Which of the following makes bread soft and spongy when baking soda is added?

Correct — C, CO₂. Baking soda is sodium hydrogencarbonate, NaHCO₃. On heating it decomposes — 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂ — and the carbon dioxide released inside the dough is trapped by the gluten network as bubbles. Those bubbles expand in the oven's heat and set as the crumb bakes, which is what makes the loaf rise and gives it a soft, spongy texture. The gas is the leavening agent; everything else the reaction produces is left behind in the bread.

• (a)Sodium salt of acid — Sodium carbonate, or the sodium salt formed when baking powder's acid reacts, stays dissolved in the dough. A dissolved solid cannot inflate anything, and in excess it gives the bitter taste bakers work to avoid.
• (b)NaHCO3 — This is the substance added, not the substance that does the raising. The question asks what makes the bread spongy, and NaHCO₃ has to decompose before anything happens.
• (d)H2O — Water is released too, but it is a minor product and mostly bakes off. The volume expansion that lifts the dough comes from the carbon dioxide.

Chemical leavening works by generating a gas inside a viscous dough faster than the dough can let it escape. Sodium hydrogencarbonate is the usual source, and heat alone is enough to break it down into sodium carbonate, water and carbon dioxide.

The trap is that three of the four options are genuinely present in the baked loaf; only one of them is a gas. Read the question as asking which product does the physical work. Bakers usually reach for baking powder rather than plain baking soda — baking powder is NaHCO₃ mixed with a mild solid acid such as tartaric acid, which neutralises the sodium carbonate that would otherwise leave a bitter aftertaste.

• Baking soda is NaHCO₃ (sodium hydrogencarbonate); washing soda is Na₂CO₃·10H₂O, a different compound.
• 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ on heating; the decomposition begins at around 70 °C.
• Baking powder is baking soda plus a mild edible acid, so the carbon dioxide is released on wetting as well as on heating.
• The same gas-release reaction makes sodium hydrogencarbonate useful in soda-acid fire extinguishers and as an antacid.

• Naming the substance added rather than the product that does the work.
• Confusing baking soda with washing soda, which is sodium carbonate decahydrate.
• Assuming water vapour is the raising agent because steam is also a gas.

Asked either as 'which gas makes the bread rise', or through the decomposition equation with one product left blank.

• practice — not a real PYQ

  The chemical name of baking soda is

  • (a)sodium carbonate
  • (b)sodium hydrogencarbonate
  • (c)calcium carbonate
  • (d)sodium hydroxide

  Answer(b) sodium hydrogencarbonate — NaHCO₃; sodium carbonate is washing soda, a different compound.

• practice — not a real PYQ

  Baking powder differs from baking soda in that it also contains

  • (a)a mild edible acid
  • (b)a strong base
  • (c)common salt
  • (d)calcium oxide

  Answer(a) a mild edible acid — such as tartaric acid, which neutralises the sodium carbonate left behind and removes the bitter taste.