure is slightly more than the external pressure. If external pressure is reduced, the boiling point will occur at a lower temperature than normal.

9. Comments • Cooking at High Elevations • atmospheric (external) pressure higher • boiling point of water is lower • food cooks more slowly at lower temperatures