21
Catalyst February 2014 www.catalyststudent.org.uk 21
Honeycomb (cinder toffee) is made using a thermal
decomposition reaction to produce the gas bubbles.
Sodium hydrogen carbonate (also known as
sodium bicarbonate or bicarbonate of soda) has
the chemical formula NaHCO3. When it is heated
above about 80°C it begins to break down, forming
sodium carbonate, water and carbon dioxide. This
type of reaction is called a thermal decomposition.
2 NaHCO3 (s) Na2CO3 (s) + H2O(g) + CO2 (g)
This reaction is used in cooking where the carbon
dioxide gas causes a number of products to rise.
The higher the temperature of the mixture, the
faster the reaction is. A toffee mixture of golden
syrup and sugar will get very hot, well over 100°C,
when heated to boiling point and sodium hydrogen
carbonate added to it will decompose very quickly.
This causes the toffee to puff up from the gas
bubbles formed. If it is cooled quickly by being
poured into a cold tin then the toffee will have all
these bubbles in it, giving a solid foam. It is known
by various names including honeycomb, cinder
toffee and hokey-pokey.
You will need:
• 100g caster sugar
• 2 tablespoons golden syrup
• ½ tablespoon sodium hydrogen carbonate
(bicarbonate of soda) – not baking powder
• Heavy-bottomed saucepan
• Wooden spoon
• Stove
• Baking tin, greased thoroughly or lined with a
tefal sheet
• Apron
What you do
Wear an apron and take care as the mixture will
get very hot.
Measure the sugar and golden syrup into the
saucepan. Mix them together then put on a low
heat, stirring until they boil and then simmer
gently for about 3 minutes. The mixture can burn
if heated too hard so keep the heat quite low
and stir occasionally. It will be ready when it has
darkened a bit.
Take the saucepan off the heat and quickly stir
in the sodium hydrogen carbonate. Watch as it
thermally decomposes, forming gas bubbles in the
mixture and causing it to foam up. Don’t over stir
or you will lose your bubbles.
Tip into the waiting tin and leave to cool while
you admire the results of your chemical reaction.
Vicky Wong is Chemistry editor of Catalyst.
The thermal
decomposition of sodium
hydrogen carbonate
(or: Making honeycomb)
Try
This
Heating the toffee mixture