# Freezing Water with Nitrogen.

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1. Apr 17, 2015

### Ethan_Tab

1. The problem statement, all variables and given/known data
*All units given are in standard SI units unless otherwise specified.*
*Assume a closed system*

How much of -200ºC Liquid Nitrogen is required to completely freeze 200mL (0.2kg) of pure liquid water at 0.00ºC (No change in heat, only a change in state)?

Given Values.
n= Liquid nitrogen
w= Liquid water
C= Specific heat capacity
Lf= Latent Heat of fusion
Lv= Laten Heat of vaporization

Cw=4.2E3
Cn= 1.1E2
Lvn=2.0E5
Lfw=3.3E5
Melting point of Nitrogen= -209.9ºC
Condensation point of Nitrogen= -196.8ºC

2. Relevant equations
Q=mcΔT
Q=mLv
Q=mLf
Conservation of thermal energy.

3. The attempt at a solution

Here is my thought process, Ideally, a logical and arithmetical check would be much appreciated.

Since we know the water cannot change temperature and instead, only states (L-S), we can state a "restriction" saying that all energy which will convert the unknown mass of nitrogen "mn" will be the absolute value of the energy the water will give off during the mw*Lfw process.

By extension, this also means that the all the stage changes which occur to the Nitrogen to get it to zero need to add up to mw*Lfw

So... (skipping two steps and common factoring the unknown mass N and isolating for it) we get:

Mn=[Mw*Lfw]/[Cn*(3.2)+Lvn+Cn*(196.8)]

Plugging in given values in section 1 I get mass as being 0.29729kg. If anyone could confirm this both numerically and the process itself logically, It would be truly appreciated.

2. Apr 18, 2015

### haruspex

The specific heat for liquid nitrogen is unlikely to be the same as for gaseous nitrogen.
I'm a bit worried by the "closed system" condition. The vaporisation temperature (and, I would have thought, the latent heat) will depend on the ambient pressure, but in a closed system that will increase as the heat transfer proceeds. Maybe it is mainly there to indicate what specific heat to use for the gaseous nitrogen.

3. Apr 18, 2015

### Staff: Mentor

The implication is that the process is carried out at atmospheric pressure. Otherwise, the given condensation temperature could be some other value. It is possible to do this process at 1 atm in a closed system (but not an isolated system) by allowing the volume to increase.

Chet

4. Apr 18, 2015

### Ethan_Tab

The implication of the "closed system", is not literal. It is simply there to show that no heat is lost to the environment. As for the heat capacity of Nitrogen Vapour; would it not remain the same as that of liquid nitrogen? I remember learning that heat capacities remain the same for all substances other then water.

5. Apr 18, 2015

### Staff: Mentor

This is not correct. The term "closed system" has a precise meaning, requiring that no mass enters or leaves the system. So the term closed system is literal. Also, the heat capacity of nitrogen vapor is not the same as liquid nitrogen. What you learned about heat capacities remaining the same for all substances other than water is obviously incorrect. If you are not sure what you are talking about, please don't give misinformation on Physics Forums.

Chet

6. Apr 18, 2015

### Ethan_Tab

Let me clarify what I meant to say: The physics course which I'm currently taking does not deal with pressures and as such the term "closed system", simply implies a system in which no heat is lost to the surroundings. With that being said, If internal pressures were negligible and the capacity of liquid nitrogen (which was specially given in the question) applied to all its states, would my work be considered correct?

7. Apr 18, 2015

### Staff: Mentor

Sure, but it is a known fact that the heat capacity of liquid nitrogen is not the same as the heat capacity of nitrogen gas/vapor. Look it up on Google.

Chet

8. Apr 18, 2015

### haruspex

Ah yes, of course. It has to be interpreted as constant pressure, or there's not enough information.

9. Apr 18, 2015

### Staff: Mentor

In terms of experiments: a closed system is not a practical thing for an actual experiment, but a well isolated box with a small hole would give a very good approximation - nitrogen can leave it at 0°C where it would not participate in further heat exchange anyway.
=> yes, assume a closed system at constant pressure

10. Apr 18, 2015

### Staff: Mentor

Conceptually, one can have an insulated massless frictionless piston, with air outside the insulated cylinder to provide the constant pressure.

Chet

11. Apr 18, 2015

### Staff: Mentor

That's exactly what I meant with "not a practical thing" :D.

12. Apr 18, 2015

### Staff: Mentor

I agree. But, given the level of conceptual idealization inherent in the problem statement (after all, how often do we ever dump water ice into liquid nitrogen in real life?), this additional level of idealization is not much of a stretch, and is typical of the idealizations that are often invoked in thermo homework problems.

Chet

13. Apr 19, 2015

### haruspex

Now I'm having doubts. If you use Cp, doesn't that mean the gas is doing work as it expands? Where is that work going, if it's a closed system?

14. Apr 19, 2015

### Staff: Mentor

In the terminology as it was taught to me (and most others), a closed system is one for which the only constraint is that no mass enters or leaves the system. A closed system is fully capable of exchanging heat and work with its surroundings. We were also taught that an isolated system is one that, in addition to not exchanging mass with its surroundings, also does not exchange heat or work. I realize that, in some books and disciplines, the term closed system is used to describe what I call an isolated system. So it is important to recognize that there is ambiguity in the terminology.