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Question Video: Explaining an Incorrect Temperature Measurement Physics

Chloe wants to measure the temperature of some water in a beaker that is near to its boiling point. She holds the thermometer above the surface of the water, as shown in the diagram, and holds it there until the reading has stabilized. She determines that the temperature of the water is 77°. Which of the following statements explains why this answer is incorrect? [A] The thermometer must be fully submerged in the water in order to measure the temperature of the water. The temperature of the water is higher than 77°. [B] The reading must be taken as soon as the thermometer is above the water rather than several minutes later. [C] The bulb of the thermometer must be submerged in the water in order to measure the temperature of the water. The temperature of the water is higher than 77°.

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Video Transcript

Chloe wants to measure the temperature of some water in a beaker that is near to its boiling point. She holds the thermometer above the surface of the water, as shown in the diagram, and holds it there until the reading has stabilized. She determines that the temperature of the water is 77 degrees. Which of the following statements explains why this answer is incorrect? (A) The thermometer must be fully submerged in the water in order to measure the temperature of the water. The temperature of the water is higher than 77 degrees. (B) The reading must be taken as soon as the thermometer is above the water rather than several minutes later. (C) The bulb of the thermometer must be submerged in the water in order to measure the temperature of the water. The temperature of the water is higher than 77 degrees.

Looking at the diagram, we can see that Chloe has placed the thermometer above the surface of the water rather than in the water. We know that the boiling point of water is 100 degrees Celsius. This question tells us that the temperature of the water is near boiling. So we should expect the temperature to be very near to 100 degrees C. Chloe must be making a mistake because the reading she takes is 77 degrees C, which is much lower. To measure correctly, Chloe should put the bulb of the thermometer inside the water, without making contact with the container and only making contact with the water.

Let’s look now at the answers and see which best reflects this practice. Answer (A) is not entirely correct because the thermometer must not be fully submerged. Only the bulb which contains the reservoir of liquid has to be submerged. Also, if the thermometer was totally submerged and the bulb was in contact with the container, this would also measure the temperature of the container, which would be different from the temperature of the water.

Option (B) is also not correct because the liquid in the thermometer that indicates temperature needs time to heat up. If you remove the thermometer too quickly, the measured temperature will be too low and immediately begins dropping. Also, the thermometer being above rather than in the water means that Chloe is actually measuring the air temperature above the water.

This means answer (C) must be correct. When the bulb of the thermometer is immersed in the water, the reservoir of liquid inside the thermometer that expands or contracts to give a temperature reading along the scale is submerged. This would indicate the correct temperature of the water and only the water. If this is done correctly, it is likely that the measured temperature would be higher than 77 degrees Celsius.

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