Slippery Ice
As we have experienced in our area recently, ice is a reality in winter. Watching out for ice on the roads and sidewalks practically becomes second nature this time of year, not to mention falling icicles from rooftops! Let’s explore ice properties a bit more – ready for an easy chemistry lesson?
Ice is solid water. In fact, there are 14 known solid phases! Commonly used, the term “ice” refers to the most abundant phase, ice Ih. It is a crystalline solid that you can see through or may be blue-white depending on what is in the air. The addition of materials changes the appearance of ice (particles, sand, dirt, salt, etc). Ice is formed when liquid water is cooled below 32 degrees Fahrenheit or zero degrees Celsius.
When solid, ice is a mineral of hydrogen oxide. Ice is unusual in that it is the only non-metallic substance that expands when it freezes. The solid state of ice is eight percent less dense than liquid water, which is why ice floats. This is an important factor in Earth’s climate – if ice sank, the entire body of water would freeze, killing practically everything in it. The water molecules in ice are packed close together, preventing it from changing shape. Ice has a very regular pattern with the molecules connected by the hydrogen bonds that form a hexagonal pattern. These crystals have a number of open regions and pockets making ice less dense than liquid water. Since ice has a lower density than water, pressure decreases its melting state and can force ice back into liquid water. In the past it was believed that slippery ice is caused by a thin layer of melted water on the ice, due to pressure. Many scientists now believe that the ice molecules that are in contact with the air cannot bond with the molecules of ice beneath – the transition molecules are in a somewhat-liquid state and act as a lubricant. This is not always the case – the extreme South Pole conditions do not make ice and snow slippery.
Ice, snow, hail, frost, and polar ice caps are examples of water in its solid state. Liquid water freezes at 0 degrees Celsius. The ice can then either stay frozen, melt as temperatures climb, or sublimate, turning directly into a vapor. Sublimation occurs naturally with strong sunlight, winds, low relative humidity and low air pressure. It also occurs in frost-free freezers with a fan and air circulation, which keeps a low relative humidity. However, ice cubes also sublimate in this environment, so they may need to be replaced regularly.
Some interesting ice facts:
- 68.7% of the fresh water on earth is stored in ice caps, glaciers and permanent snow
- Glacial ice covers 10-11% of all land.
- 99% of the ice in the world is found in Antarctica and Greenland
- Ice can create erosion by water seeping into rocks and cracking them open when it freezes and expands.
- “Black” ice can form on lakes as well as roadways, and occurs when the ice forms 6-sided, vertically organized columns with few air bubbles. The ice is transparent, and when on roadways looks simply wet, instead of the slippery stuff it is.
Now that you’ve had your chemistry lesson, go out and enjoy your winter, walk carefully, and watch out for that black ice!
By Gabi Morey, Education Outreach Director
