# Heat: equal masses of h20

Dx
Equal masses of water 20C and 80C are mixed. what is the inal temp of the mixture?

I said 60C
why is this incorrect?
Dx

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Tom Mattson
Staff Emeritus
Gold Member
It's not correct because 60o is the final (not initial) temperature.

Dx
I see
dx Last edited by a moderator:
Dx
Originally posted by Dx
Thanks Tom!
dx HallsofIvy
Homework Helper
Once again, DX, be careful of Tom's answers.
The problem with asking someone to give you the answer is that they may just give you a WRONG answer!

It's not correct because 60 degrees is the final (not initial) temperature.
is not correct. You don't know what the "final" temperature will be because you don't know the temperature of the environment. In order to do this problem, you will have to interpret "initial" temperature as the temperature immediately AFTER mixing.

What makes you think the temperature will be 60 degrees? The only way I see that you can get 60 is to subtract 20 from 80. Do you have any reason for that? If the two temperatures had been 60 and 50 would you say that the mixture will be 10 degrees? Does that even make sense?

It should make sense to you that if you mix two things the final temperature will be BETWEEN the two original ones. In fact you should think about finding the average of the two temperatures.
What is the average of 20 and 80 degrees?

You should also think about how you would "average" the temperatures there were more water at one temperature than the other.

Suppose you had 10 grams of water at 20 degrees and 40 grams of water at 80 degrees. ABOUT what do you think the temperature of the mixture would be? How would you calculate it exactly?

Dx
Originally posted by HallsofIvy
Once again, DX, be careful of Tom's answers.
The problem with asking someone to give you the answer is that they may just give you a WRONG answer!

is not correct. You don't know what the "final" temperature will be because you don't know the temperature of the environment. In order to do this problem, you will have to interpret "initial" temperature as the temperature immediately AFTER mixing.

What makes you think the temperature will be 60 degrees? The only way I see that you can get 60 is to subtract 20 from 80. Do you have any reason for that? If the two temperatures had been 60 and 50 would you say that the mixture will be 10 degrees? Does that even make sense?

It should make sense to you that if you mix two things the final temperature will be BETWEEN the two original ones. In fact you should think about finding the average of the two temperatures.
What is the average of 20 and 80 degrees?

You should also think about how you would "average" the temperatures there were more water at one temperature than the other.

Suppose you had 10 grams of water at 20 degrees and 40 grams of water at 80 degrees. ABOUT what do you think the temperature of the mixture would be? How would you calculate it exactly?
Let me see. 20 + 80 = 100 / 2 = 50 average. I think i understand better now Ivy. Thanks! Can I add you as a friend, plz? Let me try to work the problem further and if I jave anumore problems ill ask.
Dx Tom Mattson
Staff Emeritus
Gold Member
Originally posted by HallsofIvy
You should also think about how you would "average" the temperatures there were more water at one temperature than the other.
Actually, I assumed he did average them and that's how he got 60o. I didn't even notice that 60 isn't the average!

Dx
we forgive you this time Tom.
Just teasing, i am glad to see that everyone has each others back. Thanks HallsOfIvy,
I appreciate your help and toms too.
Dx 