Q.Observe the following equation of reaction of Tollens' reagent with aldehyde.
R-CHO + 2 Ag(NH3)2(+) + OH(-) --heat--> R-COO(-) + 2 Ag + 4 NH3 + 2 H2O
How do we know that a redox reaction has taken place? Explain.

Step 1. Track what happens to the aldehyde. R-CHO becomes R-COO(-): the carbonyl carbon gains an oxygen-equivalent (effectively going from one C-O bond to two C-O-type bonds in the carboxylate) and loses its aldehydic hydrogen -- this is an OXIDATION of the aldehyde carbon.

Step 2. Track what happens to silver. The diammine silver(I) complex, Ag(NH3)2(+), containing Ag in the +1 oxidation state, ends up as metallic silver, Ag(0) -- silver's oxidation state falls from +1 to 0, which is a REDUCTION.

Step 3. Recognise the redox signature. Because one species (the aldehyde) is oxidised at the exact same time that a different species (silver) is reduced, in the same overall equation, this is unambiguously a redox (oxidation-reduction) reaction -- a reaction where only one of the two happened (oxidation alone, or reduction alone) would not itself be a complete, balanced chemical change; a redox reaction always pairs the two together, electrons lost by one species being exactly the electrons gained by the other.

Step 4. Note the visible confirmation. The silver-mirror deposit itself IS the direct physical evidence of Ag(+) being reduced to metallic Ag(0) on the vessel wall.

Tracking the oxidation-state change of the aldehyde carbon versus the silver centre across the Tollens' equation, and identifying the simultaneous oxidation-plus-reduction pattern that defines a redox reaction

• Describing only the aldehyde's change (oxidation) or only silver's change (reduction) without explicitly connecting the two as a paired, simultaneous electron transfer, which is what actually makes the reaction 'redox' rather than just 'an oxidation'.
• Treating the visible silver-mirror deposit as merely a physical/precipitation change, rather than recognising it as the direct evidence of Ag(+) being chemically reduced to Ag(0).