Question

Why is the cold air and hot steam only visible? eg:from a freezer, from a hot cup respectively.

Answer 1

When you boil water in a pot, liquid water turns to gaseous water (steam), rises as bubbles through the liquid, and breaks through the surface. Steam is transparent. The little puffs of smoke like stuff you see are water turning back into a liquid (condensing) when the steam hits the cooler air above the pot. The liquid is in the form of tiny droplets suspended in the air which are visible as a mist.

Similarly, when you open a freezer, water vapor (which is also gaseous water) in the air is cooled by mixing into the cold inside. It condenses, again forming tiny drops which are visible as a mist.

It’s the same process that produces fog.