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On Diwali night a small black pellet is lit and a grey-black snake crawls out, many times longer than the tablet. It looks like magic, but it is a solid breaking down while gas escapes through it. In one Indian study, it was also the smokiest of six common crackers tested.
How does a snake tablet make a long snake?
Heat turns part of the solid into gas and leaves the rest as a light, carbon-rich residue. The gas cannot escape instantly, so it puffs the soft residue into foam, and new foam forming at the bottom pushes older ash out.
Chemist Tom Kuntzleman's explanation on ChemEdX of a calcium gluconate snake describes the foam forming as water vapour "permeates the solid products", with carbon dioxide possibly helping. The classroom sugar snake works the same way. According to the Wikipedia summary of black snake fireworks), baking soda breaks down into sodium carbonate, water vapour and carbon dioxide, the sugar leaves black carbon, and the steam "makes the snake lightweight and airy".
Shop tablets use their own formulas. The same page says that in the 1800s chemists found a black resin, made from petroleum and tar residue treated with strong acids, which "leaves a highly porous black ash" when burned.
Think of a roti puffing on a flame, but happening nonstop from the bottom, so you get a long column instead of one balloon. The same idea of hot gas needing room explains why firecrackers make a loud bang and how a Diwali rocket flies up.
How much gas can a pinch of baking soda make?
Take the sugar-snake recipe and just 1 g of baking soda (NaHCO₃, molar mass 84 g/mol), using the breakdown described on the black snake page).
The reaction is 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, so every 2 mol of baking soda gives 2 mol of gas.
1 g ÷ 84 g/mol ≈ 0.0119 mol of baking soda, so about 0.0119 mol of gas.
One mole of ideal gas fills about 24.5 litres at 25 °C and 1 atmosphere, according to the molar volume entry.
0.0119 × 24.5 ≈ 0.29 litres: roughly 290 mL of gas from just 1 g of powder.
The gas forms far hotter than 25 °C, so it takes even more room. Most escapes, but the fraction trapped in the residue is enough to blow it up many times its starting size. If moles and gas volumes still feel shaky, the Fermi Concept Library gives every chemistry concept its own lesson, from the mole to gas volumes.
Why did the old Pharaoh's serpent disappear?
The original Pharaoh's serpent used mercury(II) thiocyanate. The mercury(II) thiocyanate entry_thiocyanate) says this use "ceased in most countries in the early 20th century due to the toxicity of mercury". The black snake page) adds that a product sold in Germany was banned after several children died from mistakenly eating the leftover solid. Whatever a tablet contains, never touch or taste its ash.
Why is snake tablet smoke harmful?
In a 2019 study in Lung India, Shah and colleagues burned six common crackers five times each and measured PM2.5, the fine particles smaller than 2.5 micrometres. For the snake tablet they placed the monitor 1 foot away and 1 foot high, because "children tend to sit in proximity to this cracker".
The tablet burned for only 9 seconds.
PM2.5 peaked at 64,500 µg/m³, the highest of the six crackers.
Levels stayed raised for about 3 minutes.
For scale, the paper notes that India's 24-hour PM2.5 standard set by the Central Pollution Control Board is 60 µg/m³, and the WHO's 24-hour guideline is 15 µg/m³. A brief peak is not a daily average, but it was over 1,000 times India's daily limit. The WHO also notes that PM2.5 can penetrate deep into the lungs and even enter the bloodstream.
How can families keep snake tablets safer this Diwali?
The study's authors conclude that it "makes prudent sense to avoid the burning of firecrackers". If your family still lights them, use this plan, which draws on tips from the US Consumer Product Safety Commission:
Outdoors only, on a tile or brick, away from doors and windows.
One adult lights one tablet at a time, then moves back quickly.
Everyone watches from upwind, several big steps away, never crouching close.
Keep a bucket of water nearby; never relight or pick up a tablet that did not fully ignite.
Keep babies, pets and anyone with asthma or a heart or lung condition indoors, and follow their doctor's advice.
When it stops, soak the ash, throw it away, and wash hands.
What should you tell a younger child?
"The snake is ash and smoke, and the smoke hurts your lungs. We watch from this line, we never touch it, and when it stops I pour water on it."
If anyone has trouble breathing after the smoke, get medical help straight away. And for another kind of ash that holds its shape, read why incense stick ash keeps its shape.
