Old-time ice-cream makers used a mixture of rock salt and ice to freeze the ice-cream. The mixture got colder than the usual freezing point of water: 0°C. A seemingly opposite effect is the fact that salt is spread on the roads in winter to melt it. In this lab you will make some ice cream, observe these phenomena, and develop an understanding of what’s going on at the molecular level.
Those of us who live in climates far enough from the equator to experience regular bouts of freezing precipitation are familiar with one of the ways we cope with ice. We spread salt on it, wait for it to get brittle and melt a bit, then we scrape it away. But why does the salt make the ice melt? On a dark night in February (at about 6:30 pm) I was outside spreading salt on the inch-thick slush and ice on my driveway when my five-year-old son asked me why the salt melts the ice. Even as a teacher with years of experience teaching students about this I was at a loss. How do I explain this in a way that a five-year-old can understand? Ultimately, I had to tell him that the salt makes the ice prefer to be a liquid rather than a solid. I wasn’t satisfied with this and neither was he but it was the best I could do. Now, I don’t think I can do any better than that to explain the situation to someone as young as five but I thought it would be interesting to try to explain it to a well-educated audience. Recently, I had to discuss this very topic with my AP Chemistry class and the following discussion is the result of my attempts to explain it to them. There are two ways to think about why spreading salt on ice makes it melt faster. One involves the way that particles act collectively to change the melting point temperature. The other (to be published separately) invokes the idea of entropy, a measurement of the amount of disorder, to explain the melting.
When chemists talk about freezing point depression it is as one of several so-called colligative properties. These properties of mixtures are ones that depend only on the number of particles and not on what they are. Salt works well to melt ice but sugar would work also, though not as well. This has to do with the number of particles you get when you dissolve these substances in water. When salt dissolves it makes two particles: one sodium ion and one chloride ion. When sugar dissolves it produces just one particle: a sugar molecule. So salt is twice as effective as sugar because it makes twice as many particles. Sugar has other disadvantages compared to salt. It’s sticky and leads to tooth decay.
Before I begin to explain how salt causes water to change from one phase of matter to another I’d like to describe how different phases of matter appear at the molecular level. In the solid phase molecules are stuck in place and can’t move except to vibrate a bit. In the liquid phase they are free to move around. When molecules are in the gas phase they are moving very fast and are not in contact with one another at all.
Ice is less dense than liquid water because its crystalline shape has big empty holes. All of the molecules are pinned in place. |
This is a representation of water in the liquid phase. The molecules are stuck together by mutual attraction but move about freely. |
This is a representation of water in the gas phase. All of the molecules are far apart and moving very rapidly. |
Temperature and forces of attraction are the key to understanding the differences between the phases of matter. If the temperature is high, the molecules move faster. If the temperature is low, they move more slowly. All molecules attract other molecules with a tiny amount of force. If molecules are moving quickly enough they can break free of these attractive forces completely and zip around through empty space as a gas. If the molecules are moving more slowly then the forces of attraction keep them from careening off through space and they stay close together but remain mobile. If the molecules slow down even more then the forces of attraction force them to stay in place and a crystal forms. One place to explore these concepts is at the web site of the fine folks of the PhET project at the University of Colorado. Their State of Matter Java simulator rewards your time spent playing with it and can help you to build your understanding of how matter acts at the molecular scale. Find it here: http://phet.colorado.edu/en/simulation/states-of-matter-basics
If you looked extremely closely at the surface of a piece of ice you would find that molecules of water are constantly moving back and forth from the solid to the liquid phase. At a given temperature an equilibrium is set up so that the number of molecules sticking equals the number of molecules coming loose. At colder temperatures both rates are slower. Probably there is some temperature below which effectively no molecules are in the liquid phase but this is probably not common in places where people live.
When salt is spread on ice, even ice colder than the normal melting point, it dissolves in a thin surface layer of liquid water. The ions that make up the salt separate and act like particles of a liquid. This is when they are able to have an effect on the ice. Prior to the arrival of the salt ions, the water molecules were at their own equilibrium. The rate of sticking equaled the rate of coming loose. But now the water molecules in the liquid phase have company. For every hundred times a particle in the liquid phase hits the surface only a few are moving slowly enough to stick and become part of the ice. Let’s say 10 of the hundred move slowly enough to stick when they get near their brethren in the solid phase. Now for every hundred times a particle in the liquid phase strikes the surface of the ice only a fraction of them are even water molecules. For the sake of argument, let’s say that there are 10 salt particles and 90 water molecules for every hundred collisions. Salt ions do not become part of the solid ice. If 10 out of 100 water molecules stick when there are no salt ions then when only 90 water molecules hit the surface, only 9 are moving slowly enough to stick. This results in a reduction in the rate of sticking while the rate of coming loose remains the same.
So the situation is one of competing rates. On the one hand we have the rate of sticking–the number of molecules that stick per second might be one way to express it. Let’s call this the freezing rate. On the other hand we have the rate of coming loose–the number of molecules that come loose per second. This is the melting rate. At the normal melting point with no salt these rates equal one another and there is no change in the total amount of ice over time. Once salt is spread over the surface of the ice the freezing rate slows down relative to the melting rate. As long as the temperature remains unchanged we can imagine water molecules merrily breaking their bonds with their crystalline neighbors and bouncing away up among their liquid friends. The tired, slow molecules jostled by their liquid neighbors don’t have as many opportunities to settle down to rest. Like an exhausted parent, there is always one more thing to do before you can finally lie down. This asymmetry leads to a net change in the amount of ice: it decreases while the amount of liquid increases. That is why ice melts when you put salt on it.
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. 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 will be above 0°C after the ice and ice cream mixture have had a chance to be in contact for a while.
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. Salt makes ice melt faster 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 hand, 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 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 of the surroundings rises—at least, if you have things set up to observe this. You will not see this when water freezes outdoors on a cold day because the heat given off is small.
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 introduction 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 cooled your interest in this subject. If this makes your temperature rise, then I understand.
This experiment is intended for you to make and enjoy ice cream and to observe some things that you may not have noticed before. Pay attention to what you're doing and how it works. Do the procedure after reading the introduction so you know what to look for and pay attention to.
The recipe makes enough to make about two servings of ice cream. Feel free to add chocolate syrup to the mixture (to taste) to make chocolate-flavored ice cream. To make strawberry ice cream add frozen strawberries (this will help the freezing process, too).
| Make a cup or bowl ready to put your ice cream into by cooling it in the fridge or freezer. | ||
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1 cup cream or half-and-half
(use ½-&-½, light cream, or heavy cream) |
Mix all ingredients in a mixing bowl until the sugar is all dissolved. | Ladle or spoon about half of the mixture into a one quart zip-top plastic baggie. Seal tightly. |
| ½ cup milk (whole, 2%, or 1%) | ||
| ⅓ cup sugar | ||
| 1 tsp. vanilla | ||
| 1-quart zip-top baggie | ||
| 1-gal. plastic zip-top baggie | Fill the large baggie about half-full of ice. | Sprinkle liberally with salt. | Nestle the baggie of ice cream mixture in the midst of the salted ice so that it is completely surrounded. Keep the ice in contact with the inner baggie and keep it moving around. This tends to spread salty water around so this is best done over a sink, outside, or with the full comprehension of the mess you are about to make. | As meltwater builds up in the large baggie, pour it out. Add more salt and ice as needed to complete the freezing process. The ice cream is frozen enough when you say it is. The most you will achieve is a soft-serve consistency. If you wish to have harder ice cream then you will need to put it in a freezer for a few hours. |
| ½ gal. ice (crushed) | ||||
| ½ cup salt | ||||
| 1 baggie of ice cream mixture (see above) | ||||
When you have decided that the ice cream is frozen-enough, use a spoon to carefully extract the ice cream from the baggie and into your pre-chilled cup or bowl. Be careful not to get salty water in the ice cream. Add toppings if desired. Your assignment is to enjoy eating the ice cream, if you like ice cream.
Make sure to clean up after yourself. This experiment sometimes leads to fairly messy surroundings.
Answer the following questions in a typed document. Refer both to the introduction and to your observations in doing the experiment.