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Can You Freeze Air into a Liquid?

Yes, you can definitely freeze air into a liquid! Air is mostly nitrogen and oxygen which can be liquefied by compressing and cooling to extremely low temperatures. In this guide, let me walk you through the science behind liquefying gases, how liquid air is produced, its uses, supercool properties, and even tips to make your own liquid air at home.

The Concept of Liquefaction

Liquefaction or condensation refers to the phenomenon where a gas is converted into the liquid state. For this to happen, the gas particles need to lose enough kinetic energy so they can get close together and form molecular bonds characteristic of the liquid state.

There are two main ways to achieve liquefaction:

  1. Lowering the temperature of the gas close to its boiling point
  2. Increasing the pressure exerted on the gas

By combining temperature reduction and compression, all gases can theoretically be liquefied.

Critical Temperature and Pressure

Every gas has a critical point – the precise combination of temperature and pressure at which the gas transitions into liquid. For oxygen, this critical point is -118°C and 50 atmospheres pressure.

Once a gas is cooled below its critical temperature, applying additional pressure allows liquefaction to occur at higher temperatures. This is how gases are liquefied on an industrial scale.

The Composition of Air

Air is primarily composed of:

  • Nitrogen – 78.1%
  • Oxygen – 20.9%
  • Argon – 0.93%
  • Carbon dioxide – 0.04%

There are also trace amounts of neon, helium, methane and hydrogen.

To understand how air can be liquefied, let‘s look at the boiling points of its main components at atmospheric pressure:

Gas Boiling Point (°C)
Nitrogen -195.8
Oxygen -182.9
Argon -185.7

As you can see, all these gases transition to liquid states at extremely low temperatures.

The Process of Liquefying Air

The key steps in liquefying air are:

  1. Purification – Removal of CO2, H2O, and hydrocarbons
  2. Compression – Typically to around 20-30 bars pressure
  3. Cooling – Using heat exchangers to lower temperature incrementally
  4. Expansion – Throttling pressure to complete liquefaction
  5. Storage – In insulated vessels to maintain liquid state

Modern liquefaction plants can produce over 300 tons of liquid air per day! The compression is done using multi-stage compressors and cooling by circulating cold gaseous nitrogen in the heat exchangers.

Let me walk you through a typical industrial process for air liquefaction:

Compression

The air is pressurized to around 20 bars using a compressor. This raises the temperature to around 200°C. Additional stages of compression with intercooling can further increase the pressure.

Purification

The compressed air passes through beds of adsorbent materials like silica gel and activated alumina to remove water, CO2 and hydrocarbons.

Cooling

The air then passes through a series of heat exchangers. First it is cooled to ambient temperatures using cooling water. Then it enters heat exchangers to bring it down to cryogenic temperatures around -170°C to -190°C using cold recycled nitrogen gas.

Expansion

The high-pressure cold air is then throttled through an expansion valve to lower the pressure. This sudden expansion causes flash evaporation, rapidly cooling the air and condensing around 20% of it into liquid.

Separation

The liquid/gas mixture is put through a separator where the liquid air collects at the bottom and cold nitrogen vapor leaves from the top. This cold nitrogen gas is recycled and used to cool the incoming air.

Storage

The liquid air is finally stored in special double-walled vacuum flasks called Dewar vessels to keep it at supercold temperatures.

This entire process requires enormous amounts of compression energy and complex heat exchange systems. Therefore, liquid air production is quite energy-intensive and needs massive industrial facilities.

Applications of Liquid Air

Some uses of liquid air:

Refrigerant

The ultracold temperature of liquid air makes it ideal for cooling applications. It can maintain refrigerated trucks and railroad cars at subzero temperatures for transporting frozen foods and perishable items.

Food Freezing

Liquid air‘s low temperature can freeze foods like fish, meat, fruits, and vegetables very rapidly. This quick freezing prevents ice crystal formation and maintains food quality.

Oxidizer

Liquid oxygen as an oxidizer leads to more energetic combustion when combined with fuels. This property is harnessed as rocket propellants in spacecraft.

Preservation

Biological samples like reproductive cells, viruses, and even organs can be preserved viable for years using liquid air‘s cryogenic temperatures.

Welding

The high purity oxygen separated from liquid air helps produce clean, precision welds for metals. This includes applications like nuclear reactor components.

Chemical Processing

Reactions between gases are enhanced at the low temperatures achievable with liquid air. These are used to produce chemicals like nitric acid and methanol.

Physics Experiments

From studying material properties at supercold temperatures to visualizing complex physics concepts, liquid air has a range of uses in lab experiments and science demonstrations.

The global market for liquid air was over $13 billion in 2019 driven by surging demand from healthcare, mining, construction, and other industries.

Properties of Liquid Air

Let‘s look at some of the remarkable properties of liquid air:

  • Pale blue translucent color
  • Density around 0.9 g/ml
  • Extremely cold temperature of -194°C
  • Viscosity around liquid water
  • Boils vigorously at atmospheric pressure
  • Freezes alcohol, mercury, rubber etc instantly
  • Non-toxic but can cause frostbite on contact

When exposed to ambient air, liquid air undergoes rapid boiling and produces an intense fog as water condenses from the atmosphere. Just 1 liter of liquid air evaporates and produces around 700 liters of gas!

Liquid Oxygen

Liquid oxygen obtained from separating air has an even lower boiling point of -183°C. It is paramagnetic and exhibits a slight pale blue color. Liquid oxygen is a powerful oxidizer and needs careful handling since it can vigorously burn organic materials.

Cool Physics Experiments

Now that you understand the science behind it, let‘s do some supercool physics experiments with liquid air!

Frozen Balloon Hammer

Blow up a balloon and freeze it in liquid air. Once frozen solid, it becomes as hard as a hammer head and can be used to drive nails into wood! The balloon shatters upon impact revealing how brittle materials become at low temperatures.

Banana Hammer

Freeze a banana in liquid air and use it as a hammer to drive nails! The frozen banana is temporarily hard enough to hammer nails before disintegrating.

Bouncing Rubber Ball

A rubber ball becomes completely rigid upon freezing in liquid air. When dropped, it bounces like crazy all over the place instead of absorbing impact. This demonstrates loss of elasticity at low temperatures.

Shattering Rose

A beautiful red rose plunged into liquid air instantly freezes. Lifting it out causes the brittle frozen rose to disintegrate into fragments – a vivid visualization of cryogenic freezing.

These are just a few examples of how liquid air can inspire creative experiments to learn physics concepts. Feel free to explore ideas like magnetic levitation, freezing alcohol into solid blocks, and more!

Liquefying Air at Home

While large-scale liquefaction requires complex industrial facilities, you can liquefy a tiny amount of air at home with basic DIY tools:

What You Need

  • Bicycle pump or small air compressor
  • Pressure cooker
  • Insulated thermos containing isopropyl alcohol and dry ice
  • Funnel and jar
  • Protective gloves and goggles

Steps

  1. Fill the thermos with isopropyl alcohol and add dry ice. This will cool to around -70°C.
  2. Use the compressor or pump to pressurize air to 120 psi in the pressure cooker.
  3. Attach a funnel from the pressure cooker to the thermos. Open the valves to allow pressurized air to bubble through the alcohol-dry ice mixture.
  4. Collect the drips of liquid air from the funnel outlet in an insulated jar.

This simple process can condense a tiny amount of liquid air. While limited, it demonstrates the underlying science and is fun to try out at home.

Remember to take all necessary safety precautions when experimenting with dry ice and pressurized systems.

So in summary, yes air can be liquefied through sufficient cooling and pressurization into an amazing cryogenic liquid with unique properties! I hope you enjoyed learning about the science of freezing air and its supercool applications through my guide. Feel free to connect with me about any questions you have!