The boiling point of a substance is the temperature at which its liquid phase transitions into its gas phase, occurring when the vapor pressure of the liquid equals the external pressure exerted on the liquid.

At this temperature, the liquid turns into vapor (boils) throughout the entire body of the liquid, not just at the surface. The boiling point can vary depending on the surrounding pressure. For example, at higher altitudes, where the air pressure is lower, the boiling point of water is lower than it is at sea level.

Examples:

1. Water:
   • Boiling Point at sea level (1 atm pressure): 100°C (212°F)
   • If you’re in a high-altitude location like the mountains, water will boil at a lower temperature due to reduced atmospheric pressure.
2. Ethanol (Alcohol):
   • Boiling Point: 78.37°C (173.07°F)
     Ethanol boils at a lower temperature compared to water, which is why it evaporates faster at room temperature.
3. Mercury:
   • Boiling Point: 356.73°C (674.11°F)
     Mercury has a much higher boiling point than water due to its heavy and strong metallic bonds.

In summary, the boiling point is a key physical property of liquids that helps in understanding their behavior at different temperatures and pressures.