An Overview of Class 12 Chemistry Detection And Analysis Of Carbs Fats Proteins In Pure Samples And Foodstuffs Experiment
An Introduction
The three primary categories of macronutrients in food are carbohydrates, proteins, and fats (nutrients that are required daily in large quantities). They provide all of the diet's calories and 90% of its dry weight. Since they are made up chemically of carbon, hydrogen, and oxygen, they are known as carbohydrates. Sugars, fibre, and starches are the necessary nutrients known as carbohydrates.
They can be found in milk and other dairy products, as well as in cereals, vegetables, and fruits. They are the fundamental dietary groups that are crucial to living a healthy life.
Fats are broken down into fatty acids, which are then used for energy. Protein's primary function is to aid in the synthesis of hormones, muscle, and other proteins, while it can also be utilised as fuel.
Food Adulteration
Food adulteration is the practice of contaminating food ingredients by adding a few compounds collectively referred to as adulterants.
Food items can contain adulterants, substances or low-quality products introduced for commercial or technical reasons. Adding these adulterants, food loses nutritional value and becomes contaminated, making it unfit for ingestion. These adulterants may be present in our daily foods, including dairy products, cereals, legumes, grains, meat, vegetables, fruits, oils, and beverages.
Table of Content
Aim
Apparatus Required
Theory
Identification Test for Carbohydrates
Identification Test for Proteins
Identification Test for Fats
Procedure
Observation
Result
Precautions
Lab Manual Questions
Viva Questions
Practical Based Questions
Aim
To Detect the Presence of Carbohydrates, Fats and Proteins in the given sample of Food Stuffs.
Apparatus Required
Test tubes, beakers, a glass rod, a pestle and mortar, and a burner
Theory
By doing tests for carbohydrates, fats, and proteins with the food item's extract, it is possible to determine whether any food item contains those substances. These tests do not even conflict with one another.
Identification Test for Carbohydrates:
1. Molisch Test:
The presence of carbohydrates is confirmed by a purple or violet ring in Molisch's test, which uses a sample food and Molisch's reagent.
Fehling’s Test: Red precipitate confirms the presence of carbohydrates when given a sample of food and Fehling's reagent.
Benedict's test: Red precipitate indicates the presence of carbs when given a food sample and Benedict's reagent.
Tollen’s Test: Carbohydrates are confirmed by Tollen's test, which uses a sample food and Tollen's reagent.
Iodine Test: Sample food is provided, and the blue colour of the iodine solution verifies the presence of starch.
Identification Test for Proteins
1. Biuret Test
A biuret test is performed to check the presence of proteins. Violet colouring indicates the presence of protein when the given sample of food is treated with Aqueous copper sulphate.
2. Xanthoproteic Test
Protein presence is confirmed by the Xanthoproteic test, which involves giving a sample of the food and adding nitric acid to a yellow colour solution.
3. Ninhydrin Test:
The presence of proteins is confirmed by adding a violet-coloured solution to a pyridine solution of ninhydrin and a sample of food.
4. Millions Test:
The given sample of food, when treated with Mercuric sulphate in the presence of sodium nitrite and sulfuric acid, gives a Brick red colour solution that confirms the presence of proteins.
Identification Test for Fats
1. Solubility Test:
When a given sample of food is treated with Alcohol or Chloroform, the presence of fat is proven if it is miscible with Chloroform and immiscible with water.
2. Transparent Spot Test:
The transparent spot test confirms the presence of fats when a food sample is given and rubbed between the layers of filter paper.
3. Acrolein Test:
The acrolein test confirms the presence of fats or oils by giving a sample of food along with potassium bisulfite KHSO₄.
Procedure
To determine the presence of carbohydrates, proteins, fats, and oils in the provided sample, an extract of the food from the sample should be prepared. Dry the provided food in the mortar with the use of pastel, or boil the sample with a small quantity of water to extract the essence from the food sample. After the foodstuff has been ground, water will remove a minute amount of an organic solvent.
The extracts of some of the foodstuffs can be prepared as given under
Potatoes: Cut potatoes into slices, then boil them in water.
Butter: Directly test for butter.
Grapes: Take the juice out of the grapes.
Egg: Take the white of a boiled egg, grind it, and shake it with some water.
Observation:
Results:
The food sample that is being provided has ______ carbohydrates.
The food sample that is being provided has _______ proteins.
The food sample contains _____ oils and fats.
Precautions:
For conducting experiments, use freshly prepared reagents.
Reagents should be used in small amounts.
Lab Manual Questions
1. What are Carbohydrates?
Ans. Optically active polyhydroxy aldehydes or ketones or the compounds that create units of such kind on hydrolysis" are defined as Carbohydrates. The sucrose disaccharide, which can be obtained from either sugar cane or beets, is the substance that most people refer to as "sugar".
2. Name the types of sugar.
Ans. They are of the following types:
Monosaccharides or simple sugars. Example: Dextrose (glucose) is the major monosaccharide.
Disaccharides or complex sugars. Example: Sucrose (common sugar) is the primary example of a disaccharide.
Polysaccharides. Examples are starches, dextrin, and cellulose.
3. Name the bond which holds the monomer units of proteins together?
Ans. Proteins are made up of amino acid monomers that are joined together by covalent peptide bonds between their carboxylic acid groups on neighbouring monomers.
4. What is called the denaturation of proteins?
Ans. Proteins become denatured when their original three-dimensional structure is subjected to modifications. The proteins' hydrogen bonds can be altered by temperature, pH, or other chemical processes. This is called the Denaturation of protein.
Viva Questions
1. What colour does the precipitate take on when protein-containing food solution is treated with Millon's reagent?
Ans. When proteins are treated with Milon’s reagent, a white precipitate first forms, and when it is boiled, it turns a brick-red hue, confirming the existence of proteins.
2. What is Milon’s test used for?
Ans. Milon’s Test is used to find soluble proteins in a sample. The test solution is heated gradually after a few drops of the reagent have been added. Tyrosine residue, which is present in nearly all proteins, is indicated by a reddish-brown colour or precipitate.
3. Mention the major types of carbohydrates.
Ans. The four different forms of carbohydrates are monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
4. What is Benedict’s test?
Ans. Benedict's test can be used to determine whether reducing sugars are present in a certain sample. This test can therefore be used to identify simple carbohydrates that include a free ketone or aldehyde functional group.
5. What is the Silver Mirror test?
Ans. An aldehyde and ketone can be distinguished from one another using the Tollens test, also referred to as the silver-mirror test. It makes use of the fact that aldehydes, but not ketones, are easily oxidised.
6. What is Fehling's Test?
Ans. Detecting reducing sugars and differentiating between water-soluble carbohydrates and ketone functional groups are done using Fehling's test. To distinguish between reducing and non-reducing substances, a deep blue alkaline solution is used to identify the presence of aldehyde or any groups containing the aldehyde functional group -CHO in addition to Tollen's reagent.
Practice Based Questions:
1. Fructose reduces fehling's solution due to the presence of:
Hydroxy Group
∝-hydroxyketone group
Ketone group
None of these
Answer: b)
2. The process of adulterating food or contaminating food ingredients by adding a few substances together is referred to as-
Adulterants
Decomposed
Nutrients
Consumption
Answer: a)
3. The addition of these adulterants lowers the ________ value of food.
a) Substance
b) Quality
c) Nutrients
d) Quantity
Answer: c)
4. Use of an iodine test to identify
a) Protein
b) Fat
c) Chitin
d) Carbohydrates
Answer: d)
5. Fats and oils are not soluble in water because they are:
Polar compounds
Non polar compounds
Amphoteric Compounds
None
Answer: b)
6. Identify the test you would perform to detect the presence of Proteins:
Ninhydrin Test
Molisch Test
Tollen’s Test
All of the above
Answer: a)
7. In which of the following tests does a purple or violet ring confirm the presence of protein?
Ninhydrin Test
Molisch Test
Tollen’s Test
All of the above
Answer: b)
8. Benedict's solution is a clear blue solution of-
Sodium and copper salts
Sodium and copper nitrate
Sodium and potassium salts
None of the above
Answer: a)
Conclusion
Different dietary items contain various nutrients in varying levels, as we have already explained. Tests can be used to determine whether these nutrients are present in food (both raw and cooked), with the tests for carbs, proteins, and fats being some of the easiest.
The technique of polluting food ingredients by adding a few substances, which are referred to as adulterants collectively, is known as food adulteration.
FAQs on Class 12 Chemistry Detection And Analysis Of Carbs Fats Proteins In Pure Samples And Foodstuffs Experiment
1. What is the expected procedure for a 5-mark question on identifying the presence of carbohydrates, proteins, and fats in a given foodstuff like groundnut seeds?
For a comprehensive analysis of a food sample like groundnut seeds, you should follow this procedure:
- Preparation of Extract: First, crush the groundnut seeds using a mortar and pestle. Add a small amount of water and grind to create a paste. Transfer this paste to a test tube, add more water, shake well, and let it stand. The supernatant liquid is the food extract used for testing carbohydrates and proteins.
- Test for Carbohydrates (Starch): To a small portion of the food extract, add 2-3 drops of iodine solution. The appearance of a blue-black colour indicates the presence of starch.
- Test for Proteins (Biuret Test): Take another portion of the food extract. Add 2 drops of copper sulphate solution followed by 10 drops of sodium hydroxide (or caustic soda) solution. A violet or purple colour confirms the presence of proteins.
- Test for Fats: Take a whole groundnut seed and press it between the folds of a filter paper. The appearance of a translucent, greasy spot on the paper that doesn't dry indicates the presence of fats.
2. Which are two important confirmatory tests for detecting carbohydrates as per the CBSE 2025-26 syllabus, and what are their principles?
Two frequently asked tests for detecting carbohydrates, specifically reducing sugars, are:
- Benedict's Test: This test is used to detect the presence of reducing sugars (which have a free aldehyde or ketone group). The principle is that in an alkaline medium, the cupric ions (Cu²⁺) in Benedict's reagent are reduced to cuprous ions (Cu⁺) by the reducing sugar, forming a brick-red precipitate of cuprous oxide (Cu₂O).
- Fehling's Test: Similar to Benedict's test, Fehling's test also identifies reducing sugars. It uses two solutions, Fehling's A (aqueous copper sulphate) and Fehling's B (alkaline sodium potassium tartrate). The Cu²⁺ ions are reduced by the sugar upon heating, resulting in a reddish-brown precipitate of cuprous oxide.
3. Explain the principle of the Xanthoproteic test, a key test for detecting proteins in a sample.
The Xanthoproteic test is a crucial method for identifying proteins containing aromatic amino acids, specifically tyrosine and tryptophan. The principle involves nitration of the benzene ring present in these amino acids. When concentrated nitric acid (HNO₃) is added to the protein solution and heated, the aromatic rings get nitrated, forming a yellow-coloured compound. Upon adding an alkali like ammonium hydroxide, this colour intensifies to orange or deep yellow, confirming the presence of protein.
4. Why is it an important precaution to avoid excessive heating when testing for proteins in a food sample?
It is critical to avoid excessive heating because high temperatures can cause the denaturation of proteins. Denaturation is the process where a protein loses its native three-dimensional structure (secondary and tertiary structures) due to the disruption of weak bonds like hydrogen bonds. This change in structure can alter its solubility and chemical reactivity, potentially leading to a false negative or inconclusive result in colour-based tests like the Biuret or Millon's test.
5. How can a student experimentally differentiate between a sample of glucose solution and a starch solution?
To differentiate between glucose and starch, two different tests must be performed:
- Iodine Test: Add a few drops of iodine solution to both samples. The starch solution will turn a deep blue-black colour due to the formation of an iodine-starch complex. The glucose solution will show no colour change.
- Benedict's Test: Add Benedict's reagent to both samples and heat them in a water bath. The glucose solution, being a reducing sugar, will produce a precipitate that changes colour from green to yellow to brick-red. The starch solution will show no change, as it is a non-reducing polysaccharide.
6. Why does fructose give a positive result with Tollen's and Fehling's tests, even though it is a ketone and these tests are typically for aldehydes?
This is a frequently asked higher-order thinking question. Fructose, a ketohexose, gives a positive result because in the alkaline medium provided by Tollen's and Fehling's reagents, it undergoes a rearrangement. This reaction is known as the Lobry de Bruyn-van Ekenstein rearrangement. Through this process, fructose is converted into an equilibrium mixture of glucose and mannose, both of which are aldoses. These aldoses possess a free aldehyde group that can then reduce the reagents, leading to a positive test result like the silver mirror or the red precipitate.
7. What is the principle behind the simple 'spot test' or 'paper test' for fats, and why is it considered a preliminary test?
The principle of the paper spot test is based on the translucent property of fats and oils. When a substance containing fat is pressed or rubbed onto paper, the fat molecules seep into the paper's pores. They fill the air spaces between the cellulose fibres, reducing the scattering of light and making the paper appear translucent. This spot is greasy and does not evaporate like water would. It is considered a preliminary and not a confirmatory test because other substances, like certain volatile oils, might initially produce a similar spot, though they would typically evaporate over time.
