Air Conditioning Repair Certification (ETI) Practice Exam

At 28 inches of mercury (Hg) vacuum, the pressure is significantly lower than standard atmospheric pressure. Water normally boils at 212°F at sea level due to the higher pressure. However, as pressure decreases, the boiling point also decreases.

At a vacuum of 28 inches of Hg, which is approximately 0.93 psi, water boils at a lower temperature than 212°F. In fact, under these conditions, water starts to boil around 10°F. This phenomenon occurs because boiling is the process where the vapor pressure of the liquid equals the surrounding pressure; with the lower pressure in the vacuum, it takes less thermal energy to reach that point.

Thus, the correct temperature at which water vaporizes in a vacuum of 28 inches of Hg is approximately 10°F. Understanding the relationship between pressure and boiling point is crucial in scenarios involving vacuum systems, as it directly affects the temperatures at which liquids change to gas.