Why Can Water Boil Below 100°C?

We often say that water boils at 100°C, but that is only true at roughly one standard atmosphere of pressure. The real condition for boiling is that the liquid's vapour pressure reaches the surrounding pressure, allowing vapour bubbles to form and grow throughout the liquid.

As temperature rises, water's vapour pressure rises. But if the external pressure falls, water can reach the boiling condition without being heated to 100°C. The US Geological Survey gives a useful comparison: water boils at about 100°C near sea level, but at roughly 94.9°C at 5,000 feet because atmospheric pressure is lower. In a laboratory, reducing the pressure above water can make it boil at still lower temperatures.

This clears up a common confusion: boiling does not simply mean 'very hot'. It describes the conditions under which vapour forms throughout a liquid. Temperature strongly affects vapour pressure, but external pressure matters too. Water may boil sooner on a mountain, yet cook food more slowly because the boiling water is cooler. A pressure cooker uses the opposite principle, raising pressure so that water boils at a higher temperature.

Sources:
https://www.usgs.gov/index.php/special-topic/water-science-school/science/facts-about-water
https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/11%3A_Fluids/11.04%3A_Vapor_Pressure