The statement 'Heat lost by the hot body equals that gained by the cold one' is assumed when determining…

Analysis

The question asks us to identify which of the listed actions validates the principle of calorimetry (the method of mixtures), which states that the heat lost by a hot body equals the heat gained by a cold body. This principle is based on the Law of Conservation of Energy.

Concept Explanation

In the method of mixtures, we assume that the system is isolated—meaning no heat is exchanged with the surroundings. If heat escapes to the environment (or is absorbed from it), the calculation will be inaccurate because the heat lost will not equal the heat gained.

• Lagging the calorimeter: This involves wrapping the calorimeter in insulating material (like cotton wool or felt). This prevents heat loss to the surroundings via conduction or convection. This is essential to validate the assumption that all heat lost by the hot body is gained by the cold body and the calorimeter.
• Ensuring only S.I. units are used: While using standard units is important for calculation accuracy, it is a matter of mathematical convention and does not physically prevent heat loss or validate the thermodynamic assumption of an isolated system.
• Weighing the calorimeter, lid, and stirrer: This is necessary to determine the mass of the components so that their individual heat capacities can be calculated. While this is a crucial step in the procedure of the experiment, it does not validate the assumption that heat lost equals heat gained; rather, it allows us to account for the heat absorbed by the apparatus itself.

Step-by-Step Solution

1. Evaluate I: Lagging the calorimeter directly addresses the "isolated system" requirement. By minimizing heat exchange with the environment, we ensure the energy balance remains valid. Therefore, I is correct.
2. Evaluate II: Using S.I. units is a standard practice for consistency, but it has no physical effect on the heat transfer process. Therefore, II is incorrect.
3. Evaluate III: Weighing the components is a measurement step to ensure we have the correct data for our calculation. It does not prevent heat loss or validate the principle of energy conservation. Therefore, III is incorrect.

Since only statement I validates the assumption that heat lost equals heat gained by preventing external interference, the correct choice is I only.

Exam Tip

When dealing with calorimetry questions, always remember the "Law of Conservation of Energy." Any step that minimizes heat loss (like using a lid or lagging) is about validating the assumption, while any step involving measurement (like weighing) is about data collection.