Assessment Opinion

The Tale of a Melting Ice Cube: Have we lost the Story?

By Anthony Cody — May 03, 2010 4 min read
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A second year teacher and I began our meeting last week by looking at some student work. This teacher is a solid instructor with a good grasp of his subject, and strong rapport with his students. His high school chemistry students had drawn diagrams of what was happening when salt is added to ice. Their sketches showed small circles representing the water molecules, connected by straight lines in a rectangular grid pattern. Then when the salt was added, another drawing showed this pattern disrupted as the salt, represented by an X, was drawn in between the water molecules.

“Is this how the salt works in the water?” I asked. “Well,” he said, “Actually, the salt dissolves to form sodium and choride ions. But I didn’t think that was important and I wanted to keep it simple.”

I started thinking about how I would want to help students understand why ice melts more quickly when you add salt, and this is what I came up with:

Water is an amazing thing. Every tiny water molecule has a positively charged end where the Hydrogen atoms live, and a negatively charged end where the Oxygen atom lives. Water molecules are like tiny little magnets, each one with a positive pole and a negative pole - that is why water is called “polar.”

This makes water behave so differently from almost everything else - water is so special. We know opposite charges attract, and so the positive end of one molecule is attracted to the negative end of another. That is why the molecules stick together on the surface of a glass of water, forming weak Hydrogen bonds, which create surface tension that can float a steel needle. And when water forms a crystal, we don’t get a regular pattern like a brick wall. We get these angles formed by the angles the water molecules form as they arrange themselves - a sort of tetrahedron. So if we look at ice crystals - like snowflakes for example - we do not see a brick wall. We see all these beautiful patterns created by these angles.

And what happens when SALT is put into water? First of all, it dissolves. Salt is a compound made of Sodium and Chlorine atoms bonded together. When they dissolve in water, they form ions, Sodium ion with a positive charge, and Chloride ion with a negative charge. What do we know about a water molecule? It has a positive (Hydrogen) end, and a negative (Oxygen) end, right? So what end will the positive Sodium ion be attracted to? To the negative end of the water molecule.

So when we add salt to water, it is not just acting like a solid grain of sand between the water molecules. It is actually breaking down into two parts, and each part is attaching to opposite ends of the water molecule - and breaking the weak bonds connecting one water molecule to another - and that means the ice melts.

I shared this story I thought was hidden within the melting ice with this teacher, and he said “Yes, but I don’t think surface tension is included in the CST (California Standards Test) so I don’t think I would want to include that.” He said he tries to make sure his instruction focuses on the things that will be on the CST, so that he does not lose too much time over-complicating things with material the students won’t need for their tests.

For me, the melting ice cube is a dramatic tale.
This is why I love science - to actually have some glimmer of understanding of WHY snowflakes form such lovely shapes and patterns. Each one is unique, but they share some key features that can be predicted and explained. And when I grasp this story, then I can understand other things about how water behaves. Why does ice float? Why is water so good at dissolving other things?

We can begin to stoke this passion with our students by giving them some things to wonder about. Share photographs of ice crystals and salt crystals. Look at different crystals under a microscope. How are they similar? How are they different? What story might explain this difference? What about other crystals?

What is the story of water? This substance behaves like nothing else on earth. Once we begin to understand water, when we encounter other liquids, we can see how they are different or similar. All these behaviors are caused by what is happening beneath the surface, hidden by the tiny size of the molecules. But we can understand, when we have a story and some visual models to help us. And when we get that understanding, there is a thrill of discovering a hidden secret -- of possessing special knowledge. That is the alchemy of learning, the magic of the mind that drives us to keep on investigating and learning more -- not because it is on the test, but because it is a beautiful thing to understand!

For me, this is one of the tragedies of our test-driven educational culture. Teachers who are coming of age in our classrooms are under intense pressure to produce better test scores. They come to believe that if knowledge is not going to be on the test, it will not help their students, and it is a waste of the limited time we have to learn all the things that WILL be on the test. It is a long list, so we do not have time to waste on irrelevant material. But what is lost, I am seeing, is the narrative thread that holds these discreet bits of knowledge together. That narrative is a story with great detail, and each detail illustrates and allows us to understand why water behaves the way it does, and exactly how the ions from salt interact with the water crystals. When we strip our lessons of these details, we lose the storyline, and we end up trying to get our students to remember basic facts that lack coherence and beauty.

What do you think? Have we lost the storyline as we pursue better test scores?

Here is more information about water if this has whetted your appetite.

Snowflake image is shared under Creative Commons license, byYellowcloud:

Salt crystal is also Creative Commons, by Gurdonark:

Update: A bonus video for science lovers:

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