Freezing point

The temperature at which a liquid freezes or changes from a liquid to a solid state at atmospheric pressure is known as the freezing point. The most well-known freezing point is that of water at 32°F or 0°C.

In scientific terms, freezing, also known as solidification or crystallization, is a first-order thermodynamic phase transition in which liquids become solids when their temperature drops below their freezing point. A strongly related measure is the melting point, the temperature at which a solid changes from solid to liquid state at atmospheric pressure.

The melting and freezing temperature for most substances is approximately standard and unique to that substance. However, certain substances have variant solid-to-liquid transition temperatures, and some substances have the ability to supercool, which means they remain a liquid even when the temperature is lowered to below their freezing point. For this reason, the melting point rather than the freezing point is considered the characteristic property of a substance.

Background

Water's normal freezing point is 32°F or 0°C; however, water can be supercooled or undercooled to form a new thermodynamic phase at

-55 F or -48.3 C. Cooled at an even faster rate, water can become a non-crystalline solid or what is known as glass. Scientists estimate water's glass transition temperature to be approximately -215 F or -137 C.

Other instances of lowering the temperature of a liquid below its freezing point without it forming a solid include freezing rain (which unlike hail and ice pellets is made up of solely liquid droplets), cumulous clouds, and supercooled water droplets in the stratosphere that sometimes form ice on aircraft wings and interfere with avionic instruments.

While it is possible to cool a liquid below its normal freezing point via supercooling, there are no liquids that do not freeze. For the property of "never freezing" to occur, a liquid's freezing point would have to be the lowest temperature matter is capable of reaching, which is absolute zero and measures 0 degrees Kelvin or -459.67 degrees Fahrenheit. To date, all known liquids freeze when sufficiently cooled.

In addition to temperature, freezing and melting points are affected by the amount of atmospheric pressure. To facilitate comparison testing of chemical and physical processes in different locations, scientists have established standard sets of conditions. The International Union of Pure and Applied Chemistry (IUPAC) has established the standard temperature and pressure (STP), and the National Institute of Standards and Technology (NIST) has established the normal temperature and pressure (NTP).

The STP, which is defined as 0°C and 1 atmosphere of pressure, is used in many industrial, commercial, and chemical settings to compute thermodynamic tabulations where properties of matter such as density and viscosity vary with changes in temperature and pressure. In the field of aeronautics, the International Standard Atmosphere (ISA) is utilized, specifying temperature, pressure, density, and sound speeds at various altitudes up to 65,000 feet above sea level.

Overview

Much of what determines the freezing point of a liquid is the interaction among its molecules. Liquids with a strong force between molecules possess a high freezing point while liquids with a weak force between molecules possess a low freezing point. In what is called the Mpemba effect, warmer water freezes more quickly than cooler water because of the faster rate at which its molecules store and release energy.

Practical applications of these principles include sprinkling salt on icy roads and walkways during the winter, which lowers the freezing point of water and thus prevents new ice from forming or melted snow from refreezing. The freezing point of water is also affected by combining it with other liquids, such as adding antifreeze to a car's cooling system, which lowers the freezing point and makes it safe to drive in below-freezing temperatures.

Freezing is also referred to as an exothermic process, wherein heat and pressure are released as a liquid changes into a solid. Although no rise in temperature is noticeable in the substance, heat is continually being released throughout the freezing process. The process is known as the enthalpy of fusion, and the energy released, called latent heat, is the same energy required to melt the same amount of the solid substance.

In a related process, freezing-point depression occurs when a solute is added to a solvent in order to decrease the freezing point of the solvent. Practical technical applications include adding salt to water, adding ethylene glycol to water, and mixing two solids in a powdered drug. Freezing point depression also causes sea water to remain liquid at below-freezing temperatures (in the case of pure water, 0 C/32 F).

Liquids such as milk and soda have freezing points that are slightly lower than the freezing point of water, which is 32 degrees Fahrenheit. In the case of milk, the presence of water-soluble substances brings down the freezing point to below that of water (approximately 31.06 degrees Fahrenheit). In the case of soda, the addition of carbon dioxide lowers the freezing point of water, while the further addition of sugar lowers the freezing point even more (down to approximately 28 degrees Fahrenheit). Following these same principles, the freezing point of wines and champagne is also lower than that of water (between 15 and 22 degrees Fahrenheit) because of the liquid composition of water and alcohol. Therefore, experts recommend storing champagne bottles at temperatures of 40 to 60 degrees Fahrenheit and not chilling the bottles for more than fifteen minutes in the freezer prior to serving.

On the other end of the spectrum, the temperature at which silver changes from a liquid to a solid state (the freezing point of silver) is approximately 1,762 degrees. Silver's equally high melting point is why it remains a solid at room temperature.

Tungsten is the chemical element with the highest melting point, making it ideal for use as a filament in light bulbs. Low-temperature helium, on the other end of the spectrum, does not freeze at all under normal pressure but only at pressures twenty times greater than normal atmospheric pressure.

Finally, the food industry uses a process called flash freezing to protect perishable foods by purposely subjecting them to temperatures below water's freezing point. Placed in direct contact with liquid nitrogen or subjected to extremely low temperatures, the rapid freezing speed directly affects the crystallization process. In addition to food preservation, flash freezing is also used to freeze biological samples via submergence in liquid nitrogen or in an ethanol and dry ice mixture.

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