# The effect of air pressure on the melting point

What is the effect of air pressure on the melting point of a substance?

a) No effect
b) The melting point increases when the air pressure increases
c) The melting point decreases when the air pressure increases

The boiling point has inverse relation with vapor pressure of the liquid and positive relation with atmosphere (air) pressure. The melting point has positive relation with vapor pressure of the substance, but what about its relation with air pressure?

• Your questions are better, but please use the 'homework' tag
– user15489
Jun 13, 2015 at 11:36
• @anshabhi homework questions are allowed, just as long as there is an effort to show working made, see homework policy
– user15489
Jun 13, 2015 at 11:39
• @anshabhi, all of this 3 questions were from the test I had today. I just want to see if I wrote the right answer and also I really need to solve my problems.I Could not find the sure answers.now no matter my answers on the exams were right or wrong,I can not change them,but I need to learn Jun 13, 2015 at 12:14
• alright.... sorry..!! Jun 13, 2015 at 12:15

## 4 Answers

For most substances, higher pressure (or air pressure, in your case) will cause the melting temperature to go up. To think about it intuitively, imagine that you have a certain solid. Melting it would increase the volume of that substance because liquids take more space than solids. If you increase the pressure, it becomes harder for that transformation to occur. It's harder to change into a state that needs more volume if you are under pressure! So, you need more energy, hence temperature, to melt.

Notice that this relation is actually reverse in water. Because ice takes more volume than liquid water, it's actually easier to melt it at high pressure and it requires less heat. Try taking an ice cube and apply pressure on it with a knife, see how it liquifies at the contact point.

• Usually knifes are made of a metal, which is quite efficient at transferring ambient heat to the ice. So it is not obvious, that it is the pressure which cause the effect. To reduce the possibility of heat transfer contributing to melting in that experiment, the knife could be put in the freezer for a few hours before conducting the experiment. Jun 13, 2015 at 21:37
• @kasperd or use a ceramic knife instead Jun 13, 2015 at 22:55

The Melting Point does not go up for water until you apply over 1500psia, in which case it goes down, and then back up, and increases steadily. So for pressures under 1500psia, melting point of water is 0°C. Also water liquid is generally considered essentially incompressible, as with solid form, so I'm not sure why volumetric differences come into play... (Pressure has little affect on these, however vapor is a different story)

Also if the knife thing were true, you would see water phase change back into ice when you removed the force applied. The ice would also uniformly melt, so try doing it at a temperature of 0°C (with a knife at 0°C as well) and see what happens.

Just look up "pressure-temperature phase diagrams" if you don't believe me.

• That's incorrect. The melting point of ice decreases immediately as you increase pressure, and you can measure this change for pressure increases far less than 1500 psia. E.g, at 750 psia, the melting point is -0.4 C. In addition, the pressure-sensitivity of the melting point is due to the volume difference between ice and water; it does not require that the phases themselves be compressible. The relative compresibilities of the phases affect how the volume difference changes with pressure. Mar 8, 2020 at 23:25

For any substance to melt, it has to overcome or reduces the interaction forces that keeps the particles together in solid state. As the pressure of substance increases, particles tends to remains compacted, increasing of pressure during melting hindering in melting process, makes it difficult to overcome the strong force of attraction, i.e. more thermal energy is required. That's why the melting point increases as the pressure increase.

• That's incorrect. The melting of water requires that the "interaction forces that keeps the particles together in solid state" be overcome, yet increasing the pressure favors melting (i.e., it reduces the melting temperature). What matters is that the molar volume of ice is greater than the molar volume of water. Mar 8, 2020 at 23:19
• @theorist, how does increasing pressure reduce the melting point? It should increase it, shouldn't it? Jun 4, 2023 at 5:27
• @NatashaJ This phenomenon follows Le Chatelier's principle, which says that the equiibrium will shift to relieve an external stress. If the external stress is pressure, the equilibrium shifts to favor the smaller phase. Most substances expand when they melt, so higher pressure favors the solid phase, making it harder to melt, and thus increasing the melting point. But ice contracts when it melts. Thus increasing the pressure favors the liquid phase, which means it makes ice easier to melt => the melting point decreases. Jun 5, 2023 at 6:22
• If you want an intutitive way to think about this: As water freezes, it has to push out against the atmosphere. The higher the pressure, the more work water has to do when it freezes, making it more favorable to stay in the liquid state...which means the melting point decreases. I'll add that, formally, Le Chatelier's principle follows from the 2nd law of thermodynamics, and can be proven mathematically from the 2nd law. Some physical chemistry textbooks show this. Jun 5, 2023 at 6:36
• Okay, got it!! Thanks for explaining it so well. Jun 6, 2023 at 5:21

According to the phase diagrams of water, the freezing point and melting point is inversely proportional to increase in pressure . About the volume concept , that's actually the concept of Le Chatelier's principle that if you increase pressure on a system, the system will shift in the direction of decreasing volume. When you compare it with the concept of "Ice having greater volume then liquid water', it connects. And I think we can apply the pressure on solid and liquid here because it is the change in pressure around the liquid or solid we are talking about , not the effect of pressure inside the liquid or solid.