Dissolved Gases

• Tap water doesn't consist of just molecules of H₂O. Water also contains dissolved gases. This is because such gases are soluble in water. They readily dissolve in water at lower temperatures. When water is heated, the solubility of these dissolved gases decreases, eventually reaching the point where they are driven off. Since the gases are much less dense than the liquid, the bubbles rise and escape into the atmosphere. This occurs at below boiling temperatures.
  
  

Water Vapor

• As the water temperature approaches boiling, the cohesive forces binding the water molecules together as a liquid, are exceeded by the heating process. The molecules break free of the liquid as vapor. This occurs especially at imperfections of the vessel surfaces called nucleation sites. If there are insufficient nucleation sites (this may occur on new and especially smooth surfaces, such as glass), the water may remain liquid well past the boiling point. If that occurs, the water is said to be superheated, and may burst out of the vessel suddenly (a safety hazard). This occasionally occurs in microwave ovens.
  
  

Partial Boiling Noises

• Boiling begins nearest the heat source (usually the bottom of the vessel). As an increasing number of water molecules is converted to vapor they build bubbles. As the bubbles grow, they may reach a layer of liquid that has not yet reached the boiling point. If so, the bubbles will collapse as the vapor condenses, resembling the effect of cavitation. This produces a slight pinging sound. If the bubbles, upon reaching sufficient size, do not hit a cooler layer of water first, they will break free of their nucleation site, and rise, escaping into the atmosphere. Before the boil is rolling, rising columns of such bubbles may be apparent.
  
  

Heating Past Boiling

• If high heat is applied forming a layer of vapor beneath the liquid (such as occurs when water is dropped onto a very hot skillet), the vapor acts as an insulator between the heat source and the liquid above the vapor. This occurrence is called the Leidenfrost Effect.
  
  

Leidenfrost Effect Demonstration

• The Leidenfrost Effect has been demonstrated quite dramatically by a trained person very quickly and briefly dipping the tips of his moistened fingers into molten lead. The outermost layer of the moisture comes into contact with the lead and quickly vaporizes, while the layer of moisture closer to the skin is insulated by that vapor from being heated sufficiently to burn the fingers.