On a hot summer day, a cold drink is an absolute temptation! While enjoying it, you might notice that the surface of the drink bottle taken out of the refrigerator will develop numerous water droplets in a very short time. Could the drink have leaked through the bottle? Of course not! To understand the reason behind this phenomenon, we need to start with the air we breathe every moment. Air is a mixture composed of 78% nitrogen, 21% oxygen, and 1% rare gases and impurities. Due to the evaporation of water from rivers, lakes, and oceans in nature, or the respiration of plants and animals, the air in our daily living environment also contains water vapor. This water-vapor-containing air is called "humid air". The more water vapor it contains, the more humid it feels; sometimes, we even use the phrase "you can even wring out water from the air" to describe the humid air.
Under atmospheric pressure, the air's "absorption" capacity for water vapor is not infinite, and there is a maximum value of moisture content. The humid air at this state is called saturated air, and the pressure and temperature of water vapor are called saturated pressure and saturated temperature (the saturated temperature is the boiling point of water at saturated pressure). If the amount of water vapor in the air exceeds this maximum value, the pressure of water vapor will exceed its saturated pressure, and a gas-liquid phase transition will occur, meaning that the excess water vapor will have to condense into water droplets. In this case, there's no need to "wring it out"; the air will naturally release water.
The moisture content of saturated air is closely related to the ambient temperature. The lower the ambient temperature, the smaller the saturated pressure of water vapor, and the less water vapor the humid air can "absorb". When the ambient temperature is 34℃, the moisture content of saturated air is 34.94 g/kg. If the water vapor at this time is 34.00 g/kg, it has not reached saturation. When the weather changes and the temperature drops to 30℃, the maximum value at this temperature is only 27.52 g/kg. In this case, 6.48 g/kg of water vapor will condense into water droplets and release from the humid air, i.e., "dew condensation". In summer, the surface temperature of the drink bottle taken out of the refrigerator is much lower than the ambient temperature. There will be a heat exchange between the drink bottle surface and the air, causing the temperature of the air around the bottle to gradually decrease, and the closer to the bottle wall, the lower the temperature. As a result, the humid air will gradually become saturated due to cooling, and then dew will form on the bottle wall, releasing small water droplets and adhering to the outer wall of the drink bottle. If a bottle of drink with ice inside is placed indoors, it won't take long for a layer of frost to appear on the bottle wall. This is because the water droplets on the outer wall of the bottle wall exchange heat with the ice inside, causing the water droplets to become colder than 0℃ and freeze.