Why baking soda makes boiling sugar explode into foam
Hot syrup is almost pure sugar with no water left; baking soda breaks apart into carbon dioxide gas; the gas blows bubbles that set into glass.
What you saw
A pan of amber syrup, a spoon of white powder, and a second later a foam climbing the sides of the pan and then freezing in place. Nothing is burning. Two things are happening at once, and the recipe only works because they happen in the right order.
Sugar past the point of no return
A sugar syrup is sugar dissolved in water. As it boils, water leaves and the temperature climbs, because a stronger syrup boils hotter than a weaker one. The temperature is a readout of how much water is left:
| Stage | Temperature | Water left | What a cooled drop does |
|---|---|---|---|
| Soft ball | 235–240°F | 113–116°C | about 15% | squashes flat |
| Hard ball | 250–265°F | 121–130°C | about 8% | holds a firm ball |
| Hard crack | 300–310°F | 149–154°C | under 2% | snaps like glass |
At hard crack there is almost no water left. What is in the pan is molten sugar: it will set hard and glassy the moment it cools, and it will set in whatever shape it is in at that moment.
Baking soda comes apart in heat
Baking soda is sodium bicarbonate. It is stable on the shelf, but above about 176°F | 80°C it starts to decompose, and the reaction runs fast in a 302°F | 150°C syrup:
2 NaHCO3 → Na2CO3 + H2O + CO2
baking soda → washing soda + water + carbon dioxideEvery two molecules of baking soda release one molecule of carbon dioxide gas. Two teaspoons is roughly 9 grams, enough for about a litre of gas at the temperature of the syrup. That is the foam.
Why the bubbles stay
In a pot of water the gas would escape. In hard-crack syrup it cannot: the syrup is thick, it is cooling fast once it leaves the heat, and it sets glassy within seconds. The bubbles are trapped mid-rise. Cut a piece and you see them, round and separate, each one a pocket of carbon dioxide that was there when the sugar froze.
Whisking for only three seconds matters. Longer, and you knock the gas out before the syrup sets; the honeycomb comes out dense. Pouring without spreading matters for the same reason.
The leftover
The sodium carbonate stays in the candy. It is faintly alkaline and faintly soapy, which is the slightly bitter note at the back of good honeycomb, and the reason the recipe uses dark chocolate and a pinch of salt against it.
Where else this shows up
The same decomposition is why baking soda needs an acid in a cake batter (buttermilk, yogurt, brown sugar) to release its gas at oven temperature rather than relying on heat alone, and why too much baking soda tastes soapy.