Water boils at 100 degrees Celsius at standard atmospheric pressure, but it can boil at lower temperatures under certain conditions. The boiling point of water is influenced by the atmospheric pressure exerted on its surface. When the pressure decreases, the boiling point of water also decreases.
At higher altitudes, where the atmospheric pressure is lower, water boils at temperatures below 100 degrees Celsius. This is because the pressure exerted on the water's surface is not sufficient to keep the liquid molecules together. As a result, the vapor pressure of the water reaches the atmospheric pressure at a lower temperature, causing it to boil.
To illustrate this, let's consider the vapor pressure of water. Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature. At sea level, where the atmospheric pressure is approximately 1 atmosphere, the vapor pressure of water reaches 1 atmosphere at 100 degrees Celsius, leading to boiling.
However, if you increase in altitude, the atmospheric pressure decreases. At an altitude of, for example, 2,000 meters above sea level, the atmospheric pressure is lower than 1 atmosphere. At this altitude, the vapor pressure of water can reach the reduced atmospheric pressure at a temperature below 100 degrees Celsius. As a result, water boils at a lower temperature, typically around 90 degrees Celsius or lower, depending on the exact altitude.
In summary, the boiling point of water can be below 100 degrees Celsius when the atmospheric pressure decreases, such as at higher altitudes. The reduced pressure lowers the boiling point by allowing the water molecules to escape from the liquid phase and form vapor at lower temperatures.