Answer:
  Dr. Yasunori Tominaga of Ochanomizu Women’s University kindly answered the above question.
 
*Dr Tominaga noted, “I cannot promise that all the information in my article is correct, because this is my estimation.”  If you have any suggestions, please let us know.
 
 
  The lower boiling point is caused by two factors: the water itself and the measuring method. Although I did not actually see how the experiment was carried out, the measuring method seems to be the cause.
 
  When we measure the boiling point of water, we usually boil the water in a beaker and place the thermometer (alcohol or mercury) into the water. Elementary school science classes usually use alcohol thermometers. Most people place only the tip of the thermometer into a liquid when measuring its temperature, and a large portion of the body of the thermometer does not contact the substance they are measuring. In fact, most alcohol thermometers are made of kerosene instead of alcohol.
 
  In this type of thermometer, the liquid inside the narrow glass tube (which has a homogeneous radius) expands when heated, and rises up inside this tube. Then the length at which the liquid stops (scale) is assumed as the measured temperature. Therefore, if the temperature of all of the liquid inside the thermometer is not uniform, we cannot say that we are accurately measuring the temperature.
  
  For example, we can always measure the room temperature accurately because the whole thermometer is in the same atmosphere. However, when measuring the temperature of liquids, we must always make sure that the whole glass tube is immersed in the substance. For instance, when you measure the temperature of river water, make sure to place the entire thermometer into the water in order to have an accurate reading.
  
  Since it is not easy to place the thermometer into the boiling water, we need to use the right equipment. Use a conical flask that is slightly taller than the thermometer. Place a cork with a hole in the mouth of the flask so that the thermometer can be held in it. Place some water in the flask, boil the water, and then read the thermometer.
  
  Similar articles may be found in the teaching manuals of elementary science textbooks. 
  
  
Acknowledgement
  We would like to thank Dr. Yasunori Tominaga for his kind answer.
 
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.