Boiling with Ice
Water in a sealed flask is made to boil by inverting the flask and holding an ice cube against its bottom.
MATERIALS
• 1-liter, thick-walled, round-bottomed boiling flask
• two-hole rubber stopper
• thermometer
• glass tube
• rubber hose with pinch clamp
• Bunsen burner
• ice cube
PROCEDURE
Water can be made to boil at a temperature well below 100°C by reducing the pressure of the atmosphere above the water[1-5]. A 1-liter, thick-walled, round-bottomed boiling flask is filled about half way with water. A two-hole rubber stopper is used to seal the flask. A thermometer is placed in one hole of the stopper. In the other hole is a glass tube connected to a rubber hose which can be sealed with a pinch clamp. The water is brought to a boil with a Bunsen burner. The water is then allowed to cool for one minute. The pinch clamp is then used to seal the flask, the flask is inverted, and an ice cube is held against its bottom. The water in the flask will boil for several seconds.
DISCUSSION
The temperature at which a liquid boils depends upon the pressure of the atmosphere above it. The water will boil when its vapor pressure (see Table 2.3) exceeds the pressure of the surrounding gas. In this demonstration, the pressure is lowered from normal atmospheric pressure (760 torr) by cooling the air in the flask and by condensing some of the water vapor in the flask. As the water boils, the pressure in the flask increases to the point where it is greater than the vapor pressure of the liquid, and the water stops boiling. The flask should be no more than half full of water to provide space for the vapor produced by boiling. If too little space is provided, the vapor pressure of the liquid water may be reached before boiling is apparent.
HAZARDS
In addition to the hazard of burns from the Bunsen burner and the hot flask, the flask may implode if the pressure is lowered sufficiently. This risk is minimized...