Final answer:
The expected mid-point temperature when mixing equal volumes of water at different temperatures is not achieved due to heat loss to the environment, a concept accounted for in calorimetry.
Step-by-step explanation:
When 50 mL of water at 100°C is combined with 50 mL of water at 40°C, it might seem that the resulting temperature should be the midpoint, 70°C. However, the actual thermometer reading of 65°C indicates that heat loss to the surroundings has occurred during the mixing process. In a real-world scenario, some heat is inevitably transferred to the container holding the water and to the air, which is why the final temperature is lower than the theoretical average.
This difference in temperature can also be influenced by the specific heat capacity of the substances involved, but in this case, since both substances are water, it is primarily due to environmental heat loss. In calorimetry, the measurement of heat transfer, this is a familiar concept. When an actual physical system is considered, it is critical to remember that no process is perfectly insulated and some heat will always be lost to the surroundings.