4C31.21 - Freezing Water - Reduced Pressure

Code Number:
Demo Title:
Freezing Water - Reduced Pressure
Pressure Effects on Changes of State
Area of Study:
Heat & Fluids
Watch glass with stand, vacuum pump with bell jar, water, (if needed) concentrated sulfuric acid in a shallow dish.

Video Credit: Jonathan M. Sullivan-Wood.

Warm up the vacuum pump for 30 minutes before performing the demo so that the water will not be absorbed into the pump oil.  Place a silver dollar sized spot of water on the watch glass.  Open the gas ballast valve on the vacuum pump first, ( this helps keep water from being absorbed into the pump oil ), and then open the valve to the bell jar.  The water will appear to boil after 20 seconds and should freeze within 45 seconds.

 OLD WAY TO DO THE DEMO:  Put the watch glass stand over the dish of sulfuric acid. Put water into the watch glass or Petri dish ( 1 ml or less ). Place the entire apparatus under a bell jar with suitable seals and evacuate. With a motor-driven pump, it will take about 1-2 minutes for the water to freeze.

Note:  The sulfuric acid helps greatly when trying to freeze large quantities of water ( 1 ml to 5 ml) and also cuts down on the pumping time.  DO NOT let the sulfuric acid stand open to the atmosphere for any length of time as it will absorb water quickly.  It will then be useless when performing the demo.

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