We all feel a profound connection with the natural world. E O Wilson called this sensation biophilia: ‘the urge to affiliate with other forms of life’. That sense of connection brings great emotional satisfaction. It can decrease levels of anger, anxiety and pain. It has undoubtedly helped our species to survive, since we are fundamentally dependent on our surrounding environment and ecosystem. But lately, biophilia has spawned an extreme variant: chemophobia, a reflexive rejection of modern synthetic chemicals.
In science education, chemistry is one of the disciplines that involves regular hands-on work in a laboratory. While teaching students the intricacies of chemistry presents no exceptional risk, the very real dangers posed by many chemicals demand a higher level of safety consciousness and preparedness. This general overview outlines sensible security precautions for high school and college chemistry labs.
The Importance Of Documentation
Fortunately, in a classroom setting, all of the chemicals being used will be well understood. This means information on their potential risks is widely available. This information must be used to ensure that each substance used is treated with the proper respect for the dangers it poses.
The first source of information for any chemical is the label it carries. These always describe their hazards, but labeling may be incomplete. A more authoritative source for hazard information is the material safety data sheet (usually referred to as an MSDS) for the substance. A comprehensive reference collection of MSDSs is an integral part of every laboratory, and this collection needs to be freely available to all teachers using the classroom’s chemical supply.
Equipment And Facilities
At the high school or college level, chemistry experiments demand their own dedicated laboratory spaces. These labs should meet all state and national safety requirements and cannot be used for teaching other subjects. Even the scheduling of laboratory use must be geared towards safety. Adequate free periods must be included every day for cleaning the lab and disposing of chemicals.
Chemicals need a dedicated, lockable storage room equipped to contain them safely. A prep room is also required for teachers to use. This needs equipment similar to the lab room albeit on a smaller scale. For all three of these spaces, ventilation is a critical concern. Ventilation hoods should be used in the lab itself and all of the air removed from the lab must be vented outside.
Full safety equipment needs to be available for everyone in the laboratory while chemicals are in use. This includes both permanent safety facilities (e.g. eyewash stations, first aid kits, etc.) and personal protective equipment (PPE), including goggles. Goggles for use in chemistry labs must conform to stricter standards than other forms of eye protection to ensure that they protect against both flying debris and liquid splashes.
Planning And Preparing
Every chemistry lab needs thorough safety plans for both general and specific chemical risks. While standardized materials including the safety documentation discussed above can be used to prepare safety plans, each teacher responsible for leading classes in the lab has a responsibility to set out his or her own safety measures.
Customized safety preparations should take the specifics of the facility and the coursework into consideration. Methods for calling for help, evacuating the lab, and documenting incidents will vary based on the layout of the facility and its resources. By designing their own safety plans, teachers will be better prepared to enact them in the event of an accident.
The Teacher’s Role
A chemistry teacher has many responsibilities beyond instruction and safety planning. One of the most important of these responsibilities is teaching his or her students to share a healthy respect for the hazards posed by chemicals. Teaching and testing them on basic safety precautions and lab-specific emergency procedures is just a start.
Students should learn to understand the intricacies of chemical labeling before working with hazardous chemicals. (For example, the terms danger, warning, and caution are each distinct, indicating decreasing levels of risk.) At the college level, where students may be working independently and designing their own experiments, teaching them to read the MSDS is strongly recommended. For younger students teachers can often make use of intermediate-level warning documentation (e.g. CLIPs, Chemistry Laboratory Information Profiles) to give them adequate chemical reference materials.
Keeping students safe in the laboratory is not a difficult job. It requires a heightened sense of awareness and an amount of preparation commensurate with the hazards posed by the chemicals involved. When preparedness is combined with proper facilities, equipment, and training, schools labs can be safe places to learn through direct experimentation with all but the most dangerous of chemicals.
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About the artist: Liu Bolin imbeds himself and others into the photograph, declaring their position as individuals within the catastrophic incident, thus calling viewer’s attention to the aftermath and investigation of the disaster. Through recreating the imagery of the damage and devastation caused by the explosion, the project is Liu’s attempt to reveal social issues in China, as well as to reflect on the complex relationship between the past and the present, the reality and the illusion, as well as individuality and society. Visit Liu Bolin’s gallery page here.
As a Chemistry teacher, my initial reaction to the enormous explosions at a hazardous chemicals storage facility in Tianjin, China this week was a need to find out what exploded and why. As soon as the news broke, I started following #Tianjin on Twitter and getting alerts from Google News. Here’s what I’ve learned about the Chemistry behind these two fatal blasts. We know there were several dangerous chemicals on site. We also know that firefighters were present at the facility putting out a fire before the first explosion. The second explosion was much larger than the first, with the two blasts measuring the equivalent of 3 and 21 tons of TNT, respectively. The second, larger blast was so powerful that it caused a magnitude 2.9 earthquake in the surrounding area. For a surface explosion to cause a measurable earthquake is rare.
Here’s my understanding of what happened.
Stage 1: Fire
An unknown substance caught fire inside one of the storage containers at the facility. Firefighters arrived at the scene to douse the flames with water.
Stage 2: Water touches calcium carbide, producing acetylene gas
CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g) ΔH = -127.7 kJ/mol
Calcium carbide, CaC2(s), is an unstable compound that’s used in the production of acetylene (ethyne) and also in steelmaking. When water (or moist air) touches calcium carbide, it fizzes gently, releasing acetylene gas, C2H2(g), which, when mixed appropriately with air, explodes upon ignition. The reaction above is only slightly exothermic, and the ethyne gas released is colourless and odourless: it’s possible that the firefighters didn’t even notice that the gas was being produced.
Stage 3: Flames ignite the acetylene gas, causing the first explosion
After the ethyne had mixed sufficiently with the surrounding air, one part of this explosive gas mixture was ignited by the pre-existing flames, causing the first explosion.
C2H2(g) + 5/2O2(g) → 2CO2(g) + H2O(g) ΔH = -1299 kJ/mol
Eyewitness reports have estimated this first explosion to be equivalent to 3 tons of TNT, which equates to 12.5 million kilojoules of energy. Using n = E/ΔH, we find that around 9662 moles of ethyne appears to have exploded. Using V = n×VM, we can calculate that at 25°C and 1 atm of pressure, that explosive gas would have occupied a volume of 236719 litres. Using r = (3V÷4π)1/3, we can approximate the ethyne gas to have occupied a sphere 76 metres in diameter, which is (very approximately) consistent with what we’ve seen in the video footage.
Interestingly, we can do a simple stoichiometric calculation using m = n×Mr and calculate the initial mass of calcium carbide that decomposed: 9662 × 64.1 = 619 kilograms. At a density of 2.22 g/cm3, those 619 kilograms would have occupied 279 litres in powdered form: this is about the same size as three large luggage cases.
A quick search on Chinese wholesale directory Alibaba.com shows that very few companies offer calcium carbide in such small quantities, which might help narrow down which company was responsible. Interestingly, the raw material for that first explosion was worth a mere US$400 at 2015 wholesale prices… but the consequential damage was far more costly.
Stage 4: High temperatures caused nearby ammonium nitrate to detonate at >240°C, causing the second explosion
Temperatures of over 3000°C were generated by the combustion of the ethyne in stage 3. The immense heat from that initial fireball heated the surrounding containers to above 240°C, which initiated a runaway decomposition reaction of ammonium nitrate, NH4NO3(s), which was stored nearby. The reaction is shown below.
NH4NO3(s) → N2(g) + 2H2O(g) + 1/2O2(g) (ΔH uncertain)
The enthalpy change for the reaction above wasn’t easy to find, but this book by Sam Mannan claims it to be 0.175 million kilocalories per tonne, or 732,000 kilojoules per tonne. Analysis of the video recordings have estimated this second explosion to be around 21 tons of TNT equivalent, which equates to 88 million kilojoules of energy. Using calorimetry formula m = E/heat of combustion, we can estimate the mass of ammonium nitrate in this second explosion to be 8300 tonnes, which seems extremely high: four times as big as the Texas City Disaster of 1947. Either the second Tianjin explosion was the biggest ammonium nitrate disaster in history, or I’ve made an error in this part of the analysis. Let’s wait for more information and see.
The above reaction has caused hundreds of fatalities worldwide in the last 100 years. Smaller incidents occur every 2 or 3 years worldwide, around half of which are fatal. Gases are produced under extreme temperature and pressure, which expand outwards and destroy almost everything in their path. Ammonium nitrate is supposed to be handled and stored under very strict government regulations. These rules aren’t always followed (or understood) in rapidly-developing countries such as China.
The products of these two explosions are calcium hydroxide, carbon dioxide, water vapour, nitrogen and oxygen, which pose zero risk to nearby residents. However, the main concern now is that other (non-flammable) hazardous chemicals such as sodium cyanide, NaCN(s), might have been tossed into the air following the first two explosions. Residents living within 3 kilometres of the blast site have been evacuated as a precaution.
Fortunately, satellite imagery shows that almost all of the smoke plume was blown eastwards over the ocean, and not back westwards and back onto the city. We’ll get a clearer picture when China’s chemical experts report their findings in the next few days or weeks.