Into the vessel, at the bottom of which there was water, some air was pumped. When the valve was opened and compressed air resurfaced, the vessel got filled with water mist. Why did this happen?
With a sharp expansion of air it cools. The process is fast, and all three parameters of the system change (p, V, T),we can speak with some approximation of adiabatic expansion of gas. Then ΔQ = 0, A = -ΔU the internal energy of the gas decreases due to the commission of work by it during the expansion. The temperature of gas and the surrounding areas of the cylinder decreases. Steam in the vessel becomes supersaturated at a given temperature and it condensates in the air, cooling fog is formed.