Chemical Engineering in Life - Question 6 - Standard answers available

Boiling water requires the absorption of a large amount of heat. Even immediately after the kettle is removed from the stove, the boiling process continues; as a result, a large amount of heat is absorbed near the bottom of the kettle, temporarily lowering its temperature, which is why the bottom of the kettle isn’t hot to the touch. But after a while, the water stopped boiling and no more bubbles appeared; the bottom of the pot was at the same temperature as the water, so it became hot to touch.

Your explanation violates the second law of thermodynamics.

Although it was in direct contact with the fire, since it was placed there for only a short time before being removed, the heat absorbed was transferred to the water, so it didn’t feel hot.

That’s because the water temperature has not yet reached above 60 degrees.

Since it’s a question raised by an expert, it certainly can’t be explained by the low temperature. To put it simply, water has a high specific heat capacity, while aluminum (and even less so iron) has a low specific heat capacity. Water absorbs more heat, so the temperature change is slow; aluminum absorbs less heat, resulting in a faster temperature change. The same principle applies in reverse – when the aluminum pot is taken out, it cools down quickly, while water cools down more slowly. As a result, water seems very hot while aluminum doesn’t seem hot. I hope this explanation is satisfactory; please feel free to offer any corrections from an expert’s perspective.

I agree with this view: that’s because the water temperature has not yet reached over 60 degrees. The description of the time in the title is not precise; it’s a period of time.

This is a kind of mischief among rural children. My hometown is located in the Luan River Delta, where the groundwater hardness is low; when I went to my great-grandmother’s house in Xinkai Hutong, Xicheng District, Beijing in 1983, I didn’t dare to play that game. It’s not because one has grown up, but surely the hand will get burned. The kettle in her house is at least twice as heavy as those in her hometown in the countryside. This is a series of chemical engineering questions from real life; please think in terms of chemical engineering. The so-called chemical engineering refers to chemical engineering thermodynamics, principles of chemical engineering, chemical reaction engineering, chemical engineering calculations, and chemical engineering design. When using common sense or the opinions of experts from newspapers, you need to be careful when answering questions. Which is often wrong. Otherwise, these small events in life would not linger in my mind for half a lifetime, nor would they compel me to keep seeking to understand their meaning.

The kettle is removed from the heat, and the water continues to boil, absorbing a large amount of heat

Is it related to the low specific heat of aluminum?

Standard answer: Water has a high specific heat, so the volume of a kettle heated over coal fire cannot be very small. So it takes 15-30 minutes to boil a pot of water. The materials used for kettles are also of fairly good quality; generally it’s iron, though aluminum or copper is also used. Of course, aluminum and copper have higher thermal conductivity, iron has a lower one, while stainless steel has the lowest. Are there pots with enamel coating or kettles with protective paint? I remember there being one. Suppose there is some water adhering to the outside of the kettle as it is filled with water; therefore, when it is placed over the fire, there is a hissing sound, and even a popping sound, which is caused by water droplets falling into the fire. All these delay the increase in water temperature inside the kettle. A characteristic of water is that its density is highest at 4 degrees; it then decreases as the temperature rises. Therefore, when water is heated from below, convection between the upper and lower layers occurs quite vigorously, allowing the water temperature inside the pot to be uniform. Assume that after a certain period of time (for example, when the bottom of the pot is dry and there is no water on its outer surface), the water temperature is 40 degrees. But what is the temperature on the outside of the bottom of the pot at this time? Assuming the coal combustion temperature is >900 degrees, it is possible for the air temperature in contact with the bottom of the pot to be above 300 degrees. I think at this point, the temperature on the bottom wall of the iron pot could be 100-200 degrees. But within a few seconds of removing the kettle from the stove and then touching the bottom of the kettle with your hand, the temperature of the bottom drops to roughly the same as the water temperature inside the kettle. This is because the thermal conductivity of the pot wall is relatively high, and the pot wall is relatively thin. If you don’t believe me, find a new kettle and an old one – the old kettle will feel heavy when held, with scale inside that is more than half an inch thick. Try the old kettle again; don’t blame me if it’s hot to touch.