Dew around the cup?

Question

i was putting the ice in a cup filled with water and a seond later dew started to form.
Where does the water come from that appears on the cup? and what caused it?

Answer 1

The air around you always has water vapor in it. The amount of water vapor air can hold depends upon the temperature. When it is hot, the air can hold a lot of water. This is why you use hot air when you dry your hair. When the air is cold, it can't hold much water vapor.

When you put the ice in the glass, it chills the glass. The air in contact with the glass rapidly cools down. When it cools enough, the air ends up having more water vapor than it can keep as a gas. This water vapor condenses on the cold glass and leaves those drops of water.

The temperature at which water begins to condense out is called the dew point. It depends on the amount of water vapor in the air.

Answer 2

condensation! clouds and raindrops (not to mention snow, sleet and hail) form this way.