What Happens When Baking Soda Is Heated? NaHCO3 Thermal Decomposition

The thermal decomposition of baking soda is represented by the following balanced equation:

2NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g)

In words: Sodium hydrogen carbonate (on heating) → Sodium carbonate + Water + Carbon dioxide

This is a decomposition reaction where one compound breaks down into simpler substances on the application of heat. The reaction begins at around 50°C and proceeds rapidly above 80°C.

The following observations are made when baking soda is heated in a test tube:

1. Effervescence: Brisk bubbling is observed due to the evolution of carbon dioxide (CO2) gas.
2. White residue: A white powdery solid (sodium carbonate, Na2CO3) remains after the reaction.
3. Water vapour: Moisture (water vapour) is released and may condense on the cooler parts of the test tube.
4. Lime water test: If the gas evolved is passed through lime water [Ca(OH)2], it turns milky, confirming the presence of CO2: CO2 + Ca(OH)2 → CaCO3↓ + H2O
5. The white residue (Na2CO3) is alkaline and turns red litmus paper blue.

Baking Soda (NaHCO3):

• Common name: Baking soda, sodium bicarbonate
• Appearance: White crystalline powder
• Molecular mass: 84 g/mol
• Taste: Slightly salty and alkaline

Sodium Carbonate (Na2CO3):

• Common name: Washing soda (anhydrous form)
• Appearance: White powder
• Molecular mass: 106 g/mol
• More strongly alkaline than NaHCO3

Carbon Dioxide (CO2):

• Colourless, odourless gas
• Turns lime water milky
• Does not support combustion

Water (H2O):

• Released as steam/vapour during the reaction

The thermal decomposition of NaHCO3 has several practical applications:

1. Baking and cooking: When baking soda is heated in baked goods, the CO2 released causes dough to rise, making bread, cakes, and biscuits light and fluffy.
2. Fire extinguishers: Dry powder fire extinguishers contain NaHCO3. When heated by a fire, it releases CO2 which smothers the flames.
3. Antacid medication: NaHCO3 is used to neutralise excess stomach acid; the CO2 released provides a soothing effect.
4. Laboratory identification: Heating a substance to check for effervescence followed by lime water test is a standard method to identify carbonates and bicarbonates.