Particle model -- Air passing through a Heat Exchanger

[Jigneshbharati]

Homework Statement

Need to verify the correct answer and required expert advice on topic particle model

Relevant Equations

Have no equations

My son is in year 7. I received a test paper and his answers. Unfortunately, we don't have the correct answer for some questions. I have attached the screenshot. The topic is particle model (year 7).
I would appreciate your help. This is not a homework question but I need your expert advice so I can help him.
Please help with following:
Question 4 a. I think the correct answer should be sublimation from solid to gas for CO2. Please correct me if I am wrong.
Question 4b. I don't know why the correct answer is -210.
I read his KS3 science book but appreciate your help with this so I can help him at home as well.
Thank you in advance.

 

[mjc123]

4a: Correct. You need to make explicit that it does not become a liquid at any point, but converts directly from solid to gas, which is unusual.
4b: It isn't. The answer has been corrected from the melting point of oxygen (minor constituent of air) to that of nitrogen (major constituent of air). But you want the boiling point, not the melting point.

 

[Jigneshbharati]

Thanks. The teacher has given the correct answer in/with purple pen. I am so sorry but I don't get why the correct answer is -210 C. I tried but I am still confused.

 

[Jigneshbharati]

Is teacher wrong for both the questions?

 

[berkeman]

I received a test paper and his answers.

This is not a homework question

???

 

[Jigneshbharati]

I only put that because people assume that it is not a genuine doubt. Thank you.

 

[Jigneshbharati]

We got this paper after a month. He is in year 7. The teacher has corrected few of his answers with purple pen and the wrong ones are marked with a red cross. I hope it makes sense.

 

[Jigneshbharati]

I am not sure why the correct answer is -210 C as per teacher's correction? Thank you in advance.

 

[mjc123]

It is not. The teacher is wrong. The correct answer (from the information given) is the boiling point of nitrogen.

 

[Jigneshbharati]

Thanks. In KS3 book, they have given cooling and heating curve. How should I explain the correct answer so he understands the why? Thank you.

 

[mjc123]

First ask him why he gave the answer he did. This may reveal what he is misunderstanding (or if it was just a calculation mistake).

 

[gyorgiy]

Q4a The information given in the text is incorrect: the boiling point is correct (though I'm not certain I would use that term), but CO2 the triple point (the lowest melting point of CO2) is about -57 degC. So I doubt that the school expects the fully correct answer - below:

The CO2 will already be evaporating (sublimating) slowly at -100-degC. The rate will increase appreciably as you warm it.
If you simply heat it to -78-degC and stop adding heat it will cool as the surface material evaporates. It you continue to heat it the surface will continue to evaporate; the rate of evaporation will depending on the rate of heat addition. Boil rate is actually quite slow if you maintain the surface at boiling temperature, as "boiling point" is not for a dynamic condition - it represents the temperature at which there is equilibrium between solid CO2 and pure CO2 gas at atmospheric temperature

4b I suspect your son was confused by the carry-over of the table between the two pages. The boiling point (liquid/vapour) transition is in the right hand column. As both gases are liquid the best first estimate is the colder of the two boiling/liquefaction temperatures, which is -196-degC.
(In practice it will almost certainly be slightly cooler, as this is a dynamic situation. Note that it could theoretically be very slightly warmer if you slowed it far enough - because nitrogen is somewhaty soluble in liquid oxygen)

 

[Jigneshbharati]

Thank you so much.

 

[Jigneshbharati]

Q4a The information given in the text is incorrect: the boiling point is correct (though I'm not certain I would use that term), but CO2 the triple point (the lowest melting point of CO2) is about -57 degC. So I doubt that the school expects the fully correct answer - below:

The CO2 will already be evaporating (sublimating) slowly at -100-degC. The rate will increase appreciably as you warm it.
If you simply heat it to -78-degC and stop adding heat it will cool as the surface material evaporates. It you continue to heat it the surface will continue to evaporate; the rate of evaporation will depending on the rate of heat addition. Boil rate is actually quite slow if you maintain the surface at boiling temperature, as "boiling point" is not for a dynamic condition - it represents the temperature at which there is equilibrium between solid CO2 and pure CO2 gas at atmospheric temperature

4b I suspect your son was confused by the carry-over of the table between the two pages. The boiling point (liquid/vapour) transition is in the right hand column. As both gases are liquid the best first estimate is the colder of the two boiling/liquefaction temperatures, which is -196-degC.
(In practice it will almost certainly be slightly cooler, as this is a dynamic situation. Note that it could theoretically be very slightly warmer if you slowed it far enough - because nitrogen is somewhaty soluble in liquid oxygen)

Hi,
I you don't mind, would you please explain this in detail: As both gases are liquid the best first estimate is the colder of the two boiling/liquefaction temperatures, which is -196-degC.

 

[Chestermiller]

The correct answer is the boiling point (bubble point) of a liquid mixture of 20% oxygen and 80% nitrogen. Assuming ideal solution behavior, the bubble point can't be much higher than the bubble point of nitrogen.

 

[Jigneshbharati]

Thanks. The teacher said -210. I am confused which one is correct.
-196 C
Or
-210 C
Or
I appreciate your help.

 

[mjc123]

You have been told multiple times now that the correct answer is the boiling point of nitrogen, i.e. -196°C.
Do you understand why this is so?
"But the teacher said -210."
The teacher got it wrong. Teachers are not infallible.

 

[Jigneshbharati]

Thank you for your help.

 

[gyorgiy]

Hi,
I you don't mind, would you please explain this in detail: As both gases are liquid the best first estimate is the colder of the two boiling/liquefaction temperatures, which is -196-degC.

Hi You are correct - the answer should be the boiling point of nitrogen. I had not appreciated that the -210degC was supposed to be the correct answer.
Now I know this, my interpretation (for what it is worth) is that it wasn't just your son who misread the table then it was split between adjacent pages

 

[Steve4Physics]

Hi @Jigneshbharati. If you are still reading, and if your son is still struggling, a diagram should help him:

Having taught for KS3 (a very long time ago) I would expect the official mark-scheme to allow an answer anywhere in the range -210 $^\circ$C to -196 $^\circ$C inclusive. If I were answering this (as part of a KS3 SAT test) my answer would be -200 $^\circ$C.

If anyone doesn't like this I would note that the question is an exercise for children in understanding/applying scientific data and using reasoning skills - not a physics test!

BTW, Year 7 (age 12-ish) is pretty young for attempting a KS3 exam'. They are normally taken in Year 9 (age 14-ish)..

 

[Jigneshbharati]

Thank you so much for your answer and expert advice. I really appreciate it.