We know that, under normal circumstances, water begins to boil when heated to 100°C, after which the temperature no longer rises, hence people often say that "the boiling point of water is 100°C". When water is heated, on the one hand, it absorbs heat, and on the other hand, it loses some heat. The heat lost includes the heat carried away by water vapor evaporation and the heat dissipated to the outside due to the water's temperature being higher than the ambient temperature. When the water temperature is below 100°C, the heat absorbed by the water is greater than the heat lost, and the water temperature gradually rises. When the water temperature reaches 100°C, a gas-liquid phase transition occurs. Absorbing any more heat will only turn the liquid water into gaseous water vapor (gaseous water has higher energy than liquid water), and the temperature (100°C) remains constant. At this point, the heat absorbed by the water and the heat lost are equal, a phenomenon called boiling. However, it should be noted that this is only "usually" the case, that is, under standard atmospheric pressure conditions.
We can conduct a comparative experiment. First, boil water at the foot of the mountain, and when the water boils, measure the water temperature to be about 100°C. Then, boil water in the same way at the top of the mountain, and find that the water temperature is lower than 100°C when it boils. The difference between the ground and the top of the mountain is that the atmospheric pressure at the top of the mountain is lower. This shows that when the atmospheric pressure decreases, the boiling point of water decreases. People who have traveled to high-altitude areas such as Tibet may have noticed that the temperature of boiling water in high-altitude areas does not reach 100°C in an open environment. In fact, the altitude of Lhasa is about 3,700 meters, and the boiling point of water is only 88°C.
Since reducing the atmospheric pressure will lower the boiling point of water, increasing the atmospheric pressure will increase the boiling point of water. But how can we increase the atmospheric pressure in the environment? A typical method of increasing the boiling temperature of water is to use a pressure cooker. In a pressure cooker, water vapor cannot escape, and the accumulated heat makes the gas pressure exceed 1 standard atmospheric pressure, usually reaching nearly 1.1 standard atmospheric pressure, and the boiling point of the water inside can reach 110–130°C.