Ice is slippery because it creates a thin layer of liquid water on its surface when in contact with warmer objects, reducing friction and making the surface slippery.
Contrary to what is sometimes believed, ice is not inherently slippery on its own: it is mainly due to a very thin layer of liquid water formed on its surface. In fact, even when the air is cold, the molecules at the top of the ice block vibrate enough not to remain perfectly attached to the rest. As a result, a mini layer of water is created, only a few nanometers to a few micrometers thick. And it is this thin layer that acts as a natural lubricant, allowing everything passing over it – like a skateboard or ice skates – to glide easily. The thinner and more uniform this layer is, the faster the glide. Essentially, it is just a tiny film of liquid water that makes all the difference between failing spectacularly and executing beautiful controlled slides.
When you walk or slide on ice, the pressure exerted by your weight slightly melts the surface of the ice. Why? Because under pressure, the ice lowers its melting point: it melts at a temperature lower than 0°C. As a result, a thin layer of liquid water facilitates sliding. Be careful, this phenomenon is weak: temperature remains the key factor. When it is very cold (well below 0°C), the effect of pressure is minimal, and the ice becomes harder and grips more. On the other hand, just around 0°C, the ice is particularly slippery—just the right moment to bring out your improvised skates.
Ice has a very ordered molecular structure, formed from water molecules connected by hydrogen bonds. This particular organization creates a regular surface, solid yet surprisingly delicate. When you slide on it, this structuring greatly reduces the friction between your sole or skate blade and the surface itself. In other words, less grip, more glide! At the molecular level, these water molecules grouped in a network facilitate the creation of a thin liquid layer under pressure: as a result, you glide smoothly, as the friction forces decrease significantly.
Natural ice is never completely pure: it contains impurities like dust particles or minerals, which alter its internal structure. These tiny disturbances directly impact how water molecules arrange themselves and slide over one another, sometimes facilitating the formation of a thin layer of liquid water on the surface. The crystalline structure of the ice also plays a role: depending on how its crystals are oriented, the surface becomes more or less smooth and more or less easy to traverse. The less regular the structure, the more defects appear on a molecular scale, slightly altering the degree of slipperiness felt when walking or skating on it.
The world speed record for ice skating is 103 km/h! This achievement was made possible in part due to the thin layer of water on the surface, which significantly reduces friction.
Did you know that adding sand or salt to ice greatly reduces its slipperiness? Sand acts as an abrasive, while salt lowers the melting point, making it easier to melt.
On other planets like Mars, ice also exists in solid form, but its chemical composition differs. For example, there is carbon dioxide ice (solid carbon dioxide), which does not have the same slippery properties as water ice on Earth.
Ice skates glide efficiently due to their high pressure at the blades, temporarily melting the ice at their surface and thus creating a very thin layer of water to facilitate滑行.
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