Final answer:
Condensation nuclei are formed by particles such as ions, dust, or water vapor, which then lead to the formation of rain droplets when these particles attract water molecules due to their polar nature. Condensation occurs when gaseous water cools down enough to allow molecules to aggregate around these nuclei.
Step-by-step explanation:
Particles around which condensation nuclei are formed include molecules such as water vapor, ions, or even dust grains in the air. When the air contains these particles, and conditions are right, they can serve as nucleation centers for the formation of rain droplets. Due to the polar character of water molecules, these molecules are attracted to ions or other particles in the air, which then encourages the aggregation of water molecules around these particles.
When gaseous water becomes cool enough, the attractions between H₂O molecules allow them to stick together upon contact, a process known as condensation. For instance, raindrops form in clouds where floating dust particles or other aerosols serve as nucleation sites. Similarly, water can condense on the outside of a cold glass, demonstrating this concept on a visible scale.
Furthermore, the phenomenon known as Brownian motion, observed by Robert Brown, helps to reveal the nature of these particles. Brownian motion is the random motion of particles suspended in a fluid resulting from them being hit by the molecules of the fluid. This effect provides indirect evidence of atoms and can be used to estimate the size of particles and molecules as well.