The Right Chemistry: Water as a reagent can be useful, or dangerous

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Telling students that you are going to fry an egg in the lecture room doesn’t trigger much excitement until you tell them that you are going to do it with water. You then proceed to pour some water into an aluminum pie pan and cover it with another empty pie pan. An egg broken into the top pan will start to fry within a few minutes. Magic? In a way, yes. The magic of chemistry.

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The “trick” is placing some calcium oxide, or “quicklime,” in the bottom pan. While we recognize water as an extremely important resource when it comes to cleaning, cooking, irrigation or serving as a solvent for chemical reactions, we don’t generally think of it as a reagent in chemical reactions. Yet that simple H₂O molecule can engage in a number of very useful chemical reactions. Indeed, without one of these reactions we would not exist. Photosynthesis, upon which all life depends, is the reaction of water with carbon dioxide to produce glucose and oxygen.

There’s more. A key step in the production of sulphuric acid, the world’s most important industrial chemical, is the reaction of sulphur trioxide with water. Hydrogen gas, vital for fertilizer production, is made by reacting methane with water.

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Calcium oxide has an extreme thirst for water and reacts with it very quickly, hence the term “quicklime.” The product of the reaction is calcium hydroxide, or “slaked lime,” so-called because quicklime’s thirst for water has been “slaked.” This is a highly exothermic reaction, producing enough heat to quickly fry an egg. It is also the technology used in self-heating cans of food or drink. These cans have two chambers with one housing whatever is to be heated while the other contains calcium oxide separated from water by a barrier. Pushing a button on the can breaks the barrier and heat is quickly generated as the quicklime reacts with the water.

The ability to generate heat is far from the main use of calcium oxide. It is critical for the production of cement, which when combined with gravel and sand yields concrete, the most widely-used material in the world. Calcium oxide is produced by heating limestone (calcium carbonate) and is then fired with clay to make cement. Since heating limestone releases carbon dioxide, and the high temperatures needed to react calcium oxide with clay require burning fossil fuels, the production of cement has a colossal carbon footprint. Roughly eight per cent of all global carbon emissions caused by humans are due to cement. But life without cement is unimaginable.

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The setting of cement involves a series of complex chemical reactions that are only partly understood, but the key is the reaction of quicklime with water to form slaked lime that then slowly absorbs carbon dioxide from the air to form insoluble calcium carbonate. Although this does remove some carbon from the air, it does not compensate for the amount of carbon dioxide released in the making of cement.

While it is clear that water can act as a reagent in some extremely useful reactions, it can also initiate some reactions that can have deadly consequences. The world’s worst industrial accident, the Bhopal tragedy is a case in point. Many aspects of the disaster are debated to this day, but there is no doubt that on the evening of Dec. 2, 1984, a massive amount of highly toxic methyl isocyanate gas (MIC) was released from a Union Carbide plant in the Indian city of Bhopal. It quickly enveloped the surroundings and resulted in the immediate death of more than 3,000 people and the eventual demise of more than 15,000. Close to half a million others suffered injuries ranging from blindness to chronic bronchitis.

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Liquid methyl isocyanate was stored in a large tank connected to a pipe through a valve that when opened allowed it to be released into a chamber where it would react with napthol to produce the pesticide carbaryl. Accidental introduction of water into the MIC tank seems to be the most likely explanation for the disaster. Apparently, one day an untrained technician failed to properly shut the valve as he was cleaning the pipes with water. Methyl isocyanate reacts quickly with water to form dimethylurea and carbon dioxide. While these are not toxic, the reaction is extremely exothermic and caused the MIC in the tank to boil. The increased pressure then burst a safety valve resulting in some 40 tons of toxic MIC vapour being spread over most of Bhopal.

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The plant had been designed with a number of safety systems, but these either failed or had been made inoperative due to costs. A refrigeration system to cool the MIC tank and a sodium hydroxide scrubber had both been shut down. The latter was designed to neutralize any escaping methyl isocyanate by reacting with it to produce innocuous products but had been turned off as a cost-cutting measure. Union Carbide claimed, without any real corroborating evidence, that the tragedy was not due to an accident, but was an act of sabotage.

After much legal wrangling, the company agreed to pay $470 million to set up a fund for the victims of the disaster and to build a hospital in Bhopal dedicated to their treatment. The terrible tragedy resulted in the institution of stringent safety measures for the production of chemicals of all kinds, but without doubt has left a huge stain on the chemical industry. A sad part of the story is that carbaryl can be produced by a method that does not require methyl isocyanate at all. However, it is more expensive, and the company chose the cheaper route.

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A final note. When methyl isocyanate reacts with water, the products that form are not harmful. Had the population in the area been told to just cover their head with a wet towel in the event of a chemical leak from the plant, many injuries, particularly to the eyes, would have been avoided. The MIC wouldn’t penetrate, it would have reacted with the water and be broken down.

Water obviously can be a useful or a sometimes dangerous reagent.

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Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.