Liquid Chlorine?

What is wrong with this picture?

Is it possible for an ordinary plastic bottle with a screw cap to contain liquid chlorine?
Probably not!

Chlorine exists as a diatomic yellow-green gas at room temperature and pressure, that is, chlorine exists as Cl_2(g).
In order to produce liquid chlorine we could:

• lower the temperature of the bottle to change the gas into a liquid at atmospheric pressure.
• raise the pressure within the bottle to change the gas into a liquid at room temperature.
• lower the temperature and raise the pressure at the same time.
  

At 1 atmosphere pressure, the melting point of chlorine is about -101^oC and its boiling point is about -34^oC. So, chlorine will be a liquid at temperatures between -34^oC and -101^oC.
For comparison, your refrigerator is probably set to maintain a temperature of about 4^oC while the freezer has a temperature of around 0^oC, not cold enough to liquefy chlorine! A plastic bottle sitting on the shelf in your garage is not going to be cold enough to store chlorine as a liquid!

Gaseous chlorine could also be changed into a liquid by applying pressure. At room temperature this can be achieved with a pressure about 8 times that of atmospheric pressure, which is highly unlikely to occur in our plastic bottle with the screw cap.

So, the fluid in the plastic bottle labelled "liquid chlorine" is not chlorine. What is it?
It is most likely to be an aqueous solution of sodium hypochlorite, NaClO_(aq).
Aqueous solutions of sodium hypochlorite are produced by bubbling chlorine gas, Cl_2(g), through an aqueous solution of sodium hydroxide, NaOH_(aq) at room tmeperature:
Cl_2(g) + 2NaOH_(aq) → NaClO_(aq) + NaCl_(aq) + H₂O_(l)
When the aqueous sodium hypochlorite solution is mixed with dilute acid, chlorine gas is released:
2H⁺_(aq) + OCl^-_(aq) + Cl^-_(aq) → Cl_2(g) + H₂O_(l)

The chlorine gas that is released can kill bacteria and other microbes, so aqueous solutions of hypochlorites are often used as disinfectants.

Further Reading
Chemical and Physical Changes
Kinetic Theory of Gases

Suggested Study Questions:

1. Identify each of the changes below as either a chemical change or a physical change:
   • freezing water in a freezer
   • boiling water in a kettle
   • cooling chlorine gas to make chlorine liquid
   • boiling liquid chlorine to make chlorine gas
   • bubbling liquid chlorine though aqueous sodium hydroxide solution to form a solution of sodium hypochlorite
     
   • bubbling chlorine gas through water to make hypochlorous acid
2. Name each of the physical changes above.
3. Use the kinetic theory of matter to explain what happens to chlorine molecules when:
   • chlorine gas is cooled to produce liquid chlorine at 1 atm pressure
     
   • chlorine gas is subjected to a pressure of more than 8 atmospheres at 25^oC
   • chlorine gas is cooled to 4^oC
4. Sodium hydroxide has a melting point of 319^oC and a boiling point of 1390^oC at 1 atm pressure. Describe how you could produce sodium hydroxide liquid.
5. Which of the following pure substances could be kept in an ordinary plastic bottle with a screw cap on a shelf in your garage?
   
   • ozone (melting point -192^oC, boiling point -110⁰C)
   • potassium chloride (melting point 772^oC, boiling point 1407^oC)
   • sulfur dioxide (melting point -75^oC, boiling point -10^oC)
   • ethanol (melting point -114^oC, boiling point 78^oC)
     
   
   
   

Chlorine as a Chemical Weapon

World War I, saw the birth of the ANZAC legend*, and also the widespread use of "chemical warfare".

The first gas used by the German military as a killing agent was chlorine gas, also known as bertholite at this time.

Chlorine is a powerful irritant, which can damage the eyes, nose, throat, and lungs. Prolonged exposure to high concentrations, 1,000ppm, can cause death by asphyxiation.
Chlorine gas reacts with water in the tissues of the body to produce hydrochloric acid:

2Cl_2(g) + 2H2O_(l) → 4HCl_(aq) + O_2(g)

The chlorine gas was released from cylinders facing the enemy trenches in a favourable wind. The grey-green cloud of chlorine gas would then drift across the enemy positions.
The density of chlorine gas at 0^oC and 101.3kPa (1 atm) is 3.2g/L, while the density of air at the same temperature and pressure is 1.2754 g/L. Because chlorine gas is more dense than air, it would be more concentrated at the bottom of the trench, and less concentrated at the top. Those who suffered the worst effects were often the wounded lying on the ground or on stretchers.

Initially, German troops were issued with gauze pads filled with cotton, and bottles of bicarbonate solution. When the chlorine gas was to be released, the soldiers would dampen the gauze pad with the bicarbonate solution and breathe through it. The bicarbonate would neutralize the hydrochloric acid produced. If sodium bicarbonate solution were used, the reaction would be:

HCl_(aq) + NaHCO_3(aq) → NaCl_(aq) + CO_2(g) + H₂O_(l)

As other gases were being developed for use as chemical weapons, the need for better protection became important. One of the earliest devices was a hood with eyepieces. The hoods could be impregnated with sodium hyposulphite (sodium thiosulfate). Sodium thiosulfate reacts with dilute acids to produce sulfur, sulfur dioxide and water:

Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O

The gas mask was developed later. It was composed of an impervious mask and a box respirator or canistor. Air came through the canister which contained charcoal and granules of soda-lime, a mixture of sodium hydroxide and calcium hydroxide.

*ANZAC (Australian and New Zealand Army Corps) Day is commemorated each year on 25th April, marking the anniversary of the first major military action fought by Australian and New Zealand forces during the First World War. ANZAC Day ceremonies, which are held in towns and cities all over Australia and New Zealand, typically include an introduction, a hymn, a prayer, an address, the laying of wreaths, a recitation, the Last Post, a period of silence, and either the Rouse or the Reveille, and the national anthem. Anzac Day has evolved to acknowledge the sacrifice and service of subsequent wars.

Further Reading
Temperature Conversions
Density Calculations
Elements and Compounds
Writing Ionic Formula
Balancing Chemical Equations
Molecular Mass
Definitions of a Mole
Ideal Gas Law
Acid-Base Titration Calculations

Study Questions

1. Draw a table listing each element and each compound mentioned in the article above.
2. In April 1915, the German Army is said to have stockpiled 168 tons of chlorine which was contained in 5,370 cylinders. on average:
   • how many kilograms of chlorine gas was contained in each cylinder?
   • how many moles of chlorine gas was contained in each cylinder?
   • what volume would this moles of gas occupy at 25^oC and 101.3 kPa (1 atm)?
     
3. Using your calculations in part 2, calculate the density of chlorine gas at 25^oC and 101.3 kPa (1atm) in g/L.
4. Compare the density of chlorine gas calculate in part 3, to the density of chlorine gas given in the article above. Account for the difference in the two density values.
5. Convert 1,000ppm chlorine gas to a concentration in:
   • mg/L
   • g/L
   • mol/L
6. What mass of HCl could be produced from 1L of 1,000ppm chlorine gas?
7. What mass of sodium bicarbonate would be necessary to neutralize the amount of hydrochloric acid produced in question 6 above?
8. What mass of sodium thiosulfate would be necessary to neutralize the amount of hydrochloric acid produced in question 6?
9. Write a balanced chemical equations for:
   • the reaction between hydrochloric acid and calcium hydroxide
   • the reaction between hydrochloric acid and sodium hydroxide
10. Explain why a gas mask containing soda-lime might be preferable to one containing sodium thiosulfate.