Test for Carbohydrates Theory Material Required Procedure Result

Carbohydrates are referred to as the most abundant class of organic compounds that occur in nature. Carbohydrates play a key role in the establishment and evolution of life on earth by creating a direct link between the sun and chemical energy. Among the three main nutrients, carbohydrates are one of them. Our body breaks down carbohydrates into glucose and simple sugar. Simple sugar or blood sugar is the main source of energy in our body tissues, cells, and organs, while the body stores glucose for later usage.

Key Terms: Carbohydrates, Glucose, Energy, Nutrients, Starch, Tollen’s Test, Molisch’s Test, Fehling’s Test, Iodine Test, Benedict’s Test, Sugar

Read More: Difference Between Glucose and Fructose

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What are Carbohydrates?

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Carbohydrates are the macronutrients found in certain foods and drinks. Carbohydrates are organic and play an important role in the evolution of life and energy. Carbohydrates are broken down into sugar and thus are used by our bodies. Carbohydrates are essential as they are a source of energy for our bodies. Fruits, vegetables, and whole-grain products are rich sources of simple carbs whereas the keto diet is low in carbs and high in fats.

Carbohydrates

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Types of Carbohydrates

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There are three different types of carbohydrates: 

• Fibre- Fiber is a complex carbohydrate and our body cannot break down fibres easily. Hence fibrous food is healthy and our body feels energetic after its intake.
• Sugars- Sugars are simple carbohydrates and hence have the most basic form, sugar can easily be fined in food such as dessert, candy, etc. fruits, and vegetables contains natural sugar.
• Starches- Lots of simple sugar is made up of complex carbohydrates that as starch. To use starch as energy our body needs to break down it into sugar.

Read More: Sources of Carbohydrates

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Aim

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To study a few simple tests of carbohydrates in the given sample.

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Theory

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Carbohydrates are a combination of chemical carbon elements and hydrogen plus oxygen. The word carbohydrates are made up of carbon plus hydrogen. Here are the chemical tests used to identify the presence of carbohydrates;

• Tollen’s Test
• Molisch’s Test
• Fehling’s Test
• Iodine Test
• Benedict’s Test

Tollen’s Test: Carbohydrates when reacts with tollen’s reagent forms a silver mirror on the inner walls of the test tube, this test is for reducing sugars. The formation of silver mirrors confirms the presence of reducing sugar. The ions of the silver present are then reduced to metallic silver.

Tollen’s reaction is given by the following equation-

The formation of a silver mirror on the inner sides of test tubes indicates a positive test and shows the presence of reducing sugar.

Molisch’s Test: This test is a colourimetric method for the analysis of the presence of carbohydrates in any given analyst, molisch’s reagent is used in this test. A solution of naphthol in ethanol (95%) is known as molisch’s reagent. Concentrated sulphuric acid is also used in this test. This test gives a positive result for all carbohydrates. Monosaccharides react fast whereas disaccharides and polysaccharides react slowly with molisch’s reagent

The chemical reaction is as follows-

The formation of the purple product indicates a positive result.

Fehling’s Test: Fehling’s test is used to identify the presence of reducing sugar. Fehling’s solution is added to an aqueous solution of carbohydrates and thus heated in a water bath. The formation of a red precipitate indicates the presence of reducing sugar. The copper ions present in Fehling’s solution in the +3 staree are reduced to the +2 oxidation state and in an alkaline medium, it is precipitated as red cuprous oxide.

Iodine Test: The iodine test is used to identify the presence of starch when starch reacts with iodine it gives the blue colour solution, and glucose reacts with glycogen to form a reddish-brown complex. On heating, the bicolour disappears and after it’s cooled down the colour appears again.

The chemical equation of the reaction-

The appearance of dark blue colour in the solution confirms the presence of starch

Benedict’s Test: Sodium carbonate converts into glucose into enediol and cupric is reduced to cuprous hydroxide by enediol. the formation of red precipitate takes place when the solution is kept in sodium citrate at boiling temperature.

The chemical reaction is as follows-

Read More: Difference Between Starch and Cellulose

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Material Required

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• Molisch’s reagent
• Fehling’s reagent
• Benedict’s reagent
• Tollen’s reagent
• Iodine solution
• Concentrated sulfuric acid
• Sodium hydroxide solution
• Test tubes
• Test tube holder
• Test tube stand
• Bunsen burner
• Water bath
• Dropper
• Stirrer

Classification of Carbohydrates

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Procedure

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Preparation of Reagents

• Fehling’s Reagent- Fehling’s reagents is a mixture of both 1ml of Fehling’s A and Fehling’s B solution.
• Fehling’s Solution A- It is dissolved copper sulfate in distilled water and a few drops of sulfuric acid.
• Fehling’s Solution B- Sodium hydroxide and sodium-potassium tartrate dissolved in 150 ml of distilled water.
• Molisch’s Reagent- It is prepared by adding α-naphthol in about 10% alcoholic solution.
• Benedict’s Reagent- Distilled water is added to a solution of sodium citrate 0.25g of anhydrous sodium carbonate and copper sulfate. To make volume up o 125ml by adding distilled water.
• Tollen’s Reagent- Add ammonium hydroxide solution dropwise to sodium hydroxide solution to silver nitrate solution till precipitate dissolves.
• Iodine Solution- Dissolve iodine in potassium iodide solution to contain iodine solution.

Tollen’s Test:

• Take a sample in a clean test tube
• Add 2-3 ml of tollens reagent and keep it in a boiling bath for about 10 minutes.
• If the silver mirror appears in the inner walls of the test tube, it confirms the presence of reducing sugar.

Molisch’s Test:

• In a clean test tube, take 2 ml of a given sample
• Add 2-3 drops of Molisch reagent slowly
• Now add concentrated sulphuric acid, and the acid forms a layer at the bottom.
• If there is a formation of a violet ring between the junction of two layers then the presence of carbohydrates is confirmed.
• Thus carbohydrate is present.

Fehling’s Test:

• Take 2 ml of a given sample in a clean test tube
• Add about 2ml of Fehling’s solution A and feelings solution B to the 2ml of a given solution
• Now set up a water bath and keep the solution in a water bath for about 10 minutes.
• Now wait and look at the test tube closely, notice if the formation of red precipitate forms then the presence of carbohydrates is confirmed.

Iodine Solution:

• In a clean test tube, take a sample of the given solution to be tested.
• Add about 2-3 drops of iodine solution to the solution
• Now observe closely the colour change in the solution
• If you notice an appearance of blue or say light violet colour then the presence of carbohydrates is confirmed in the solution.
• Hence, the presence of carbohydrates is confirmed.

Benedict’s Test:

• Take the given sample solution in a clean and dry test tube.
• Add about 5ml of benedicts reagent to the test tube solution
• Now in a water bath, boil the solution for about 2-3 minutes.
• After boiling, cool down the solution and observe the change in the solution
• If there is a formation of red, yellow, or green precipitate then there is the presence of sugar in the solution.

Read More: Precipitation Reaction

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Observation and Inference

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The inferences from the tests performed are 

• Molisch’s test is positive for all types of carbohydrates.
• The iodine test is specific only positive for starch and glycogen.
• Bial’s test is used to differentiate between monosaccharides and disaccharides.
• Benedict’s test is positive for reducing or say pentose sugar
• Tollen’s reagents only react positively with glucose and starch.
• Fehling’s reagent tests positive for glucose and lactose and is negative for starch and sucrose.

Read More: MCQs on Carbohydrates

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Result

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The organic compound that has been given is a _________ (reducing sugar/starch/carbohydrate) compound.

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Precautions

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• Make sure that the test tube in usage are clean and are properly dried.
• Handle the reagents with care.
• When using concentrated acid-like sulphuric acid, be cautious.
• Wear gloves and eye protection glasses.
• Always close the lid of the reagents’ property so that they don’t react with atmospheric particles.
• Always use a dropper to mix reagents.
• Perform the reactions under expert supervision.

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Things to Remember

• Tollens test is given positive by lactose also, hence to differentiate between glucose and lactose we perform Bareford’s test. Also, remember not to boil the compound for a prolonged time as it will give a false test.
• Benedict’s test is a semi-quantitative test as the colour of the precipitate is proportional to the reducing sugar concentration in the test tube.
• In the iodine test, the colour disappears on heating and reappears when the solution is cooled. The test is highly specific for polysaccharides.
• In Molisch’s test, in order to have a positive test, the carbohydrates must have at least five carbon. Impurities in the reagent will give a false-negative test that is green colour.
• In Fehling’s test, the formation of red precipitate confirms the presence of red precipitate in reducing sugar. And In benedict’s test, there is a formation of a red precipitate of cuprous oxide.

Read More: Galactose Formula