Final answer:
Water can boil above or below 100°C due to variations in environmental pressure. For example, higher pressures increase the boiling point, while lower pressures, such as at high altitudes, decrease it. The presence of solutes also elevates the boiling point due to what is known as boiling point elevation.
Step-by-step explanation:
Water can boil at more or less than its normal boiling point of 100°C depending on the environmental pressure. At pressure levels above 1 atm, water will boil at a temperature higher than 100°C because the vapor must have more kinetic energy to escape the increased pressure. Conversely, at pressures below 1 atm, water will boil at temperatures lower than 100°C, since the surrounding pressure is lower and vapor needs less energy to escape. This concept also applies to boiling point elevation. When solutes, such as salt, are added to water, the boiling point increases because the solute particles inhibit the water molecules from escaping into the gas phase, requiring more heat to reach boiling. In high altitudes, such as Denver, Colorado, or on the summit of Mount Everest, the lower atmospheric pressure leads to a decreased boiling point for water.
In a closed system containing an equal mix of water and steam at 100°C and 1.00 atm, the two will coexist. However, in an open system, as is typically the case when boiling water in a pot, the presence of air reduces the number of water molecules that can condense, leading to continual boiling.
The presence of impurities or dissolved gases in water can also cause bubble formation before it reaches the boiling point, as these can form air pockets that expand and rise to the surface, which we perceive as boiling.