# Thermodynamics phase changes steam and water

## Homework Statement

In each case, the following are mixed in a perfect calorimeter with no heat lost or gained from the system. Find the final temp in each case.

12.5 g of steam at 100C with 344 g of water at 22.2C.

## Homework Equations

Q=Mc(Tfinal-Tinitial)
M is the mass in kg
C is the specific heat given by whatever substance we are dealing with
And the parantheses is the change in temperature

## The Attempt at a Solution

So I didn't really know where to start with this problem I realize that one of the substances is going to undergo a phase change but am unsure how to find which one does it. If I could help with that I believe I could calculate the final temperature.

PeterO
Homework Helper

## Homework Statement

In each case, the following are mixed in a perfect calorimeter with no heat lost or gained from the system. Find the final temp in each case.

12.5 g of steam at 100C with 344 g of water at 22.2C.

## Homework Equations

Q=Mc(Tfinal-Tinitial)
M is the mass in kg
C is the specific heat given by whatever substance we are dealing with
And the parantheses is the change in temperature

## The Attempt at a Solution

So I didn't really know where to start with this problem I realize that one of the substances is going to undergo a phase change but am unsure how to find which one does it. If I could help with that I believe I could calculate the final temperature.

Steam exists at 100o or higher.
Water exists at 100o or lower. [but not below 0o]

Given that one quantity starts at 100o and the other starts at 22.2o, it is reasonable to assume that the final temperature will be somewhere in between those two - so which one do you now think will change phase.

The steam should become water since water boils at 100C. So it should just be common sense? Or is there a calculation I should be making?

So since the steam is undergoing a phase change to become water. I'm not sure what to multiply the mass for since it isn't fusion or vaporization.

PeterO
Homework Helper
So since the steam is undergoing a phase change to become water. I'm not sure what to multiply the mass for since it isn't fusion or vaporization.

Have you heard of melting and condensation?

Yes my physics book doesn't give those values though so I hadn't even thought I could use those.

PeterO
Homework Helper
Yes my physics book doesn't give those values though so I hadn't even thought I could use those.

Your book does give that - you just didn't recognise it.

The energy input need to vaporise some water equals the energy returned when that (now) vapour condenses.

Same thing with fusion and melting.

So it's just the sign that changes
. Thanks very much