Hey there! Have you ever wondered why ice is slippery? We use ice in so many ways, like in our drinks or for skating, but there’s something mysterious about it that scientists are still trying to figure out. Let’s dive into the science behind why ice is so slippery!
Ice is just frozen water, or H2O. It’s a solid form of water that we see all the time. But even though it’s common, ice has some strange properties. One big question is why it’s so slippery. You might think it’s because there’s a thin layer of water on top of the ice, but there’s more to it than that.
One idea is that when you step on ice, your weight creates pressure that melts a tiny bit of the ice, making it slippery. This idea goes back to the 1840s when a scientist named Michael Faraday did an experiment with ice cubes. He thought there was a layer of water between them that froze. But here’s the thing: for pressure to melt ice, you’d need to weigh as much as an elephant! So, pressure alone doesn’t explain why ice is slippery.
Another idea is friction. When you move across ice, like when you’re skating, friction creates heat, which can melt a thin layer of ice. This does happen, but it doesn’t explain why ice is slippery even when you’re standing still.
The real answer lies in something called a “quasi-liquid layer.” This isn’t exactly liquid water or solid ice. It’s a thin layer on the surface of the ice where the water molecules are not tightly bonded. Imagine marbles on a dance floor; they can roll around easily. This layer is what makes ice slippery.
Water molecules in ice are usually held together in a solid pattern by hydrogen bonds. But on the surface, these bonds are weaker, allowing the molecules to move around more freely. This creates the slippery layer that we experience.
Even though ice is something we see every day, scientists are still learning new things about it. This shows us that science is always evolving, and there’s always more to discover. So, keep asking questions and stay curious!
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Thanks for joining me on this icy adventure! Remember, science is all about exploring and discovering new things, even in the most ordinary places.
Try this at home: Place an ice cube on a flat surface and press down on it with your finger. Observe if any water forms underneath. Discuss with your classmates why pressure alone isn’t enough to make ice slippery, considering the weight needed to melt ice through pressure.
Use a small piece of ice and rub it quickly between your hands. Feel the heat generated by friction and observe any melting. Discuss how friction contributes to ice’s slipperiness and why it doesn’t fully explain the phenomenon.
Create a model using marbles on a tray to represent water molecules on the surface of ice. Gently shake the tray to see how the marbles move freely. Discuss how this movement relates to the quasi-liquid layer and ice’s slipperiness.
Research another everyday phenomenon that seems simple but has a complex scientific explanation. Prepare a short presentation for your class, highlighting the mystery and the science behind it, similar to the slippery ice concept.
In groups, discuss how scientific understanding evolves over time. Use the example of slippery ice and other scientific discoveries to illustrate how new evidence can change our understanding of the world. Share your thoughts with the class.
Thank you to Nord VPN for supporting PBS. Hey smart people, Joe here. I’m willing to bet you’re familiar with this stuff — ice, good old frozen H2O. If you ever really stop to think about it, we put it in our drinks, we slip on it, we slide on it, we skate on it… And we’re worried climate change is going to melt every last bit of it. But even though ice is such a common and ordinary substance, it still has a few weird properties and mysteries that we’re just now figuring out. One of those mysteries is one of the most obvious questions about ice — why is ice slippery?
The most intuitive explanation for ice’s slipperiness is that there must be a thin layer of liquid water on the surface of the ice. This idea goes back to the 1840s when the physicist Michael Faraday conducted an experiment. He took two ice cubes, and when they were placed in contact for just a few seconds, they became one ice cube. Faraday thought that there must be a layer of liquid water between them that froze. But I don’t see a layer of liquid water, so maybe something has to create that layer.
This is what’s called a phase diagram, and this one is for water. For a particular temperature and pressure, it tells you which phase — solid, liquid, or gas — the substance will be in. Looking at the diagram, we see that at minus five degrees Celsius and 1 atmosphere of pressure, like your typical ice rink, we have solid ice. This phase diagram also shows us something else: there’s more than one way to melt ice. You can raise the temperature, which we are all familiar with. But water is special because you can also melt it by increasing the pressure. For most substances, their solid form is denser than their liquid form, so putting the solid form under more pressure only makes it more solid. But water ice is different; its liquid form is more dense than the solid form, and that’s why ice floats.
So if we take solid ice and squeeze it under a lot of pressure, we can force it to become a liquid again, even at much colder temperatures where we’d usually expect it to be solid. The idea is this: when you step onto the ice, you exert pressure on the area under your feet, which lowers the melting point of the ice and creates a thin layer of liquid water. Now, pressure does lower the melting point of ice. For a long time, people thought that this “pressure melting” was the reason ice was slippery — a person’s weight, concentrated on the area under their feet, melted the top layer of the ice. But something doesn’t add up here.
I weigh about 88 kilograms, and the area under these skates is about 900 square millimeters. Plugging that into the appropriate equations shows that it only lowers the melting point of ice by a fraction of one degree Celsius. If you’re wearing regular shoes, you’d lower the melting point by even less. Basically, for pressure melting to work, you’d have to weigh over 3,000 kilograms, which is only half as heavy as most elephants.
There’s another answer that feels closer to the truth — friction. My ice skates glide across the ice, creating friction. That friction generates heat, which can create a thin layer of liquid water on top of the ice. We know this happens; ice skates gliding across ice can create friction that melts the ice beneath them. However, it doesn’t explain the whole story. Anyone who’s tried skating for the first time knows you don’t have to move very much for your feet to slip out from under you, and it also doesn’t explain why ice is slippery even when you’re not moving.
So does friction explain it? Well, maybe in part, but it doesn’t explain the whole story of why ice is slippery. There’s a fundamental issue here that we haven’t talked about. Imagine a layer of water on a hard surface, like a puddle on a smooth floor. It’s pretty obvious that ice is actually way more slippery than just water. There must be something else going on.
Several theoretical and experimental studies in the last few years have found the real cause of ice’s slipperiness — that thin layer of liquid water on the surface isn’t really liquid water at all. It’s also not solid ice; it’s something very different and very strange. Do you remember the weirdest property of solid water? It floats. Water’s solid form is less dense than its liquid form. Think about it; that’s weird. Solid rocks don’t float on lava.
Now let’s think about this on the molecular level. Water is a polar molecule. The slightly positive hydrogens are attracted to the slightly negative oxygens. This is known as hydrogen bonding. A single molecule in liquid water will only be hydrogen bonded to a few other water molecules, but a single water molecule in ice will have more molecules hydrogen bonded to it than in liquid water. This makes ice a crystalline solid with water molecules bonded together in a consistent repeated pattern.
But what about a molecule on the surface of the ice? Those may be bonded to as few as just one other molecule in the crystal, and it’s this lack of structure that allows the surface molecules to tumble around in disorder, detaching and then reattaching themselves to the surface and to each other. These water molecules deeper in the ice are held in place rigidly by hydrogen bonds, but those on the surface don’t have as many hydrogen bonds holding them in place, allowing them to roll around. This creates something on the surface that isn’t really a liquid because, according to water’s phase diagram, water is solid under these conditions, but it’s not totally a solid either — at least not the way we normally think of solids. Scientists call it a quasi-liquid layer.
One researcher who studied its properties compared it to marbles scattered across a dance floor. You just take one step, and the marbles roll out from under you. Scientists still have a lot of questions about the properties of this quasi-liquid layer since the border between it and ice is really hard to see. We’re not even sure exactly how thick it is; all we know is that it’s somewhere between the size of a single bacterium and a thousand times smaller than that, talking about just a few molecules.
So ice is slippery because of a non-solid quasi-liquid layer of water on the surface. It’s not caused by pressure, and it’s not totally caused by friction. It’s caused by the H2O molecules coming loose on the molecular level, and those H2O molecules don’t act like liquid water; they act like something much weirder.
I know it’s strange to say here in the 21st century that we don’t know everything about something as ordinary as frozen water, but this demonstrates something fundamental about science — that the more resolution we have to gaze into the universe, whether it’s on the biggest or the smallest scales, we will always keep uncovering new discoveries and updating what we knew yesterday, even in the places we’ve been looking all along. Stay curious.
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Hey, how’s it going? This intro is going to be a challenge! Did you get it?
Ice – A solid form of water that occurs when water freezes at 0 degrees Celsius or below. – When the temperature drops, water in a pond can turn into ice.
Water – A liquid substance composed of hydrogen and oxygen, essential for all forms of life. – Scientists study how water moves through the environment in the water cycle.
Pressure – The force applied perpendicular to the surface of an object per unit area. – The pressure inside a balloon increases as more air is added.
Friction – The resistance that one surface or object encounters when moving over another. – Friction between the tires and the road helps cars to stop safely.
Molecules – The smallest units of a chemical compound that can exist; composed of two or more atoms bonded together. – Water molecules consist of two hydrogen atoms and one oxygen atom.
Layer – A sheet, quantity, or thickness of material, typically one of several, covering a surface or body. – The Earth’s atmosphere is made up of several layers, each with different characteristics.
Science – The systematic study of the structure and behavior of the physical and natural world through observation and experiment. – In science class, we learned about the laws of motion and how they apply to everyday life.
Hydrogen – A colorless, odorless gas that is the lightest and most abundant element in the universe. – Hydrogen is one of the two elements that make up water.
Bonds – The connections between atoms in a molecule, formed by the sharing or exchange of electrons. – Chemical bonds hold the atoms in a molecule together, determining its structure and properties.
Slippery – Causing or tending to cause slipping or sliding, often due to a smooth or wet surface. – Ice can be very slippery, making it difficult to walk on without slipping.
