Why dust generally settles down in closed room?

The door of an operating refrigerator kept open in a closed room. Will it make the room cool?

An electric fan is switched on in a closed room. Will the air of the room be cooled?

The sprinking of water reduces slightly the temperature of a closed room because,

Atomic sizes generally increase as we come down a group of the periodic table. But in the 4th group of the periodic table, Zr and Hf have almost the same atomic sizes. Why?

Gelatin is generally added to ice creams. Why?

Why is the length of a food chain in an ecosystem generally limited to 3-4 trophic levels ?