Q. When heating increases the temperature of water, why do bubbles appear?

A. When we pour water in a pan and heat it, after a while a vapor rises from the surface of water. This is because vapor (gas) produced from water (liquid) evaporating from the surface is cooled down by air and condenses as small water drops. You cannot see the vapor itself with your eyes. If the temperature rises more, bubbles form on the bottom and sides of the pan. This is because the air molecules in very small holes of the pan that we cannot touch or see and vapor that has evaporated into such holes increase their volume at higher temperatures come up to the surfaces forming bubbles. If the temperature increases to 100 degrees Centigrade, water not only on the surface but also in the pan becomes vapor rapidly. The temperature for boiling is lower in higher elevations. For example, water boils at around 87 degrees Centigrade on the top of Mt. Fuji.