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.
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.
Norton 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.