How do fireworks work?
Get ready for the Swiss national day’s fireworks with a bit of chemistry
The Swiss national day has been celebrated on the first day of August for the past 127 years. This day has traditionally been accompanied by breathtaking display of colorful fireworks. In Basel, the celebrations take place by the river Rhine on 31st July and include synchronized fireworks, which are lit from two boats in the middle of the river.
It comes by no surprise that fireworks represent one of the most known uses of chemistry, at least when it comes to children and general public. The magnificent explosions are presented in a wide array of colors, shapes, and patterns with precisely timed sound and smoke effects. If these components are combined well, the overall effect ignites powerful and beautiful memories.
How is it all possible? The answer is quite simple; the secret of a successful fireworks show lies in great planning and precise science.
The basic recipe
Unlike with other types of explosives, fireworks require quite a complicated recipe. First, you need fuel. The so-called black powder — a mixture of charcoal, powdered sulphur and potassium nitrate — is often used. Alternatively, flash powder, consisting of powder aluminum and potassium permanganate, is used for the effect of very fast and bright flashes or light, capable of fast ignition of the remaining materials.
The mixture also contains an oxidizing agent (nitrates or perchlorates), causing other substances to lose electrons and thus become oxidized.
A typical reaction of the gunpowder combustion is the following:
74 KNO3 + 30 S + 96 C + 16 H2O → 56 CO2 + 35 N2 + 14 CO + 3 CH4 + 2 H2S + 4 H2 + 19 K2CO3 + 2 K2S + 2 KSCN + 7 K2SO4 + 8 K2S2O3 + (NH4)2CO3 + C + S
This reaction is exothermic — this means that a lot of energy is released. It is exactly this energy that is necessary for creating color.
The rest of the complicated mixture contains various additives, binders, propellants, sparklers and initiators inducing smoke and sound effect.
Heavy metals and colors
The most important part of a fireworks mixture if the source of the vivid colors. Several metal elements come into play here, thanks to their ability to get their electrons excited by absorbing thermal energy.
Upon excitation, the electrons in the metal atoms get into a high-energy state. His is an unstable state for them where they can stay only for a small fraction of time. The electrons will soon go back into their original state, releasing the absorbed energy as photons, carriers of light.
The difference between the electron energy levels determines the wavelength of the emitted light — or in other words, its color. Since the difference between the two energy levels (base vs. excited electron level) varies from element to element, we can produce a whole array of colors, from red and green to white and blue.
For creating very bright and intense colors, powdered magnesium is often used. Strontium (Sr) and sodium (Na) salts are used for red, calcium (Ca) salts for orange fireworks. Green belongs to Barium (Ba) salts, blue to copper (Cu) and pink to lithium (Li) containing compounds.
Planning the patterns and shapes
As each used material needs different time for combustion, fireworks are manufactured in layers and various 3D patterns.
Aerial fireworks come in form of shells, each containing four basic components. The paper container is filled with black powder containing the so-called stars. These pyrotechnic pellets contain the color-inducing metals and metal salts. Last component is the fuse, which allows the pellet to be first brought high in the sky before the actual colorful explosion takes place.
Layering these materials allows for creating fireworks that explode in waves. Alternatively, positioning the stars within the shell in a pre-planned 3D pattern can create various shapes, from bubble-like explosions to the famous willow firework, evoking a shower of golden sparks.
Health risks
You might notice this Tuesday that soon after the first fireworks explosions, the air will be filled with heavy grey smoke, causing people to cough and get irritated eyes. Upon explosion up in the sky, pollutants, particles of unburned metal salts and other materials are dispersed onto the watching crowd and can be detected in the environment hours after the show.
No matter how beautiful, fireworks pose a serious health risk. Beyond the noise pollution, the composition components are a source of great concern. For instance, several toxic substances, such as barium oxide (BaO) or barium hydroxide (Ba(OH)2) are released upon burning barium chloride (BaCl2). Both of these compounds have a high solubility in water, which makes them easy to infuse into the ground, rivers and lakes.
Ironically, the compound responsible for creating beautiful green-colored fireworks represents a huge environmental problem from the perspective of green chemistry. Multiple research groups all around the world are therefore working on the development of less toxic alternatives for creating barium-free green fireworks. These new organic compounds are often nitrogen-rich and combined with less polluting metals, such as copper. Other approaches based on tetrazoles are targeting the smoke issue and can create smoke-free explosions.
Nonetheless, the implementation of these innovative compounds is slow and years may pass until we see them being used on a commercial level. When you will be watching the Swiss national day’s fireworks in a few days, make sure to stay safe. Limit the dangers by only watching the fireworks, do not bring your own. In addition, perhaps think about an alternative to chemically illuminated sky in the form of a much safer laser show.
