Question

your assignment
air pressure decreases as elevation increases. identify how the boiling point of water on top of a mountain is different from its boiling point at sea level. draw two models to show how the particle motion and the state of water on the mountain top and at sea level would change if you kept adding thermal energy to water that was already at 100°c. label your models to show what happens to the temperature as the energy is added. use the \insert image\ button to upload your models.

Explanation:

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the external pressure. At sea level, the atmospheric pressure is relatively high (standard atmospheric pressure is 1 atm), and the boiling point of water is \(100^{\circ}C\). On top of a mountain, the air pressure is lower. According to the relationship between vapor pressure and boiling point (lower external pressure means lower temperature is required for the vapor pressure of the liquid to equal the external pressure), the boiling point of water on a mountain top is less than \(100^{\circ}C\).

When adding thermal energy to water at its boiling point (whether at sea - level or on a mountain top):

• The temperature of the water does not increase. The added thermal energy is used for the phase change (from liquid to gas).
• In terms of particle motion, at the boiling point, water particles (molecules) gain enough energy to overcome the intermolecular forces. At sea - level (higher pressure), the rate of evaporation (conversion from liquid to gas) is such that it balances the condensation (gas to liquid) at \(100^{\circ}C\). On the mountain (lower pressure), at its lower boiling point temperature, water molecules also have enough energy to escape into the gas phase, and the added thermal energy just continues to fuel this phase change without increasing the temperature.

For the models:

• Model 1 (Sea - level, \(T = 100^{\circ}C\)):
• Draw water molecules in the liquid state (closer together, some random motion). As thermal energy is added, show some molecules escaping into the gas phase (more spread out). Label the temperature as remaining \(100^{\circ}C\).
• Model 2 (Mountain top, \(T=t_{b}<100^{\circ}C\)):
• Draw water molecules in the liquid state (closer together, random motion). As thermal energy is added, show molecules escaping into the gas phase. Label the temperature as remaining at \(t_{b}\) (the boiling point at that altitude).

Answer:

The boiling point of water on a mountain top is lower than \(100^{\circ}C\) (its boiling point at sea - level). When adding thermal energy to water at its boiling point (either at sea - level or on a mountain), the temperature of the water does not increase, and the added energy is used for the liquid - to - gas phase change. For the models, one would show the phase change at \(100^{\circ}C\) (sea - level) with no temperature increase upon adding thermal energy, and the other would show the phase change at a temperature \(t_{b}<100^{\circ}C\) (mountain top) with no temperature increase upon adding thermal energy.