When I make pasta or potatoes, I dispose of the hot water (around 80-100°C) in a metal sink. Am I justified to first pour regular hot tap water on the sink to reduce the amount of steam generated, or is this pure superstition ? If it's not, what is the physical explanation ?
By running hot water in the sink first and thus pre-heating the metal sink, you are increasing the amount of steam. The reason is explained in the other answers, the visible "steam" (not gaseous) consists of fluid water droplets that condensed from vapour that previously evaporated due to high vapour pressure due to high temperature of water. So if the sink is hot the water is cooled less and more of it evaporates. If you run cold water in the sink and keep the tap on, that's usually enough to completely stop such a wave of hot mist on pouring in hot water. It also prevents the banging noises that some sinks produce because of thermal expansion when pouring hot water into them. Neither heating nor cooling the sink is necessary or objectively useful before pouring in boiling water (the exception is some sinks for example in camping cars which aren't heat proof and you aren't allowed to put boiling water in them at all).
The other answers are fine from a standpoint of physical correctness, I just thought explitcitly pointing out that what you are doing is backwards made sense.
If you fill the sink with colder water there will not be much steam, but if you just pour a layer of colder water it will not have much effect. The reason is that the mass of boiling water is much larger than the layer, so the temperature reduction will be much smaller than for mixing it with an about equal volume of colder water.
Typically boiling water will stop boiling nearly instantly when you take it off the stove: it is kept boiling by adding energy, and quickly goes below 100C due to energy loss due to steam. The remaining steaming is because it is still warm enough to have an appreciable vapour pressure, but going down to 80C about halves it. So even a fairly small temperature reduction can reduce the steaminess noticeably.
Steam is water in the gaseous phase and is invisible. It occurs when water boils which, at one atmosphere, is a temperature of 100 C.
What you are seeing coming up from your sink is not steam, but water vapor that is condensing into liquid water droplets. You can see the same thing above a hot, but not boiling, cup of coffee. It is an accelerated form of evaporation of water at the surface of the liquid due to its high temperature. Evaporation occurs at the surface of the liquid even for liquids at room temperature, though the rate is slow. It is due to the escape of the higher kinetic energy water molecules that exist at the surface. The higher the temperature of the water, the greater the kinetic energy of the molecules at the surface and the greater the rate of evaporation. Then when the water vapor mixes with the cooler air above the surface it condenses into water droplets.
Hope this helps.