Ken Hovanes, age 11, of Huntsville, Ala., for his question:
WHAT MAKES BUBBLES IN BOILING WATER?
Most boilers are filled with water, and they are used to heat buildings, drive turbines and produce steam used for refining oil or drying paper. In the boiler, water is changed into a vapor steam. The steam provides the energy which can drive electric generators and even propel ships. Some boilers have furnaces below them while others are completely surrounded by furnaces.
When the temperature of a liquid reaches its boiling point, bubbles are formed as the liquid is turned into vapor. The bubbling, brought about by applying heat, is called boiling.
The bubbles in boiling water are actually a collection of gas which is then released into the air.
Atmospheric pressure actually determines the boiling point of any liquid. And the boiling point is that temperature at which the liquid's vapor pressure equals the atmospheric pressure.
At sea level water boils at 212 degrees Fahrenheit. At this location the atmospheric pressure is 14.7 pounds per square inch.
At higher altitudes, where the pressure of the atmosphere is lower, water boils at lower temperatures. The boiling point is about one degree Fahrenheit lower for each additional 550 foot increase in the altitude. At extremely high altitudes, the boiling point of water is so low that usually food cannot be cooked in open pots.
Not every liquid will boil at the same temperature. But the boiling point of a liquid will always stay the same under the same atmospheric pressure. The boiling point of a liquid may be found by holding a thermometer in the vapor just above the surface, but not in the liquid.
In open kettle boiling, once boiling of the water has started you can save heat by adjusting your gas flame or electric temperature to a point just sufficient to keep the liquid boiling. No amount of extra heat will increase the temperature after the liquid has started to boil. Steam pressure cookers, for example, save time and fuel because pressure from the confined steam raises the temperature and reduces cooking time.
Boilers used for heating homes also use the pressure system. One in your home, for example, might operate at 10 to 15 pounds per square inch. Power plants have larger boilers and operate at 100 to 250 pounds per square inch.
Boilers used to provide energy to power turbines in electric power stations can operate at pressures as high as 3,500 pounds per square inch.
Fire tube boilers, used in some homes and small factories, have hot gases flowing through tubes that are surrounded by water. The water is then changed into steam. Boilers requiring higher pressures are of the water tube type. In this type of boiler the hot gases flow over tubes which are filled with water.