Tell Me Why
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Have you been curious about something you observed and want to know why? Ask JM! This is the first of a new series where we answer readers' questions. So do write in to us at jantarmantar.why@gmail.com. Be sure to write your name, class and address so we can publish it along with your question.
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I was eating a toasted stuffed sandwich that my mother had just made. I waited till the bread had cooled a little and took a bite. But I almost got burned because the stuffing inside was still very hot. I waited for a while and cut open the sandwich. But the tomato stuffing was still too hot to eat. Why did this happen?
Answer: First of all, if the sandwich was tightly closed it is possible that steam was trapped inside. After you cut open the sandwich, the steam would have escaped. But as you observed so nicely, the tomato was still extremely hot. This is because of the large specific heat of water (which tomato is mostly made of). Specific heat is the amount of heat lost (or gained) when the temperature of unit mass of the substance changes by 1 degree. This is 4.2 Joules for 1 gm of water. Let us assume the tomato had about 5 gm (1 spoon) of water content. For it to go from (say) 90C to just 89C, it needs to lose 4.2x5=21J of energy. In contrast, the specific heat of oil is about half of that. This means that even if hot water is radiating away a lot of energy, temperature-wise, it takes a long time to cool down. (The energy released when steam at 100C converts to water also at 100C is about 2200 J/gm! It is called the latent heat of steam. That's why steam can really burn you).
By the way, this is the same reason why sea water by the beach is cooler than the hot sands in the afternoon and warmer than the cool sands at night. Can you figure it out?