Science of Fireworks – Diwali is festival of lights and colours. We all enjoy the lights and a sound of a bursting cracker once in awhile. Let’s find out what causes the plethora of colours made by fire crackers and learn about other interesting scientific facts you need to know about fire crackers.
1. How do fireworks work and produce sound?
Pic courtesy: https://cdn.instructables.com
The main chemical ingredient used in fireworks is Gunpowder. Explosive reaction of Gunpowder creates huge amounts of gases in a confined space to cause the explosive sound. An explosion takes place when a number of chemical reactions take place simultaneously.
Activation Energy: This is the energy required in the chemical system for reactants to react with each other and result in a chemical reaction.
This same concept is used in the fire crackers, which makes the chemical compounds packed inside the fire cracker combust with oxygen in the air and convert them into other chemicals, further releasing smoke and harmful exhaust gases like Carbon Dioxide, Carbon Monoxide and Nitrogen in the process.
The chemical reaction that happens when the main gunpowder (Potassium Nitrate, Sulfur and Charcoal) charge burns:
2KNO3 (potassium nitrate) + S (sulfur) + 3C (carbon in charcoal form) → K2S (potassium sulfide) + N2 (nitrogen gas) + 3CO2 (carbon dioxide)
How do firecrackers produce sound?
As the gases produced by the above reaction have a very small volume to expand, which is a result of careful engineering maintains a high pressure on the gases which at last produces an explosive sound when the pressure of gases exceeds the forces holding the cartridge. This results in the loud bang produced by the firecrackers.
As we all are aware that not all firecrackers produce the same sound. By controlling the shape of the firecracker cartridge and chemicals inside the tube, we can control the sound made by the firecrackers. As we know different chemicals have a different reaction time. The reaction times to the chemical being burned can be either fast or slow. Slow reacting chemicals are behind the vibrating sound. On the other hand, fast reaction gives an explosive sound. This creates a shockwave of sound causing bangs and thuds.
- Loud bangs, which are the most common, are created by confining the explosion inside a small shell. The gases expand much quicker than the speed of sound and thus, the cartridge explodes creating a bang.
- Crackling noise in the firecrackers comes from lead oxide, which turns into lead atoms as the firecrackers explode. These lead atoms vaporize and the vapours make the crackling sound.
- Whistling sounds takes place due to the narrow tube, which ensure a slow burning of the chemicals. Potassium Benzoate is the Potassium salt of Benzoic acid which is also used as a food preservative. The oxidiser and the Potassium Benzoate burn together, one layer at a time, and is slower than the speed of sound. This emits the gases produced in spurts in the empty half of the narrow tube, which vibrates and thus makes a whistling sound.
2. How do Fireworks produce various colors and multicolored patterns/effects?
Different elements and metal compounds in the fire crackers impart different colours to them. Barium gives green color whereas Magnesium gives a bright white color. The Blue-greens and vivid violet-blues are the most dangerous and difficult to formulate and hence are the most expensive. They are highly unstable and very dangerous. Here is a full list of elements and the corresponding color/effect they impart.
Pic credits; https://www.stevespanglerscience.com
How to create multicoloured patterns/shapes?
As we know that elements impart a color unique to them when combusted, combination of these elements are used to create attractive color combinations and effects in the sky. The shells inside the fire cracker are filled with multiple sections which are ignited individually. After the first section bursts, the next is ignited and bursts and thus the third and it continues further.
We also have seen fireworks making a lot of shapes when bursting in the sky like faces, flags etc.These kinds of shapes are achieved by making a pattern (prenatal image of the fire cracker) on a piece of cardboard. If we have a smiley face pattern on the cardboard, for example, they will explode into a smiley face in the sky.Various different elements are used for different colors and they are placed on the cardboard along the pattern made. The layer which is used to launch the rocket in the air ignites first and the layer containing the pattern ignites while in the air. In fact, you may see several smiley faces in the sky at one time. With shaped fireworks, pyrotechnicians often set off several at the same instant to ensure the shape can be seen from all angles.
3. Why are Sparklers so different from other firecrackers?
Sparklers, which most of us have handled are quite different from the other firecrackers. A sparkler burns for a longer time and produces an extremely bright light. The basic components of the sparklers are fuel, oxidiser, iron/steel powder and a binder. The fuel is charcoal and sulfur. The binder is sugar or starch and these chemicals are mixed with water and form a slurry that can be coated on a wire or poured into a tube. Once it is dried, you have a sparkle. Once lit the sparkler burns from one end to another. The fuel, oxidiser and other chemicals are proportioned in such a way that the sparkler burns slowly unlike an exploding firecracker. The metal fragments start detaching from the resin while combusting, forming a “Sparkling effect”. Also, the combustion reaction in the case of sparkler happens at atmospheric pressure and smoke directly goes into air than going into a small volume chamber preventing the explosion.
A firework contains a variety of chemicals, like the aluminum, iron, steel, zinc or magnesium dust in order to create bright, shimmering sparks. The metal flakes heat up until they are incandescent and shine brightly or, at a high enough temperature, actually burn. A variety of chemicals can be added to create colors.
4. Pollutants released due to fire crackers and why they are so deadly?
Perchlorates:
The gunpowder mentioned above can prove to be unstable and messy thus, the oxidizer in the mixture is often replaced by perchlorates, viz. Potassium perchlorate and ammonium perchlorate, which have become popular in the pyrotechnics industry. Perchlorates in high dosage limit the human thyroid gland’s ability take iodine from the bloodstream, potentially resulting in hypothyroidism.
- Children, infants and especially fetuses suffer the worst from hypothyroidism.
- Perchlorates have been shown to cause thyroid cancer in rats and mice. Though low exposure to the perchlorates does not have a significant effect.
- The smoke from crackers burned charcoal and sulfur fuel contains particulate matter that can get lodged into people’s lungs, an immediate danger for those with asthma or chemical sensitivities. Prolonged exposure to the same can cause lung cancer.
- But the only sign of respite is that both perchlorates and particulates do not have an essentially long term threat on mild exposure.
Metallic Compounds:
The metallic compounds used in to impart colors in the firecrackers also have adverse effects on the environment. These include:
- Strontium, a silvery-yellow metal turns red when it burns, and it is extremely reactive with air and water. Low levels do not have a significant effect but it can be dangerous in high dosages. It is known to cause impaired bone growth in children.
- Aluminum, which turns white when it burns, at high levels can be a threat to the brain and lungs.
- Copper compounds which impart blue colour turn into dioxins when perchlorates in the crackers burn. Dioxins are vicious chemicals that are by products of certain chemical reactions. The most noted health effect of dioxin is chloracne, a severe skin disease. It is a human carcinogen and has shown to disrupt the hormone production and glucose metabolism in the human body.
- Rubidium which imparts a purple colour is a skin irritant.
- Cadmium which is used abundantly in fire crackers is a human carcinogen. Breathing high levels of Cadmium can seriously damage the lungs and consuming it can fluster the stomach, resulting in vomiting and diarrhea. Long exposure can also cause kidney disease, lung damage and fragile bones.
5. Eco-friendly Alternatives and New age fire crackers
A lot of effort is being put into finding the eco friendly alternatives for the fireworks.
One of the method is not to use a charge to launch the firecracker in air but to use the compressed gas. Since 2004, Disney has been using compressed air to launch fireworks at Disneyland in California, which reduces at least the issues of smoky particulates in the air and perchlorates in the water. Research is going on, on fine-tuning alternative propellants that use nitrogen-rich materials in place of perchlorates.
Research is also going into making eco-friendly crackers which cause less harm to the environment. These eco-friendly crackers are made of recycled paper and they do not contain as much chemicals as conventional firecrackers, and thus emit less smoke and noise. Unlike the normal cracker making method, the eco-friendly crackers are based on vacuum combustion method. These crackers produce colourful sparks with a considerable sound and less smoke.
There are still new technologies developing but the best way to avoid pollution is not burning fire crackers this diwali. Say no to firecrackers.
We wish you all a Happy and Safe Diwali.