When water droplets are placed on extremely hot, flat surfaces like stoves, they don't instantly evaporate due to several factors:
High heat capacity of water: Water has a relatively high heat capacity, which means it can absorb a significant amount of heat energy before its temperature rises. When a water droplet is placed on a hot surface, the surface rapidly transfers heat to the droplet. However, due to water's high heat capacity, it takes some time for the droplet to absorb enough heat to reach its boiling point.
Latent heat of vaporization: Water undergoes a phase change from a liquid to a gas (vapor) when it reaches its boiling point. This phase change requires the absorption of a considerable amount of heat energy, known as the latent heat of vaporization. The latent heat of vaporization for water is relatively high, so even when the surface is extremely hot, it takes time for the water droplet to absorb enough heat to convert into vapor.
Formation of a vapor barrier: When a water droplet is placed on a hot surface, it can create a thin layer of vapor close to the surface, known as a vapor barrier or Leidenfrost effect. This vapor barrier acts as an insulating layer between the hot surface and the water droplet, slowing down the heat transfer process. The vapor barrier reduces direct contact between the water and the surface, which helps to prevent rapid evaporation.
Surface tension: Water molecules are cohesive, meaning they are attracted to each other, and this results in surface tension. Surface tension causes water droplets to form spherical shapes due to the minimization of surface area. The spherical shape helps to maintain the droplet's integrity by reducing contact with the hot surface and minimizing heat transfer through direct contact.
While these factors contribute to the persistence of water droplets on extremely hot surfaces, it's important to note that they will eventually evaporate as the heat continues to transfer to the droplet, raising its temperature to the boiling point.