What are the main reasons for receiving a negative temperature from a calorimeter? I recently did an experiment with HCl and Sodium Bicarbonate but the temperature went down, which is not expected as the reaction should be exothermic. This result was not always occuring, but occasionally a negative temperature was obtained. I now need to explain this in my report but I am unsure the true reason. I believe it has something to do with either cleaning properly or the mixing method but I am not entirely sure.