A Chilling Effect

by James Morris

What melts more quickly – ice in fresh water or ice in salt water? Read on to find out.

Recently, I attended a conference on Digital Education at the Google Headquarters in New York. Ironically, one of the lessons I took away from the conference was not digital at all. And this lesson seems particularly fitting at this time of year, with winter storm Jonas battering the east coast and leaving mountains of snow in its wake.

One of the presenters at the conference did a demonstration. She placed some ice cubes in salt water and some in fresh water, and then asked a simple question:

Which ice cubes will melt more quickly?

Take a moment to think about it and then come up with an answer. Perhaps even write it down. Got it?

Let’s use our everyday experience to answer this question. At this time of year, many of us put salt on our driveways and roads to melt the ice. Therefore, it is logical to assume that the ice cubes in salt water will melt more quickly than ice cubes in freshwater. Salt, after all, melts ice.

Well, not quite. It looks like salt melts ice, but salt instead reduces the freezing point of water. Instead of 32˚F (or 0˚C), salt water melts/freezes at a colder temperature, depending on the amount of salt in the water. This means that your driveway will be free of ice at say 30˚F if salt is present, but frozen solid at this same temperature without salt.

It’s also why lakes freeze before the ocean, and why we add salt to water when making ice cream. The salt allows the water to stay liquid at a colder temperature than if the salt was not present, which in turn speeds up the process of making ice cream.

But what about our ice cubes? In the salt water, the freezing point is less than in the fresh water. So, at 30˚F, for example, we would expect more liquid water than frozen ice in the salt water compared to the fresh water. Based on this fact alone, you might conclude that the ice in the salt water should melt more quickly than in the fresh water.

But it’s the other way around. Try it for yourself.

FrostWere we wrong about the effect of salt on the freezing point of water? No, that’s correct. But there is more going on here.

Don’t forget that salt water is more dense than fresh water. Think about how you float more easily in the ocean compared to a lake. So, as the ice melts in salt water, there is a thin layer of very cold fresh water (from the melted ice cubes) above the more dense salt water below. This layer of cold water surrounds the ice cube, keeping it nice and cold, and causing it to melt slowly.

Now consider the ice cubes in fresh water. There is a density issue here too, but a different one. Cold water, it turns out, is more dense than warm water. Therefore, cold water sinks, and warm water rises. So, in the glass of fresh water, the cold water that just melted from the ice cube sinks to the bottom and relatively warm water rises to the top. This is the same process that occurs in the oceans, with cold water sinking and warm water rising, affecting weather patterns across the globe.

In fresh water, then, the warm water comes into contact with the ice cubes, speeding up their rate of melting, and causing them to melt much faster than they do in the salt water.

The lesson I took away from this simple experiment is to be careful about jumping too quickly to conclusions. When we know something, or think we know something, it sometimes blinds us to other possibilities.

The same is true in medicine – doctors look for patterns, and when they start to see one, they may put aside other diagnoses. Dr. Jerome Groopman in his book How Doctors Think warns of the problem of what he calls “anchoring,” where a doctor makes a snap judgment or diagnosis which colors all of his or her thinking from that point on.

This got me thinking about other, more famous experiments.

Take ATP. ATP is often called the energy currency of the cell. Cells need energy for all kinds of tasks – to divide, move around, pump nutrients in and waste out – and ATP provides this energy in a readily accessible form, in the same way that we use coins and bills for our daily transactions.

For some time, it was not known how ATP is synthesized in the cell. Chemists knew how chemical reactions work: Molecules come together to form other molecules. So, it made sense that ATP would be built by some kind of simple, conventional chemical reaction.

But that’s not the case. Peter Mitchell in the 1960s proposed a completely different way that the cell could make ATP. He hypothesized that protons (positively charged hydrogen atoms) build up on one side of a membrane, storing energy in much the same way as a dam in a river, and that ATP could be made using this energy.

This idea was widely rejected. Why? In part because it was novel. In part because there was no evidence for it. And in part because we already knew how chemical reactions worked. But Mitchell was correct, and in 1978 he was awarded the Nobel Prize in Chemistry for his insight. Like with the experiment of the ice in the glass, a little bit of knowledge led us for a time in the wrong direction.

PhantomTollboothNorton Juster in his famous and beloved The Phantom Tollbooth also warns us of the danger of jumping to conclusions. Taking this misstep will land you quite literally on the Island of Conclusions. As Canby explains to the protagonist Milo, “To be sure…you’re on the Island of Conclusions. Make yourself at home. You’re apt to be here for some time.”

“But how did I get here?” asked Milo…

“You jumped, of course,” explained Canby. “That’s the way most everyone gets here…”

If we jump too quickly to a conclusion – the ice will melt more quickly in the salt water, it’s just the common cold, ATP is built through chemical reactions that we are already familiar with – we might not consider what else is possible. And who wants to spend too much time on the Island of Conclusions?

It’s hard to get off that island once you get there.

© James Morris and Science Whys, 2016.

6 thoughts on “A Chilling Effect

  1. Sarina Tcherepnin

    Can’t wait to try the ice experiment in my Kindergarten classroom! It is great to finally understand why we use salt to “melt” ice – very interesting! Good reminder for us to take care not to jump to conclusions!

  2. James Morris Post author

    One way to extend the experiment I described above is to ask the students to make a prediction (which cubes will melt fastest) and then watch. The ice cubes really do melt much faster in the freshwater compared to the salt water. Then, once students see a difference but before you explain why, add a drop of food coloring. In the freshwater, it will quickly sink to the bottom, following the flow of cold water. In the saltwater, it will sit in a thin layer at the top – again, a very dramatic and obvious difference. After students see the food coloring, they may be able to figure out on their own about the cold water sinking and warm water rising in the freshwater. Let me know if you give it a try.

  3. Torbjörn Larsson

    ATP is a very good example, because it could very well have been true. Cells produce ATP in the conventional way as well in the cytosol, by glycolysis. And gluconeogenesis/glycolysis may very well have been the first metabolic pathways. [ http://advances.sciencemag.org/content/advances/2/1/e1501235.full.pdf ]

    So why isn’t cytosol ATP production dominant?

    The very same conditions that makes the above so compelling – alkaline hydrothermal vents produce the substrate pyruvate under Hadean conditions with the catalyst mineral greigite – would make chemiosmosis compelling. The only known way to evolve modern non-permeable cell membranes is to have a protocell with semipermeable membranes evolve chemiosmosis at the alkaline/acidic boundary of an alkaline hydrothermal vent in a Hadean carbonated ocean. Such a cell type would be much more hardened against the environment (which may be the decisive factor), and it would be much more energy efficient to boot.

    So while both cytosol and membrane ATP production may be remnants of our geological ancestry, the situation isn’t an immediate conclusion to jump to!

    1. James Morris Post author

      Thanks for such a thoughtful reply. What an interesting observation. I really like how you put it in an evolutionary perspective.

  4. steven karel

    About the ice cube experiment…

    What’s important, it seems to me, is to encourage students to continue to poke at the problem to increase their understanding. It’d be just as bad to come out of the demonstration thinking “ok, so my intuition is all wrong”.

    Get the students to start proposing variants of the experiment. Get them to start wondering about what other variables there are. Does it matter what the starting temperature of the water is? How big the ice cube is relative to the container? What if I apply some forced convection? What if I insulate the container?

    I once taught chemical engineering at a big-name university. The students were really smart, but distrustful of their instincts when solving problems. They’d learned, in many courses in high school and college, that professors like to give lectures about, and assign as homework, “gotcha” problems in which intuition leads you the wrong way. As a result, they’d solve equations and happily report their answer even if it was many orders of magnitude off what you’d expect from intuition (it’s so easily to accidentally misplace a digit or a minus sign).

    In the class, we worked on trusting our intuition again, and trying to delve into why it failed us when it did. I encouraged them to start with a guess, and then say something at the end about why they thought the guess failed or succeeded. It worked for some of the students — you can’t win them all.

    Applying that to this case, it’d be great to follow up an experiment where intuition often fails with additional tests in which students can reclaim some confidence in their intuition (and maybe even get a little more insight into heat and mass transfer)

    1. James Morris Post author

      That’s a really good point and an important reminder. We definitely want students to trust their instincts. It’s a great starting place. And then we want to encourage them to delve further, as you suggest.


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