# What Is the Difference Between Triple Point and Melting Point?

• Paul Snasel
In summary, the conversation discusses the conditions under which three phases of matter can coexist, specifically at the triple point where temperature and pressure are equal to the triple point values. The conversation also mentions the different situations that can occur when there is a mixture of gas and vapor, depending on the temperature of the mixture. The question then arises whether the rounded value of "0°C" mentioned in literature and diagrams refers to the temperature of the triple point or the melting point. The conversation also mentions that the melting point is pressure-dependent and that the triple point is where all three phases are in equilibrium. It is also noted that NTP is not exactly the same as the triple point.
Paul Snasel
Hello,

Im new here and I hope someone of you can answer this probably trivial question. I tried to find the answer in many of phyisc/termodynamic texbooks but in vain.

http://imageshack.us/photo/my-images/62/123002h.jpg/
http://imageshack.us/photo/my-images/62/123002h.jpg/

According to the phase diagram 3 phases (fig. II) of matter can coexist only by conditions of triple point, so when:
- temperature = temperature of triple point
- preassure = preassure of triple point at the same time.
Now let's have mixture of gas and vapour, for example mixture of dry air and watter. In literature or in Moillier diagram (fig. I) are determined 4 possible situations:
- the air is not saturated (area A, fig. I),
- the air is saturated (RH = 100%, fig. I),
- the air is over-saturated: watter coexists in phases liquid + gas (area B, fig. I),
- the air is over-saturated: watter coexists in phases solid + gas (area C, fig. I),
- the air is over-saturated: watter coexists in phases solid + liquid + gas (area D, fig. I).
It depends on relation of temperature of mixture to temperature "0°C", which of three possible situations in area of over-saturated air comes. There is alwas mentioned probably rounded value of temperature "0°C" in literature or in Mollier diagram, but without further explanations. So I am asking, is it temperature of triple point Ttp or temperature of melting T12(p)? Or is the problem more complicated? I know, that difference between value of Ttp and T12(p) of watter is negligible, but there are matters where the difference is significant.

Ill appreciate any advice or suggestion. I am sorry for my bad english.

Best regards
Paul

The melting point is pressure-dependent. If you want to have all 3 phases at the same time (in equilibrium), you are at the triple point, which is equal to the melting point at the pressure of the triple point. In addition, it is the boiling point at this pressure and the point where sublimation is in equilibrium.

I was taught that NTP is 0C and 760mm Hg, which is not quite the same as the triple point.
Mind you, that was a long time ago.

## 1. What is the triple point?

The triple point is the temperature and pressure at which a substance can exist in all three phases (solid, liquid, and gas) simultaneously.

## 2. What is the melting point?

The melting point is the temperature at which a solid substance changes into a liquid state.

## 3. How do the triple point and melting point differ?

The triple point is a specific combination of temperature and pressure at which a substance can exist in all three phases, while the melting point is the temperature at which a solid substance turns into a liquid state.

## 4. Can a substance have a triple point and a melting point at the same time?

Yes, a substance can have both a triple point and a melting point. The triple point represents the conditions at which the substance can exist in all three phases, while the melting point represents the temperature at which the solid phase turns into the liquid phase.

## 5. How are the triple point and melting point used in scientific research?

The triple point and melting point are important properties that are used to characterize and identify substances. They are also used in various experiments and studies to understand the behavior and properties of different materials.

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