Chemistry Experiment: Tests of Carbohydrates Fats and Proteins in Pure Samples and Detection of Their Presence in Given Food Stuffs
Carbohydrates provide energy to your brain, kidneys, heart muscles, and nervous system. Fibre, for example, is a carbohydrate that aids digestion, makes you feel full, and helps keep blood cholesterol levels in check. Proteins are the building blocks of living tissues and are coded for by our genes. They are also important in biological processes.
Proteins, for example, catalyse reactions in our bodies, transport molecules like oxygen, keep us healthy as part of the immune system, and communicate messages from cell to cell. Fats also aid in the transport of fat-soluble vitamins such as A and D. They cushion and form protective pads around sensitive organs like the heart, liver, and kidneys, and the layer of fat beneath the skin helps insulate the body against excessive heat loss.
Table of Contents
Aim of the Experiment
Apparatus Required
Theory
Procedure
Observations
Result
Precautions
Aim of the Experiment
To test the presence of carbohydrates, proteins and fats in pure samples
Apparatus Required
Molisch’s Reagent
Fehling’s Reagent
Benedict’s Reagent
Tollen’s Reagent
Iodine Solution
Copper Sulfate Solution
Sodium Hydroxide
Nitric Acid
Mercuric Sulfate
Sodium Nitrite
Alcohol
Chloroform
Filter Paper
Potassium Bisulfate
Concentrated Hydrochloric Acid
Furfural Solution
Test Tubes
Test Tube Holder
Water Bath
Dropper
Stirrer
Bunsen Burner
Theory
The presence of protein, carbohydrates and fats in any food is detected by performing protein, fat, and carbohydrate tests on the food extract. The benefit is that these tests do not interfere with one another.
Test for Carbohydrates:
Molisch’s test – Molisch’s reagent + Given sample food → Purple or violet ring confirms the presence of carbohydrates.
Fehling’s test – Fehling’s reagent + Given sample food → Red precipitate confirms the presence of carbohydrates
Benedict’s test – Benedict’s reagent + Given sample food → Red precipitate confirms the presence of carbohydrates.
Tollen’s test – Tollen’s reagent + Given sample food → Silver mirror confirms the presence of carbohydrates.
Iodine test – Iodine solution + Given sample food → Blue colour solution confirms the presence of starch.
Protein Test in Food:
Biuret test – Aqueous copper sulfate + Given sample food → Violet colouration confirms the presence of Proteins
Xanthoproteic test – Nitric acid + Given sample food → Yellow colour solution confirms the presence of proteins.
Millions test – Mercuric sulfate in the presence of sodium nitrite and sulfuric acid + Given sample food → Brick red colour solution confirms the presence of proteins.
Ninhydrin test – Pyridine solution of ninhydrin + Given sample food → Violet colour solution confirms the presence of proteins.
Test for Fats:
Solubility test – Chloroform or alcohol + Given sample food → Miscible with chloroform and immiscible with water the fat presence is confirmed.
Translucent spot test – rubbed between the folds of filter paper + Given sample food → presence of translucent spot then the presence of fats is confirmed.
Acrolein test – Potassium bisulfite $${{K}{H}{S}{O}_{4}}$$ + Given sample food → Pungent irritating odour, then the presence of fats or oil is confirmed.
Procedure
To determine the presence of carbohydrates, proteins, fats, and oils in a given food sample, an extract of the food should be prepared first. After grinding the foodstuff, dry it in a mortar with a pestle or by boiling it with a small amount of water and extracting it with a small amount of an organic solvent.
Some examples of food extract preparation are provided below.
Cut potatoes into slices and boil in water.
For butter, conduct a direct test.
Extract the juice from grapes and test.
Take the white portion of an egg and grind it with water to make boiled eggs.
Observations
Result
The given sample of food may contain carbohydrates, proteins and fats.
Precautions
To test the sample food, always use the smallest amount of reagents that are freshly prepared.
Take the reagents from the bottle with droppers.
During the experiment, wear lab aprons and hand gloves.
Lab Manual Questions
1. How will you distinguish between sucrose and glucose?
Ans. Glucose on heating with Tollens' reagent gives a silver mirror while sucrose does not.
2. Explain why fructose reduces Fehling’s solution and Tollens reagent inspite of the presence of a ketonic group?
Ans. In alkaline medium fructose rearranges to glucose and the two are in equilibrium with each other.
3. What is the role of tartrate and citrate ions in Fehling’s reagent and Benedict’s reagent respectively?
Ans. Using Fehling's solution or Benedict's solution, complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide. Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution.
4. What are the reagents used in this experiment?
Ans. Following are the reagents used in this experiment:
Molisch's reagent
Fehling's reagent
Benedict's reagent
Tollen's reagent
Viva Questions
1. Which chemical is used for carbohydrate testing?
Ans. Concentrated sulphuric acid is used in testing carbohydrates.
2. Why is testing for carbohydrates important?
Ans. To characterise carbohydrates present in an unknown solution based on various chemical assays.
3. What is the purpose of testing known samples for lipids, carbohydrates, and proteins?
Ans. To detect the presence of carbohydrates, fats and proteins in the samples of the given food.
4. What colour does protein go?
Ans. Proteins are detected using Biuret reagent. This turns a mauve or purple colour when mixed with protein.
5. Why are carbohydrates polar?
Ans. Carbohydrates are more polar than lipids due to the presence of many -OH groups in carbohydrates.
6. What organism makes carbohydrates?
Ans. Green plants form carbohydrates from carbon dioxide and water during the process of photosynthesis.
7. What are proteins made of?
Ans. Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains.
8. What is the protein factor?
Ans. The protein factor is the numeric factor (6.25) by which the nitrogen content of a protein is multiplied to approximate the amount of protein.
9. What is the chemical formula of protein?
Ans. Proteins general chemical formula is $${{R}{C}{H}{(}{N}{H}_{2}{)}{C}{O}{O}{H}}$$.
10. How is protein measured?
Ans. The most frequently used methods for measuring protein content in foods include the Kjeldahl method, Dumas method, direct measurement methods using UV-spectroscopy and refractive index measurement.
Practical Based Questions
Which of the following Biomolecules simply refers to as “Staff of life”?
(a) Lipids
(b) Proteins
(c) Vitamins
(d) Carbohydrates
Answer: (d)
Which of the following is the simplest form of carbohydrates?
(a) Carboxyl groups
(b) Aldehyde and Ketone groups
(c) Alcohol and Carboxyl groups
(d) Hydroxyl groups and Hydrogen groups
Answer: (b)
Protein, carbohydrates and fat are examples of macronutrients.
(a)True
(b)False
Answer: (a)
Which of the following monosaccharides is the majority found in the human body?
(a) D-type
(b) L-type
(c) LD-types
(d) None of the above
Answer: (a)
Which of the following is the most abundant biomolecule on the earth?
(a) Lipids
(b) Proteins
(c) Carbohydrates
(d) Nucleic acids.
Answer: (c)
Which of the following are the major functions of carbohydrates?
(a) Storage
(b) Structural framework
(c) Transport Materials
(d) Both Storage and structural framework
Answer: (d)
Which of the following is the smallest carbohydrate – triose?
(a) Ribose
(b) Glucose
(c) Glyceraldehyde
(d) Dihydroxyacetone
Answer: (c)
Which of the following is the simplest carbohydrate?
(a) Gulose
(b) Glucose
(c) Dihydroxyacetone
(d) Glyceraldehyde
Answer: (d)
Which of the following factors is not responsible for the denaturation of proteins?
(a) Heat
(b) Charge
(c) pH change
(d) Organic solvents
Answer: (b)
Carbohydrates and fats are called energy food
(a)False
(b)True
Answer: (b)
Conclusion
Carbohydrates provide energy to the body's cells. Protein is the most important functional and structural component of all body cells. Fats and oils are the most abundant fatty acid-containing energy sources. Carbohydrates provide energy to your brain, kidneys, heart muscles, and nervous system. Proteins are the building blocks of living tissues and are coded for by our genes. They are also important in biological processes. Fats also aid in transporting fat-soluble vitamins such as A and D.
FAQs on Tests of Carbohydrates Fats and Proteins in Pure Samples and Detection of Their Presence in Given Food Stuffs
1. What are some important questions from the food tests experiment for the Class 12 Chemistry board practical exam?
For the CBSE Class 12 practical exam for the 2025-26 session, some important questions focus on the underlying principles and specific observations. Key areas include:
- The chemical reactions involved in tests like Tollen’s test or Fehling’s test.
- The reason why certain carbohydrates are classified as reducing or non-reducing sugars.
- The specific function of each component in a reagent, like the role of sodium potassium tartrate in Fehling's solution.
- Distinguishing tests to differentiate between biomolecules, for example, glucose and sucrose.
- The chemistry behind the colour changes observed, such as the purple complex in the Biuret test.
2. How do you perform the test for proteins in a food sample and what are the expected observations?
The most common and important test for detecting proteins is the Biuret test. To perform it:
- Take a small amount of the liquid food extract in a clean test tube.
- Add 2 mL of sodium hydroxide solution to make the medium alkaline.
- Add 2-3 drops of 1% aqueous copper sulfate solution and shake well.
Observation: The appearance of a violet or purple colour confirms the presence of protein in the food sample. This colour is due to the formation of a coordination complex between Cu²⁺ ions and the peptide bonds of the protein.
3. Explain the chemical principle behind Tollen’s test. Why is it called the 'silver mirror' test?
Tollen’s test is a crucial method for identifying the presence of aldehydic groups. The principle is based on a redox reaction where the aldehyde is oxidised to a carboxylate ion, and the silver ions in Tollen's reagent are reduced to metallic silver.
Tollen's reagent is an ammoniacal silver nitrate solution, [Ag(NH₃)₂]⁺. When heated with an aldehyde, the Ag⁺ ions are reduced to elemental silver (Ag), which deposits on the inner surface of the clean test tube. This uniform, shiny coating of metallic silver acts like a mirror, which is why the test is famously known as the silver mirror test.
4. How can you distinguish between a reducing sugar like glucose and a non-reducing sugar like sucrose in the lab?
You can distinguish between a reducing sugar (glucose) and a non-reducing sugar (sucrose) using Fehling's test or Benedict's test. These are important distinguishing tests for the board exam.
- Procedure: Add a few drops of Fehling's or Benedict's reagent to both the glucose and sucrose solutions in separate test tubes and heat them in a water bath.
- Observation with Glucose (Reducing Sugar): A red or reddish-brown precipitate of copper(I) oxide (Cu₂O) is formed. This is a positive test.
- Observation with Sucrose (Non-Reducing Sugar): The blue colour of the solution remains unchanged. This is a negative test.
This difference occurs because glucose has a free hemiacetal group that can open up to form an aldehyde, while the anomeric carbons in sucrose are involved in a glycosidic bond and are not free to react.
5. Why does fructose, despite having a ketonic group, give a positive result with Fehling’s and Benedict’s solutions?
This is a frequently asked conceptual question. Although Fehling's and Benedict's solutions are mild oxidising agents used to detect aldehydes, fructose (a ketohexose) still gives a positive test. The reason is that in the alkaline medium of these reagents, ketoses like fructose undergo a rearrangement (tautomerization via an enediol intermediate) to form an equilibrium mixture containing aldoses (glucose and mannose). It is this rearranged aldehydic form that then reacts with the Cu²⁺ ions in the reagent to give a positive test, forming a red precipitate of Cu₂O.
6. What is the specific role of Rochelle's salt (sodium potassium tartrate) in Fehling's solution?
The role of Rochelle's salt (sodium potassium tartrate) in Fehling's solution is critically important. In the alkaline solution, copper(II) ions would normally precipitate as blue copper(II) hydroxide, Cu(OH)₂. Rochelle's salt is a chelating agent. The tartrate ions form a deep blue, soluble complex with the Cu²⁺ ions. This keeps the copper ions in the solution, allowing them to be available to react with and be reduced by the aldehyde group of a reducing sugar.
7. Describe two important tests for detecting fats or oils in a given foodstuff.
Two simple yet important tests for detecting fats are:
- Translucent Spot Test: A small amount of the food sample is pressed or rubbed between the folds of a filter paper. The paper is then dried. The appearance of a translucent, greasy spot that does not disappear on drying indicates the presence of fats or oils.
- Solubility Test: A small amount of the sample is shaken with water and then with an organic solvent like chloroform or ethanol. Fats are insoluble in water but soluble in organic solvents. If the sample forms an immiscible layer with water but dissolves in chloroform, the presence of fat is confirmed.
8. What precautions are crucial to get accurate results in the biomolecule tests and avoid errors in the board practical exam?
To ensure reliable results and score well, observing the following precautions is very important:
- Always use clean and dry test tubes for each test to prevent cross-contamination.
- Use freshly prepared reagents, as some, like Tollen's and Fehling's solution, are unstable.
- Add reagents in the prescribed quantities using a dropper to avoid excess.
- When heating is required, as in Benedict's or Fehling's test, use a water bath for gentle and uniform heating. Direct heating can char the sample.
- Carefully observe the colour changes and precipitate formation, as these are the key indicators for your results.
9. Why is the Biuret test not given by individual amino acids, but is positive for proteins?
The Biuret test is a specific test for the presence of peptide bonds. A positive test (violet colour) requires the formation of a coordination complex between the Cu²⁺ ion and the nitrogen atoms involved in the peptide linkages. For this complex to form, a molecule must have at least two peptide bonds. Since individual amino acids do not have any peptide bonds and dipeptides have only one, they do not give a positive Biuret test. Proteins, being polypeptides with many peptide bonds, react readily to give the characteristic colour.
10. For the CBSE Class 12 practical exam, how are marks typically awarded for this experiment?
The marks distribution for a qualitative analysis experiment like the food tests is designed to assess a student's overall understanding and practical skills. While the exact breakup can vary, a typical distribution of marks for the 2025-26 board exam would be:
- Theory/Principle: Writing the correct chemical principle behind the tests.
- Procedure: Correctly listing the steps for performing the tests.
- Observation: Accurately recording the results in an observation table (e.g., colour changes, precipitate formation).
- Inference/Result: Correctly identifying the biomolecules present in the given sample.
- Viva Voce: Answering oral questions based on the experiment's theory, procedure, and precautions.
