When does water boil?

Water boils when its vapor pressure equals the atmospheric pressure. At sea level, where the atmospheric pressure is approximately 1 atmosphere (101.3 kPa), this occurs at 100°C (212°F). However, the boiling point of water changes with altitude and atmospheric pressure. At higher altitudes, where atmospheric pressure is lower, water boils at a lower temperature. Conversely, in a high-pressure environment, water boils at a higher temperature.

Among the given options:
– [A] “When its vapor pressure is one gram per square centimeter” is incorrect because boiling is not related to this specific pressure unit.
– [B] “When its vapor pressure is equal to 76 cm of mercury” is incorrect as this is specific to standard atmospheric pressure but doesn’t cover all conditions.
– [C] “Static vapor pressure of water is equal to atmospheric pressure” is correct.
– [D] “When the temperature of water reaches 100°C” is true only at sea level.

Thus, the correct answer is [C].