Dry ice

Dry ice is solid carbon dioxide (CO2) that sublimates under atmospheric conditions at -78.48 ° C, which leads directly into the gas phase without first melting.

On earth is not dry naturally, in the CO2 - rich atmosphere of Mars but it freezes in winter in higher latitudes and forms the well-known polar caps. Some meteoroids contain large quantities.

Properties

Dry ice is a white, eisähnlicher and odorless solid. Dry ice sublimates at atmospheric pressure at -78.48 ° C ( 194.67 K) and not required for this phase transition 571.1 kJ / kg (or 25.1 kJ / mol). Since the triple point of carbon dioxide in Pt Tt = 216.58 K ( -56.57 ° C) and pt = 5.185 bar is located, carbon dioxide can melt only above 5,185 bar. The density is about 1.56 g · cm -3, which is above that of water. Pc is the critical point of carbon dioxide is at a temperature of Tc = 304.13 C ( 30.98 ° C) and a pressure of 73.75 bar = pc.

1 m³ of dry ice depending on the predominant compression pressure about 1500 kg for 1 m³ of gaseous carbon dioxide weighs only 1.98 kg at 0 ° C and 1013 hPa. This means a change in volume of the 760 -fold in the sublimation.

Production

Dry ice is produced by the liquefied carbon dioxide is expanded under pressure. A portion of the carbon dioxide vaporizes, wherein the rest extracts the heat required for evaporation and thus cooling. The result is this so-called frozen carbon dioxide snow. This is then, depending on the application, is pressed into the desired shape.

Small amounts of dry ice can be produced by placing a CO2 bottle upside down and turning up. The discharged liquid CO2 evaporates for the most part immediately and thereby cools so much on that part freezes as CO2 snow. In addition to the displacement of atmospheric oxygen - - According to this principle, CO2 extinguishers that work to escape the heat, and the burning materials to cool below flash point.

Use

Dry ice will be used primarily as a coolant.

Industry

Medicine and Biology

Event Technology

• In the event and stage ground fog effects are rare today produced by dry ice, is poured on the hot water, as by smoke machines. The fog that is produced by dry ice, but has the advantage that it remains cold due to its near the ground and does not fill the entire space as fluid mist. See: fog machine. Increasingly come here fog fluids for use.
• Because located below water dry ice visually creates the impression of bubbly boiling water, it is occasionally used in film technology to do so. From real boiling water, the process differs by filled with fog and CO2 water bubbles.
• In restaurants and hotels to slow the melting of ice sculptures.

Hazards

When dealing with dry gloves and goggles should be worn. In case of accidental skin contact, it sublimates Although on its surface and forms a thin protective layer of gas, the direct contact to the skin prevents ( Leidenfrost effect). Gets dry ice still for several seconds a firm contact with the skin, the sublimation of the ice is broken and remains, similar to the touch extremely cold metal objects stick to the skin. To prevent long-term damage to the skin and the underlying tissue layers, it must be immediately removed (demolished ), since otherwise there will be a cold combustion, in which the tissue dies off in a few seconds. Such damage takes no account of tissue layers and spreads conically inwards from. This process is used medically in the treatment of warts.

When storing indoors, the resulting carbon dioxide displaces the air on the ground due to its higher density. During sublimation, the gas expands to the 760 times the original volume, and therefore small amounts of dry ice can completely fill a space with gas. At concentrations above 5 % it is suffocating at even sufficient oxygen content of the air we breathe. In closed containers, a dangerous gas pressure can build up.

Commercial dry ice for industrial use may contain impurities and therefore should not be used for cooling of food ( drinks ). Ingestion of dry ice is extremely dangerous due to risk of suffocation and frostbite.