Tech UPTechnologyWhy does ice cream melt?

Why does ice cream melt?

In 1761 the Scottish distilleries were facing an important dilemma. The recent union of Scotland and England had opened the doors to the English and American markets. The distillation process used wood as a source of heat and the continuous exploitation of the forests had made it a scarce commodity and, therefore, expensive. If they wanted to increase production, they needed to find a way to reduce the ever-increasing cost of fuel . Concerned, they decided to go to the University of Edinburgh and there they found Joseph Black. The famous chemist quickly realized that improving the efficiency of the distillation process involved understanding in detail how a substance changed state. In short, to clarify why the snow was slow to melt when spring arrived.

What happens to snow when we heat it? The answer to this question involves first figuring out what happens when two substances with different temperatures come into contact. Joseph Black decided to start there. Experience tells us that if we wait long enough, they both end up having the same temperature. Black called this ultimate state of uniform temperatureheat balance, what we call todaythermal equilibrium.The problem is that this physical principle says that the system, once left to its own devices,reaches an equilibrium temperature but says nothing about what that final temperature is. Common sense and everyday experience lead us to suspect that it is between the initial temperatures of both bodies and that it depends on the amount of each of them that we have: the final temperature of a thousand liters of water at 90 ºC and a liter of water at 20 ºC will not be the same as a thousand liters at 20 ºC and one at 90 ºC. Let us now do the following experiment. Suppose we have a kilo of water at 70 ºC and another at 10 ºC and we mix them. Our initial suspicion is that the final temperature will be somewhere in between 40ºC; that’s what we measure. Now let’s put two kilos of water at 70ºC and one at 10ºC. Intuitively we see that they will not reach an equilibrium temperature of 40 ºC, but one closer to 70 ºC; maybe two-thirds of the way, about 50 degrees. Also this is correct. There is nothing special about it, but reflecting on it a little we discover that a new concept has been subtly introduced into our calculations: two kilos of water at 70º have more thansomethingthat a kilo of the same water at 70º, and that water at 70º has more of thatsomethingthan the same amount of water at 10º.It is clear that thatsomethingit is not the temperature but it is related to it. black called himheat matter.

latent heat and sensible heat

The solution to the problem of distilleries was to make quantitative measurements of that heat, so Black took ice and measured the amount of heat needed, first, to convert it into water, and then, the amount needed to boil it. In doing so, he discovered a detail that greatly intrigued him: while the ice was melting, the thermometer did not indicate any increase in temperature and only began to rise when all the ice had turned into water. The temperature was kept constant throughout the duration of the fusion! Why was this happening? What happened to the heat? Where was he going? Did it disappear?

Black resolved the paradox by introducing the concept of latent heat -as opposed to sensible heat , whose effect is to change the temperature of the object and which can be detected by touching it, hence sensible- . According to the Scottish chemist, the thermometers did not register any increase in temperature because all the heat was invested in changing the shape of the water . Only when all the ice had melted did the heat produce an increase in temperature. Thanks to these experiments, Black was able to tell distilleries the minimum amount of wood needed to evaporate a given volume of whiskey and how much cold water to use to condense it in the coil. And not only that. What happened next is one of those things that proves the profound interrelationship between the different fields of scientific research. The idea of latent heat made James Watt – on the other hand, a good friend of Black’s – realize in his Glasgow workshop that the essential drawback of the Newcomen engine was that the water was not able to cool the cylinder completely, so that after each cycle it was hotter and hotter and the steam condensed to a lesser extent until the machine was unable to create the necessary vacuum to work. The only feasible solution was for the steam liquefaction to occur elsewhere. Watt modified Newcomen’s steam engine by introducing the condenser , thereby making it a more efficient instrument, ushering in the Industrial Revolution .


Goldstein, M. y Goldstein, I. F. (1993) The refrigerator and the universe, Harvard University Press

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