# What Causes a Lower Freezing Point?

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Two kinds of changes, one chemical and one physical, can affect the freezing point of a substance. You can lower the freezing point of some liquids by mixing a second, soluble substance into them; this is how road salt keeps melt-water from refreezing in cold temperatures. The physical approach, changing the pressure, can also lower a liquid’s freezing point; It can also produce unusual solid forms of a substance not seen at normal atmospheric pressure.

#### TL;DR (Too Long; Didn't Read)

Antifreeze lowers the freezing point of water, keeping it liquid at low temperatures. Both sugar and salt will do this as well, although to a lesser extent.

## When Molecules Freeze

Electrical forces between molecules determine the temperatures at which a substance freezes and boils; the stronger the forces, the higher the temperature. Many metals, for example, are bound by strong forces; the melting point of iron is 1,535 degrees Celsius (2,797 degrees Fahrenheit). The forces between water molecules are considerably weaker; water freezes at zero degrees C (32 degrees F). Solvent mixtures and pressure variations reduce the forces between molecules, lowering the freezing point of liquids.

## Mixing it up

By mixing one liquid with another compatible substance, you lower the freezing point of the liquid. The substances must be compatible to ensure complete mixing; oil and water, for example, separate and will not change the freezing point. A mixture of table salt and water does have a lower freezing point, as does a water-alcohol mixture. Chemists can predict the freezing-point temperature difference by applying a formula that takes into account the amounts of the substance involved and a constant associated with the second substance. For example, if you calculate for water and sodium chloride and the result is -2, that means the mixture’s freezing point is 2 degrees C (3.6 degrees F) lower than for pure water.

## Taking the Pressure Off

Changes in pressure can raise or lower the freezing point of a substance. Generally, pressures lower than 1 atmosphere lower the temperature at which a substance freezes, but for water, a higher pressure gives a lower freezing point. The force from a pressure change figures into the molecular forces already at play in a substance. For water at low pressures, vapor directly turns to ice without becoming a liquid.

## Amazing Hot Ice

Water has several solid phases, each observed at different amounts of pressure. Standard ice, which scientists call “Ice I,” exists at atmospheric pressure and has a characteristic hexagonal crystal structure. At temperatures below minus 80 degrees C (minus 112 degrees F), cubic ice crystals can form from vapor at 1 atmosphere of pressure. At high pressures, exotic types of ice form; scientists identify them as Ice II to Ice XV. These forms of ice can remain solid at temperatures exceeding 100 degrees C (212 degrees F) -- the boiling point of water at 1 atmosphere of pressure.

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