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The statement ‘heat lost by the hot…

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PHYSICS

The statement ‘heat lost by the hot body equals that gained by the cold one’ is assumed when determining specific heat capacity by the method of mixtures. Which of the following validates the assumption?

  • I. Lagging the calorimeter
  • II. Ensuring that only S.I units are used
  • III. Weighing the calorimeter, the lid and the stirrer
  • A. I only
  • B. I & II only
  • C. I & III only
  • D. II & III only

QUICK ANSWER…

A

EXPLANATION…

A calorimeter is an instrument used in calorimetry to determine heat capacities of physical materials or to determine energy changes in physical or chemical reactions.

For example, if we want to find the heat capacity of iron, the experiment is set up such that a ball of iron is heated into a hot ball, and then transferred into a calorimeter filled with cold water whose mass, heat capacity, and temperature are known. After the hot ball is transferred and introduced into the cold water, some quantity of heat energy will then be transferred into the water, making it hot. Now, to properly understand this question, most experiments assume that all the heat lost by the hot ball equals that absorbed by the water. But this is not perfectly true, some of the heat will be lost to the container (also called calorimeter), some to the stirrer, and the lid (or cover).

In order to validate the assumption that no heat is lost to other objects, the calorimeter is lagged to reduce heat loss. the stirrer and lid are made of materials with poor heat conductivity. If the assumption is true, then there will be no need to weigh the calorimeter, stirrer and lid.

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Let’s analyze each statement:

  • When performing calorimetry, heat losses to the calorimeter, lid, stirrer and the surrounding are very much unwanted, this is because what we actually want to measure is the heat interactions between the contents of the calorimeter and not with the calorimeter itself, this is why the calorimeter is lagged to prevent heat loss to, or through it.
  • Ensuring the use of only S.I units has nothing to do with heat loss and gain by the calorimeter contents.
  • Weighing the calorimeter, lid, and stirrer only suggests they must have acquired some portions of the heat being transferred between the two contents of the calorimeter. This is no good news, as a fraction of heat which the colder content would have absorbed has been lost to these instruments.

So, only statement I is correct.

Now for the right answer to the above question:

  1. Option A is correct.
  2. Option B is incorrect.
  3. C is incorrect.
  4. D is not the correct answer.

HELPFUL TIPS…

You may please note these/this:

  • Heat is supposed to be exchanged between the two contents (for example water and hot ball) of the calorimeter.
  • If heat loss by the hot ball must be equal to that gained by the water, then heat loss to the calorimeter, lid, stirrer, and environment must be zero. This is why lagging is done to reduce heat loss to the calorimeter.

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/ culled from 2019 JAMB-UTME physics question 30 /

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