# Does atmospheric pressure affect the thermal energy generated by steam?

## Recommended Posts

Hello all:

So I had this question in a physics exam a few days back:

Quote

This question is about heating a liquid.

(a)     Suggest why, in terms of the molecular model, the energy associated with melting is less than that associated with boiling.

(b)    Milk in a cup is heated to boiling point by passing steam through it. Whilst cooling subsequently, some milk evaporates.

(i)      Distinguish between evaporation and boiling.

(ii)    The cup contains 0.30 kg of milk at an initial temperature of 18 °C. Estimate the minimum mass of steam at 100 °C that is required to heat the milk to 80 °C.

Specific latent heat of vaporization of water           = 2.3 × 106 J kg–1
Specific heat capacity of water                    = 4200 J kg–1 K–1
Specific heat capacity of milk                      = 3800 J kg–1 K–1

(iii)    State two reasons, other than evaporation, why the answer to (b)(ii) is likely to be different from the actual mass of condensed steam.

For (b)(iii), I had stated that the atmospheric pressure was a factor as to why the mass of condensed steam would be different than calculated. This is because the boiling point of water would change according to the pressure, which would thereby affect the boiling point, hence impacting how much energy is transferred by steam while its temperature reduces to the boiling point. Besides, if the atmospheric pressure were higher than 1atm, then the boiling point of water would be greater than 100 degrees centigrade, which would mean the steam wouldn't exist at 100 degree celsius! Is this a valid reason?

##### Share on other sites

You are over thinking the question. My answers would be 1) there's heat loss to surroundings 2) condensed water is not saturated and may be lower in temperature.

Pressure was never mentioned in the problem. I think it's possible that they accept "not all heat was used to condense water, some was used to lower temperature of steam or water" as ONE of the points. If you have written the exact answer as you posted here up to "Besides" then you should be able to get a point. Stating simply boiling point changes or atmosphere pressure will not be enough (in fact pressure of gas, not atmosphere pressure, is what changes the boiling point.)

Finally as a side note, it would never be acceptable to say steam does not exist when question states steam was used

##### Share on other sites
4 hours ago, kw0573 said:

If you have written the exact answer as you posted here up to "Besides" then you should be able to get a point.

My exact answer was: The atmospheric pressure would not be the same due to which the boiling point of water would change.

Do I deserve a point?

##### Share on other sites
2 hours ago, Abhirup Ghosh said:

My exact answer was: The atmospheric pressure would not be the same due to which the boiling point of water would change.

Do I deserve a point?

I said that saying atmosphere pressure and boiling point is Iikely not enough.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.