Ice

Everyone knows that water is important in our live. Without liquid water we could not live. Water also occurs naturally as a solid called ice ( and as gas, water vapor). Ice, too, is important in our lives.

We usually think of ice as something used cool beverages in summer, but it has other important uses. For example, we use it to keep food from spoiling.

Ice can also be dangerous. Ice in a frozen harbor can crush the hulls of ships. Floating island of ice the great icebergs of the North Atlantic have sunk many ships and caused much loss of life. And you know how dangerous icy road can be.

Ice from water

Let’s take the case of a small pond and see what happens as it freezes. During the summer the water becomes warmer. It absorbs heat from the sun, and it is in contact with the warm air. As winter approaches, the air temperature drops. The water then loses heat to the air, and its temperature falls.

To understand what happens next, you have to know another fact about water. It is heaviest at 40C (390F). At temperatures higher or lower than 40C, it is lighter.

As the surface of the pond cools to 40C, the water on the top becomes heavier and sinks to the bottom. When it sinks, it pushes warmer and lighter water up from below. The new surface water cools, sinks, and pushes more bottom water the surface. This circulation continues until all of the pond reaches a temperature of 40C.

When all water is at 40C, further cooling of surface water only makes it lighter. Therefore it stays at the surface. As a result, the pond starts to freeze at the surface, while the water at the bottom remains at 40C (390F).

Because water is heavier than ice, plant and animal life can survive in the water at the bottom. If ice were heavier than water, ponds and lakes would freeze from bottom. Few living things would survive in them.