Here's why:

* Ingredients: Sponge toffee typically contains baking soda (sodium bicarbonate, NaHCO3) as a leavening agent and an acidic ingredient like cream of tartar (potassium bitartrate, KHC4H4O6).

* Reaction: When the mixture is heated, the acid reacts with the baking soda. This reaction produces carbon dioxide gas, which gets trapped in the toffee mixture, creating the airy, bubbly texture.

The chemical equation for the reaction is:

NaHCO3 (baking soda) + KHC4H4O6 (cream of tartar) → KNaC4H4O6 (potassium sodium tartrate) + H2O (water) + CO2 (carbon dioxide)