CDS (II) 2020 General Knowledge · Question 112 of 120

Correct — A, Liquid nitrogen. Long-term storage of living material — blood, semen, embryos, stem cells, tissue samples, seeds in a gene bank — needs a temperature low enough to stop every chemical and enzymatic process, and in practice that means below about minus 130 degrees Celsius. Liquid nitrogen boils at about minus 196 degrees at atmospheric pressure, comfortably past that threshold, and it is the medium the whole field of cryopreservation is built on. What makes it the practical choice rather than merely a possible one is that nitrogen is the most abundant gas in the atmosphere, so liquid nitrogen is cheap and available anywhere, and it is chemically inert, non-flammable and non-toxic, so it will not react with the material it protects. A dewar of it needs no power supply, only occasional topping up as the liquid boils away.

• (b)Liquid helium — Colder — about minus 269 degrees Celsius — but wrong for the job. Helium is scarce and very expensive, and nothing about tissue storage needs that extreme a temperature. It is reserved for work that genuinely requires it, such as cooling superconducting magnets in MRI scanners and particle accelerators.
• (c)Liquid argon — Argon is inert and does liquefy at about minus 186 degrees Celsius, so in principle it would work, but it is far more expensive than nitrogen and offers no advantage over it. Argon's real uses are as a shielding gas in welding and as the filler in some incandescent and specialised lamps.
• (d)Liquid bromine — The odd one out on the list, and dangerous rather than merely unsuitable. Bromine is liquid at room temperature — nothing is being cooled — and it is a corrosive, highly reactive halogen that would destroy any tissue put into it.

Cryopreservation is the storage of cells and tissues at temperatures low enough to halt biological activity. Below about minus 130 degrees Celsius water no longer forms growing ice crystals and diffusion effectively stops, so a sample can be held for years without measurable degradation. Cryoprotectants such as glycerol or dimethyl sulphoxide are added before freezing to stop ice crystals from puncturing cell membranes, and the cooling rate is controlled for the same reason. The word cryogenics covers this whole low-temperature field, including the liquid-oxygen and liquid-hydrogen propellants used in rocket upper stages.

All four options name a liquid, so the item is decided by matching a temperature and a chemistry to a purpose. Bromine goes first because it is not cold at all and is chemically hostile. Helium and argon are both cold enough and both inert, so the deciding factor becomes cost and availability, and nitrogen wins decisively: the atmosphere is nearly four-fifths nitrogen, so the raw material is free and the liquefaction is routine. This is a good example of a question whose answer is settled by economics rather than by physics, which is why eliminating on the chemistry alone leaves three plausible options. In India the same liquid is used in semen banks for cattle breeding, in the National Gene Bank at New Delhi and in hospital blood and stem-cell storage.

• Liquid nitrogen boils at about minus 196 degrees Celsius at atmospheric pressure.
• Cryopreservation requires temperatures below roughly minus 130 degrees Celsius, at which biological activity effectively ceases.
• Nitrogen makes up about 78 per cent of the atmosphere, which is why liquid nitrogen is cheap and widely available.
• Liquid helium boils at about minus 269 degrees Celsius and is used for superconducting magnets in MRI scanners and accelerators.
• Cryoprotectants such as glycerol and dimethyl sulphoxide are added before freezing to prevent ice crystals from damaging cell membranes.

• Choosing helium because it is colder; the question is which liquid is used, and cost and availability decide that.
• Assuming any inert gas will do — argon works in principle but is far dearer than nitrogen.
• Overlooking that bromine is a room-temperature liquid and a corrosive halogen, not a coolant at all.

As this direct application item, as a boiling-point comparison, or through a cryogenics question spanning rocket engines and superconductors.

• practice — not a real PYQ

  The approximate boiling point of liquid nitrogen at atmospheric pressure is

  • (a)−78 °C
  • (b)−196 °C
  • (c)−269 °C
  • (d)−40 °C

  Answer(b) −196 °C — the temperature at which nitrogen boils at one atmosphere, and the working temperature of ordinary cryogenic storage.

• practice — not a real PYQ

  Liquid helium is used chiefly for

  • (a)storing blood and tissue samples
  • (b)cooling superconducting magnets in MRI machines
  • (c)welding as a shielding gas
  • (d)filling weather balloons

  Answer(b) cooling superconducting magnets in MRI machines — the one application that genuinely needs a temperature near absolute zero.