How water freezes - water freezing explained

How water freezes

When a lake freezes the water on top is chilled until it reaches 4⁰ C or 4 degrees above freezing point. It then drops to the bottom and the water underneath comes to the top. This takes place until all the water in the lake is at 4⁰ C at which time the surface water is chilled below the 4 point as soon as it reaches 0⁰ C ice begins to form on the surface and the lake freezes over. If it were not for this unusual property of water, a lake would freeze solid since the ice thus formed would sink to the bottom, and gradually the entire lake would be ice, becoming so from the bottom up; actually it freezes from the top down.

In freezing, water always expands. Since pressure opposes expansion, pressure opposes freezing, and water will not freeze at 0⁰ C. Under pressure; hence any pressure on ice will melt it. That is why you can skate on ice and not on glass. The pressure of the skates which bear your weight, distributed over such a small area on the ice, melts the ice directly under your skates and causes a thin film of water to form between the ice and the slates. On this thin film, you slide. The extremely thin film of water created by your weight freezes the instant you leave it.

 This same principle, known as regulation, can be illustrated by attaching two heavyweights to the ends of a wire and putting the wire on a chunk of ice. In a short time, the wire will go clean through the cake of ice leaving it whole as before.

When salt crystals are thrown upon ice, the ice melts. That is how it helps to remove ice from pavements and steps in the wintertime. Why does the ice melt when salt is added? To begin with, it is well known that when any soluble solid is added to water, the boiling point of the solution is raised and its freezing point is lowered. Now, unless the air surrounding the ice is at 0⁰ C or lower, there will be a thin film of water around the ice. When the salt is added, a saturated solution of salt is formed. But the freezing point of a saturated solution of salt is formed. 

However, the freezing point of a saturated solution of salt is 21⁰ C. As a result, the ice continues to melt, absorbing heat from its surroundings, which happen to be the salt, until the temperature reaches -21⁰ C. or at least considerably below 0⁰ C if insufficient salt is used. This fact is taken advantage of in the old-fashioned homemade ice cream freezer. The liquid milk and cream, sugar, and flavoring are poured into a cylinder which is rapidly revolved while surrounded by a mixture of ice and salt. The temperature of the liquid in the cylinder is gradually brought below the freezing point and the liquid freezes into ice cream.