Most people buy their ice cream at the store, ready-made. On special occasions some of us still make it at home. Mostly, we do this using specially designed ice cream machines with a frozen insert to make the delicious dessert. But sometimes we use the power of salt and ice to steal the heat away from the ice cream. But just how does it work? Why isn’t just ice enough to do the job and why does adding salt to the ice clinch the deal?
First, a little background about ice cream making. A simple ice cream recipe involves nothing more than cream, milk, sugar and vanilla. Because of all the sugar and other solubles, this mixture has a freezing point below the normal freezing point for water, which is 0°C. (See my other post about why ice melts when salt is poured on it.) In order to make a dessert that is a treat to eat the liquid mixture has to be frozen while being stirred. It turns out that some of the texture we like in ice cream is due to the inclusion of air pockets. More importantly, by stirring while the ice cream freezes we can limit the size of ice crystals to very small sizes. High quality store-bought ice cream has very little in the way of air pockets but is still creamy and smooth because the ice crystals are all small.
Ice alone is not enough to freeze ice cream. There are two reasons for this. First, and easiest to understand, the ice just isn’t cold enough. At zero degrees Celsius the ice doesn’t have a temperature low enough to cause freezing in the ice cream mixture. When substances at different temperatures come into contact they will eventually arrive at the same temperature by exchanging heat. The temperature they arrive at depends on the mass of each substance and the substances’ heat capacity. The ice cream mixture probably has a temperature starting at 4°C (refrigerator temperature). The ice is around 0°C. Suffice to say that whatever effect mass and heat capacity have, the temperature at which ice cream mixture meets the ice will be above 0°C.
Second, and this will require some further explanation, the ice is absorbing heat not just because it is colder but also because a phase change is going on. A certain amount of heat energy is absorbed or released without a change in temperature during melting or freezing. In physics class this is called ‘latent heat of fusion’ and in chemistry class it is just called ‘heat of fusion’. (Fusion is an old-fashioned word for melting). Melting requires an input of heat energy: imagine an ice cube on a hot plate. Freezing requires that heat energy be given up–heat must be taken away from water to make it freeze. This may sound strange at first but it makes sense because heat is something that can be added or taken away but ‘coldness’ is not. So objects become colder when heat is taken away from them. There is a great practical use of this fact in citrus farming. When temperatures drop below 0°C citrus growers can spray trees with water until the frost event passes. The heat released into the fruit and leaves by the freezing water keeps the crops and trees safe from damage. There is a technical paper at the Univ. of Arizona site about this: http://ag.arizona.edu/pubs/crops/az1222.pdf.
Surrounding the ice cream with plain ice won’t make it freeze. Adding salt to the ice, though, makes all the difference. In a previous post I wrote about why salt makes ice melt faster. It does so, in short, because the salt serves to slow down re-freezing while leaving the melting rate unchanged (at constant temperature). Since melting absorbs heat and freezing releases heat the temperature of a mixture of ice and water will remain at the freezing point until the ice has all melted or the water has all frozen. But when the equilibrium is disturbed and melting is faster than freezing, the energy balance is also disturbed. More heat is absorbed than is released and this leads to a decrease in temperature. This is why the ice gets colder when salt is sprinkled on it.
This last part of the explanation is counterintuitive. On the one hand, I am saying that “objects become colder when heat is taken away from them”. On the other I claim that ice gets colder when more heat is absorbed. It appears that I have contradicted myself. To understand that I have not requires a bit of background.
When a pure liquid or a solution absorbs heat, its temperature rises. If it loses heat, its temperature falls. But when a liquid or a solution freezes or melts then the the exchange of heat requires a more subtle understanding. I will consider melting first because it allows a more intuitive grasp of the idea. Molecules in a solid are held together by intermolecular bonds. These bonds form due to electrical attractions between the molecules. On an everyday-life scale you can observe this type of attraction when you rub a balloon on your hair and the hair clings to the balloon. In order for the solid to melt, the bonds holding molecules together have to be broken. Breaking a bond requires energy. This energy comes from neighboring molecules either in the solid phase or the liquid phase. When a molecule in the liquid phase strikes a molecule in the solid two things can happen. First, it can transfer only a little energy to or from the solid molecule and so make it vibrate a bit more (or less). If the solid molecule absorbs the energy then it raises the average molecular motion of the solid molecules and reduces the amount in the liquid. No change in temperature results from this exchange of energy. Second, the molecule may hit hard enough that there is enough energy to break the bonds holding the solid molecule in place. The fast-moving molecule slows down but the solid molecule does not speed up as much—some of the energy given to it was used to break bonds and cannot make it move faster. Because bonds were broken, some energy was ‘used up’ and the molecules are moving more slowly than before. When molecules move more slowly we measure that as a reduction in temperature. So when a solid absorbs heat in the process of melting, temperature goes down.
The exchange of heat when a liquid freezes is a bit harder to understand. Pulling molecules apart to make them melt requires an input of energy. When this energy is taken away from nearby molecules they slow down and the temperature drops. But the opposite happens when molecules freeze: they are put together and when they are they release energy. This causes nearby molecules to move faster and so the temperature rises.
The mechanism for this release of energy might go something like this: A fast-moving molecule in the liquid strikes a slower molecule and gives up some of it energy. This makes the second molecule move faster. If the first molecule loses enough energy it may form a bond with a molecule in the solid phase. It has to lose some energy because the attraction between the molecules is only so strong and won’t make a molecule stick unless it is moving slowly enough. The energy it lost didn’t disappear; the energy made another nearby molecule speed up. When this happens to enough molecules it causes the average speed of the collection of molecules to increase. An increase in average speed is what we measure as an increase in temperature.
To sum up, when ice is sprinkled with salt the freezing process slows down and the melting process stays about the same. The energy balance is disturbed and more heat is absorbed than is released since freezing releases heat and melting absorbs it. Since the heat is used up breaking bonds it does not make the ice hotter; it makes it colder. So the temperature of ice decreases when salt is sprinkled on it. Ice alone won’t freeze ice cream, but ice sprinkled with salt gets so cold that it takes heat away from the liquid and makes it form ice crystals, which, if carefully stirred, makes an irresistible treat.
Eating ice cream can lead to ‘brain-freeze’ but if this article was too long and obscure it may have melted your brain instead. If so, I am sorry. In my defense, writing about freezing is a very hard topic. Putting my thoughts about melting into smoothly flowing words is also difficult. In the heat of my hurry to write down my thoughts, I may have been less than clear. If this makes your temperature rise, then I understand and I hope you’ll forgive me.