boiling a liquid is by definition transforming it into a gas. it takes an energy to heat up to the boiling temperature, and far more energy to transform the liquid into a gas. this is called the latent heat of a substance and is material dependent. so when water hits boiling temperature, gradually molecules of the water begin to transform into gas, and exit in the form of steam. whereas the water at the bottom will be heating up and rising to the top in the form of bubbles, ready to detach from the liquid by becoming a gas.
