Question

How will last molecule evaporate without colliding with other molecule that is how will it get energy to escape?

Answer 1

When water meets dry air, it is not in equilibrium; water molecules evaporate off the surface until the amount of water in the air creates enough vapour pressure to achieve equilibrium. When water is heated to a temperature of 100°C, the water turns into steam, escaping into air.

During evaporation, water is not turning into a gas or steam, but random movement of molecules on the surface of water provides some of the molecules enough energy to escape from the surface into the air. Evaporation takes place only on the surface of the body of water.

The amount of evaporation depends on several factors - like, the temperature of both air and water, the humidity of the air, and the size of the surface exposed. There is always some amount of water vapor that can exist as gas mixed in with the air. This is called the saturation pressure of water at that temperature. The relative humidity is the amount of water vapor, expressed as a percentage of the saturation pressure. As you increase the temperature, the saturation pressure increases.

Water molecules are vibrating, and when heat is applied, they vibrate more energetically. At 100°C at sea-level, the molecules vibrate and jiggle so hard that they start escaping into air as STEAM. Scientifically speaking, water boils when the thermal energy in the water - which is a type of kinetic energy - exceeds the strength of the hydrogen bonds between the molecules, causing them to separate from the other molecules. Under normal conditions, much of the water vapor almost immediately turns to steam, which is actually composed of droplets of water because it is away from the source of heat. The steam cools when it comes into contact with the air, increasing the relative humidity above 100%, so the water vapor condenses into mist. Steam is water in the gaseous phase.