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In my chemistry class, we had a problem akin to the following:

If an object with mass bla and specific heat bla is placed into a bla g calorimeter with an initial temperature of bla and a specific heat of bla, and the temperature of the calorimeter rises to bla at the end of the experiment, how much heat was lost by the object?

I performed the calculations and got an answer of, let's say, $8 J$. When I submitted this answer, I was told that it was wrong, and the correct answer would have been $-8 J$. Wouldn't this response to the above question mean that the object gained heat?

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  • $\begingroup$ Sign conventions have always been a bit of an issue in chemistry, especially in introductory courses. I agree with your interpretation in this case, but what might have been meant by the question was to state the heat change of the object, in which case -8J follows standard convention. $\endgroup$ – chipbuster Nov 3 '13 at 8:52
  • $\begingroup$ It is better to say it as a convention, than saying it as a customary. It is convention that, one hand we consider as right, and the other as left. One may assume the vice versa case. It would not be wrong. $\endgroup$ – Immortal Player Nov 3 '13 at 21:30
  • $\begingroup$ I guess what I'm asking is whether I'm right or wrong. $\endgroup$ – Undo Nov 3 '13 at 21:31
  • $\begingroup$ Note that physicists and chemists use different sign conventions for work, and I've seen some variation in the convention for heat, too. $\endgroup$ – ManishEarth Nov 5 '13 at 10:58
  • $\begingroup$ In cases such as these, I think it can be useful to explicitly write out that "$8 J$ of heat is released", or "considering the convention that negative energies correspond to heat release, the energy involved in the process is $-8 J$" to cut through any ambiguity. $\endgroup$ – Nicolau Saker Neto Nov 5 '13 at 11:47
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Your assumptions are correct, and from the wording of the question, the "correct" answer doesn't make sense, unless like you said, the object gained heat, which is possible; people have negative profits all the time. Sign conventions are not flawed in chemistry, but you just have to be careful when wording a question. To answer the title of your question, yes it is not only customary, it is necessary to express heat loss as a negative number, but the question still has to be phrased correctly--using the term "lost" serves as a negation in this use of language, the same way the unary operator in -1 serves as negation for a number. If heat loss were not expressed negatively, it wouldn't make sense because the system of an exothermic reaction must lose energy for its surroundings to gain it.

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