Question

Fig. 4: Decomposition of sodium hydrogencarbonate and evolution and testing of carbon dioxide gas explain it
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Answer 1

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) Na2

CO3 (s) + H2

O(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.

Answer 2

Answer:

NaHCO3= Na2CO3+H2O+CO2

TEST FOR CARBON DIOXIDE GAS

When CO2 gas is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate. If the gas is in excess, milkiness disappears due to the formation of soluble calcium bicarbonate.

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