Demonstrations for Chapter 6: Condensed Phases And Phase Transitions

11. Pressure and melting point. As you might predict (on examining the phase diagram for H₂O), an increase in pressure should lower the normal melting point. Explain why a piano wire can be pulled through a block of ice (by two weights suspended from the ends) without cutting the block in two.

12. Boiling at Reduced Pressure. When the phase transition from the liquid to the vapor state takes place such that the pressure of the escaping liquid molecules equals the pressure of the atmosphere, you are at the boiling point. Boiling point, therefore, is a function of the atmosphere over the liquid surface, as well as the temperature and the nature of the liquid.

13. Vapor Pressure of Liquids: (a) Vapor pressure of water, alcohol and ether in a barometer experiment. (b) The classic "drinking bird" is an old time kid's toy based on well-worn scientific principles governing the properties of liquids, especially vapor pressure.

14. Crushed dry ice ... solid CO₂ .... is placed into a length of Tygon tubing, a polymeric material made of silicon-containing polymers that is an "elastomer," a material with rubbery elastic properties, and the ends are tightly sealed with Hoffmann (screw-type) claps to ensure no leakage of the gas and liquid that forms as the ice warms to room temperature. The explosive result is the work of expansion. What is the liquid that forms and how do you explain its presence?

15. Capillary action is a manifestation of surface tension and the balance of adhesive and cohesive forces at work between a liquid and the walls of its container. Here, in this demonstration, five connected columns of different internal diameter are filled with a colored (for visual effect) aqueous solution.

16. Supersaturated solutions are thermodynamically unstable. In this case, such a solution of sodium acetate (NaOAc) can be easily "nudged," causing the crystallization of long, often strikingly visual needles of the hydrated salt.

17. Colloids: Aqueous sodium thiosulfate (Na₂S₂O₃) solutions are unstable in the presence of acids, decomposing in such a way as to yield colloidal suspensions of sulfur. Such finely divided particles scatter light passing through the solution, producing the same effect observed at sunset as light passes through the particle-filled lower atmosphere, giving the sun its characteristic and increasingly red color as it sets.

18. Osmosis and osmotic pressure are properties governed by laws that look very much like the gas laws. Here we see examples in the behavior of a egg that has had its hard, outer coating of calcium carbonate removed by pretreatment with acid, and a prune.