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Tollens and Fehling (aldehyde Tests)
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different).
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Tollens and Fehling (aldehyde Tests)
What is it?
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
In simple words
- Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
- 1. TOLLENS: Clean test tube, warm.
- 2. FEHLING: Blue to brick red.
- Exam reminder: Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
Important terms
TOLLENS AND FEHLING (ALDEHYDE TESTS): Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
1. TOLLENS: Clean test tube, warm.
2. FEHLING: Blue to brick red.
What you should learn
- Explain tollens and fehling (aldehyde tests) in simple chemical language.
- Connect 1. tollens with 2. fehling using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Examples
- Ethanal yes
- Propanone no
- Mirror named
TOLLENS
Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link.
FEHLING
Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test.
Recommended for Class 11
Full school-level explanation
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
Silver, then copper:
WHAT — TOLLENS AND FEHLING (ALDEHYDE TESTS)
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
WHY — Why this topic matters
TOLLENS AND FEHLING (ALDEHYDE TESTS) is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
TOLLENS: Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link. Examples: Ethanal yes; Propanone no; Mirror named. FEHLING: Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test. Examples: Brick red; Ketone no; Heat. Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
TOLLENS
Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link.
Remember
- Ag+ -> Ag
Examples
- Ethanal yes
- Propanone no
- Mirror named
FEHLING
Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test.
Remember
- Cu2+ -> Cu2O
Examples
- Brick red
- Ketone no
- Heat
Remember
- aldehyde: mirror or red ppt
- ketone: no
- mirror = aldehyde
- tests only
- Ag+ -> Ag
- Cu2+ -> Cu2O
- aldehyde: mirror or red ppt
- ketone: no
How the ideas connect
Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
Deeper understanding
Go beyond memorising Tollens and Fehling (aldehyde Tests). Connect the definition to reasons, relationships, exceptions, and exam wording.
How the ideas connect
Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
Common mistakes
Exam focus
Core Idea
1. TOLLENS
2. FEHLING
Exam Example
Exam focus
Quick revision
- aldehyde: mirror or red ppt
- ketone: no
- mirror = aldehyde
- tests only
- TOLLENS AND FEHLING (ALDEHYDE TESTS): Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
- 1. TOLLENS: Clean test tube, warm.
ID tests. No silvering SOP, no clinical dosing.
For long answers on Tollens and Fehling (aldehyde Tests): define clearly → explain the main parts → give one example → state one exam caution.
Exam Focus
Exam Focus
Important points
- aldehyde: mirror or red ppt
- ketone: no
- mirror = aldehyde
- tests only
Important formulas
- aldehyde: mirror or red ppt
- ketone: no
- mirror = aldehyde
Common exam traps
- Do not confuse 1. TOLLENS with 2. FEHLING; state the difference before giving an example.
- Writing only “aldehyde: mirror or red ppt” is not enough. Explain what it means in a complete sentence.
Long question structure
Definition → key parts → example → exam caution
Common Mistakes
Common Mistakes
- Mistake
Do not confuse 1. TOLLENS with 2. FEHLING; state the difference before giving an example.
- Mistake
Writing only “aldehyde: mirror or red ppt” is not enough. Explain what it means in a complete sentence.
- Mistake
ID tests. No silvering SOP, no clinical dosing.
- Mistake
ID tests. No silvering SOP, no clinical dosing.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
aldehyde: mirror or red ppt
ketone: no
mirror = aldehyde
tests only
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat).
mirror = aldehyde
- Definition
- Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat).
- Formula
- mirror = aldehyde
- Example
- Ethanal yes
- Critical warning
- Do not confuse 1. TOLLENS with 2. FEHLING; state the difference before giving an example.
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Exam Practice
Define
Q1. Define Tollens and Fehling (aldehyde Tests).
Explain
Q2. Explain: TOLLENS.
Example
Q3. Give one clear example of Tollens and Fehling (aldehyde Tests) and say why it fits.
Mistake
Q4. State one common mistake students make about Tollens and Fehling (aldehyde Tests).
Exam note
Q5. Write a short exam note on Tollens and Fehling (aldehyde Tests): definition, two points, one example.
Sources & References
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.