An experiment is carried out using calorimetry to compare the heat released from a
range of different fuels. Which of the following factors does not need to be kept constant when repeating the
experiment for each different fuel? (A) Starting temperature of the water. (B) Volume of the water. (C) Same burner and wick. (D) Thermometer. (E) Same mass of fuel burnt.
Calorimetry is the study of heat transfer during physical and chemical changes. We are told that the purpose of the calorimetry experiment in this question is to
compare the heat released from different fuels. So, we will be observing a chemical change in this experiment. The fuels will be burned, causing a combustion reaction, which is exothermic and
releases heat. During the experiment, we will need to use a calorimeter, which is a device used to
measure the heat transferred.
The following setup could be used for an experiment like this. A burner containing the fuel to be tested is placed below a calorimeter holding
water. The calorimeter has an insulating lid and is fitted with a thermometer so that its
bulb is below the surface of the water. The wick is lit. And while the fuel burns, heat is transferred to the water through the metal walls of
the calorimeter. This causes the temperature of the water to rise, which we can measure using the
We can use the rise in temperature caused by the burning of each fuel to calculate
how much heat was released by the fuel and transferred to the water. To complete this calculation, we would also need to know the mass of the water in the
calorimeter and the specific heat of water, which has a value of 4.18 joules per
gram degrees Celsius. We can recognize this as the equation typically used in calorimetry experiments.
In the experiment in this problem, 𝑞 represents the heat released by the fuel and
transferred to the water. We can calculate 𝑞 by multiplying the mass of the water in the calorimeter by the
specific heat of water and the change in temperature. When comparing fuels, the larger the temperature increase observed, the more energy
that was released by the fuel. However, we can only make this comparison if the mass of each fuel burned is the
same. In addition, we are making the assumption that none of the heat transferred to the
calorimeter escapes to the surroundings and that the other equipment present, such
as the thermometer, do not absorb any of the heat.
In many experiments, it’s very important that the experiment be conducted under fair
test conditions. What this means is that the conditions and setup used when burning each fuel should
be controlled and be as similar as possible. Since we now know that the purpose of the experiment is to measure and compare the
temperature increase in the water by the fuels, we should hold the mass of the water
in the calorimeter constant when burning each fuel. Water has a constant density. So, if we know the volume of the water, we can calculate its mass. Knowing this allows us to eliminate answer choice (B).
We must also use the same mass of each fuel. Knowing this allows us to eliminate answer choice (E). The specific heat of water is a constant value, so it is not a variable that we need
to hold constant. It’s also important to think about the starting conditions of the experiment. Ideally, at the beginning of each experiment, the calorimeter and the water in the
calorimeter should start at the same temperature. and the fuels to be burned should
also start at the same initial temperature. Knowing this allows us to eliminate answer choice (A).
Of course, we also want to use the same equipment when burning each fuel. We should use the same burner and wick to burn each fuel. We should also use the same calorimeter if possible and the same type of
insulation. Therefore, we can eliminate answer choice (C). This leaves only answer choice (D). While we certainly could use the same thermometer for each experiment, it’s not
required. A thermometer is a standard measuring device, which means we can use different
thermometers and we should get the same temperature reading.
In conclusion, the factor that does not need to be kept constant when repeating the
experiment for each different fuel is the thermometer, or answer choice (D).