Question

as altitude increases, the air pressure decreases. how do you think the lower pressure will affect the following? (circle your answers.)
increases\tstays the same\tdecreases
freezing poin\t○\t○\t○
melting point\t○\t○\t○
boiling point\t○\t○\t○

Explanation:

• Freezing Point: For most substances (like water, though water has an anomaly), pressure has a small effect on freezing point. For water, increasing pressure raises freezing point, but in general, for many substances, the change is minimal or follows specific phase - rule trends. However, in the context of typical atmospheric pressure - altitude relationships and common substances, the freezing point is considered to stay the same (or change very little) with a decrease in air pressure at normal altitudinal changes.
• Melting Point: Similar to freezing point (since freezing and melting are reverse processes), the effect of pressure on melting point is complex. For most substances, the change in air pressure with altitude (a relatively small pressure change in the context of daily altitude changes) has a negligible effect on the melting point, so it stays the same.
• Boiling Point: Boiling occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. As altitude increases (and air pressure decreases), the surrounding atmospheric pressure is lower. So, the liquid needs to reach a lower temperature for its vapor pressure to equal the now - lower atmospheric pressure. Thus, the boiling point decreases.

Answer:

• Freezing Point: $\boldsymbol{\text{Stays the Same}}$ (circle the "Stays the Same" option for Freezing Point)
• Melting Point: $\boldsymbol{\text{Stays the Same}}$ (circle the "Stays the Same" option for Melting Point)
• Boiling Point: $\boldsymbol{\text{Decreases}}$ (circle the "Decreases" option for Boiling Point)