At nearly 70°C, sodium bicarbonate shows the property of gradually decomposing, which makes it usable in bakery products. The product of decomposition responsible for this use of sodium bicarbonate is
- A.Carbon dioxide
- B.Hydrogen
- C.Water vapour
- D.Oxygen
Correct answer
Explanation
The correct answer is A, Carbon dioxide. On heating, sodium bicarbonate releases carbon dioxide gas, whose bubbles make dough rise and turn cakes and bread soft and spongy.
Sodium bicarbonate (NaHCO₃) is baking soda. When heated it decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The carbon dioxide gets trapped in the dough and expands in the heat of the oven, leaving the baked product light and porous. Baking powder is baking soda mixed with a mild edible acid such as tartaric acid, which also neutralises the sodium carbonate formed, so the food does not taste bitter. Baking soda is also used as an antacid and in soda-acid fire extinguishers.
B is wrong: hydrogen is not a product of the decomposition, as there is no reaction that frees it. C is wrong: water vapour is formed, but in a small amount and it does not make the dough rise. D is wrong: oxygen is not released when sodium bicarbonate breaks down.
Exam tip: baking soda = NaHCO₃, gives CO₂ on heating; washing soda = Na₂CO₃·10H₂O.
Practice Questions
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- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
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- A.electrical signals to sound waves
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- C.propagating in straight line
- D.reflected towards the opposite direction
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