Biomolecules - Lab (Finals Q&A)

Learn and Practice Test in Multiple Choice only

Functional group plays a vital role in determining the nature of the inorganic compounds and their properties. A particular functional group will always display its distinctive chemical behavior when it is present in a compound; thus, the identification of functional groups is a very important part of qualitative analysis.

Both statements are incorrect.

Why did olive oil, turpentine oil, oleic acid, and castor oil test positive in Baeyer’s test?

They contain unsaturated carbon-carbon bonds

Which of the following is a limitation of Baeyer’s test?

It cannot differentiate between alkenes and alkynes

Which inference best explains why acetone and formic acid showed decolorization in the bromine test, despite being saturated compounds?

They may undergo side reactions or enolization, enabling bromine to react.

If a compound tested positive in both the Baeyer’s and Bromine tests but showed no reaction in the Lucas test, what can be concluded about its structure?

It is an unsaturated hydrocarbon with no alcohol group.

During the Iodoform test, a sample with a known secondary alcohol structure gave a negative result. Which factor BEST explains this outcome?

The test conditions may have lacked sufficient iodine or heat.

In the Lucas test results, 2-butanol showed slower or incomplete turbidity formation compared to 2-methyl-2-propanol. What structural feature most likely accounts for this difference in reactivity?

2-butanol is a secondary alcohol, forming less stable carbocations.

In the Iodoform test, only certain alcohols such as ethanol and 2-propanol gave positive results, while others like 1-butanol did not. Which conclusion best explains this selectivity?

The presence of a methyl group directly bonded to the carbon bearing the OH group is necessary.

Despite being saturated, acetone gave a positive result in the Iodoform test. What aspect of acetone's molecular structure allows this reaction to proceed?

It contains a methyl ketone group susceptible to halogenation.

During a chemistry test, a student overheats the sample by mistake in the Lucas test. The turbidity that is formed is immediate. What is the best interpretation?

The alcohol is tertiary, but the heating artificially accelerated the reaction.

All samples that were subjected to Schiff's test came out magenta or violet, even a known hydrocarbon like cyclohexane. This indicates some limitations in this testing procedure as outlined in the paper.

The test is non-specific and prone to false positives due to impurities or environmental factors.

Formaldehyde was sodium bisulfite test positive but Fehling's test negative. What is the most likely explanation, as discussed?

Experimental error or faulty Fehling’s reagent prevented oxidation-reduction.

How can Schiff’s test yield a false positive when testing compounds like formic acid or aromatic alcohols?

They contain functional groups structurally similar to aldehydes, which can interact with Schiff’s reagent.

Given the high sensitivity but limited specificity of Schiff’s test, which of the following would best enhance the accuracy of identifying aldehydes in a sample?

Performing complementary tests or spectroscopic analysis to confirm the presence of aldehydes.

Why do alcohols and carboxylic acids typically test negative in the sodium bisulfite test despite some containing polar groups?

The hydroxyl and carboxyl groups do not form bisulfite adducts due to differences in reactivity and solubility.

Why is it necessary to employ certain chemical reagents when testing for the existence of functional groups in organic compounds?

To allow visible chemical reactions that prove the existence of specific functional groups

A compound was reacted with 2,4-DNP and yielded a yellow precipitate. What can be concluded regarding the functional group in the compound?

It has a carbonyl group (C=O) as in aldehydes or ketones

While carrying out the experiment, a student observed that the tested compound reacted positively with Tollens' reagent but failed to react with 2,4-DNP. What conclusion is most appropriate?

The compound has an aldehyde group but is possibly impure

Which of the following reagents are used to identify the presence of aldehyde groups in a compound?

Tollens’ reagent

What is the primary use of Baeyer’s (potassium permanganate) test in functional group analysis?

To test for unsaturation (double or triple bonds)

Which of the following is not typically considered a visual indicator in chemical tests for functional groups?

Change in molecular weight

Why might a non-alcoholic compound like turpentine oil test positive in both Baeyer’s and bromine tests?

It has pi bonds that allow it to undergo addition and oxidation reactions.

In the Bromine test, compounds like cyclohexene rapidly decolorize bromine water, while cyclohexane did not. What does this observation confirm about the nature of the test?

The test selectively detects unsaturation via electrophilic addition.

Based on the principle of the sodium bisulfite test, which compound below is most likely to yield a false negative, and why?

A highly polar carboxylic acid that is poorly soluble in organic solvents.

In Benedict's test, which of the following color changes would indicate the lowest concentration of reducing sugars?

Blue to green

Benedict's test is a qualitative assay used to detect the presence of reducing sugars. Sucrose is a reducing sugar under standard Benedict's test conditions. Which of the following statement/s is correct?

Statement 1 is true, Statement 2 is false

Monosaccharides and disaccharides can be dissolved freely in water. Polysaccharides cannot be dissolved easily in water because they have low molecular weight. Which of the following statement/s is correct?

Statement 1 is true, Statement 2 is false

What happens when reducing sugars are heated with Benedict’s reagent?

Copper (II) ions are reduced to copper (I) ions, forming a red precipitate of copper (I) oxide.

In Benedict’s test, what is the correct sequence of steps to follow after preparing the sugar sample solutions?

Add 1 mL of sample in a test tube, add 2 mL of Benedict’s reagent, heat the mixture in boiling water for 5 minutes, then observe

What is the main indicator of a positive result in the Molisch test?

Formation of a purple ring

Monosaccharides give a faster reaction in the Molisch test than polysaccharides. A purple ring always confirms the presence of proteins in the Molisch test.

The first statement is correct; the second is incorrect.

Which sample produced the slowest reaction in the Molisch test and what does this imply about its structure?

Starch; it’s a complex polysaccharide with strong glycosidic bonds

Which of the following statements best describes the difference between monosaccharides and disaccharides in the Molisch test?

Monosaccharides reacted more quickly due to the absence of glycosidic bonds.

Which of the following best explains the formation of a purple or reddish-purple ring in a positive Molisch test?

Carbohydrates are dehydrated by sulfuric acid to form furfural derivatives, which then react with α-naphthol to produce a colored complex.

Which of the following best explains why galactose gives a positive Barfoed’s test?

It is a monosaccharide that reduces Cu²⁺ to Cu₂O in acidic conditions.

Why do starch test negative in Barfoed’s reagent?

It is a polysaccharide with no free reducing groups in its native form.

Which observation confirms that lactose is a disaccharide during Barfoed’s test?

No red precipitate formed, and the solution is pale blue.

What is the purpose of Barfoed’s test in carbohydrate analysis?

To differentiate monosaccharides from disaccharides

What is the expected result of Barfoed’s test for monosaccharides?

A red precipitate forms.

Among the common qualitative tests used for experiment 2, this method monosaccharides and oligosaccharides from polysaccharides, primarily starch, by detecting the presence of amylose and amylopectin in the carbohydrate samples.

Iodine Test

In the Iodine test of carbohydrate samples, the appearance of a blue-black color upon adding iodine indicates the presence of starch in the sample. Temperature changes can affect the iodine test; a decrease in temperature leads to a decrease in the blue color intensity, as lower temperatures cause the amylose-iodine complex to dissociate.

Statement 1 is true, Statement 2 is false

What are the possible factors attributed to the dark brown color of the starch sample’s positive reaction in experiment 2 of the Iodine Test, instead of the expected dark purple or blue color?

I. Presence of glucose in the sample.

II. A weaker reaction to the iodine.

III. Low concentration of starch in the sample.

IV. Exposure of the iodine reagent to heat before use.

II & III

In experiment 2, which of the following conclusions can be drawn from the reaction of carbohydrate solutions with NaOH and glacial acetic acid?

I. NaOH alone turns all solutions transparent.

II. The reaction with acetic acid indicates possible gas formation and chemical breakdown.

III. The glucose solution turned purple, indicating the presence of starch.

IV. The iodine test alone is sufficient to identify all carbohydrate classes.

I and II

In experiment 2, what was the color of the carbohydrate solutions after NaOH was added during the Iodine test?

Clear and transparent

Fehling’s test involves a redox reaction where Cu²⁺ is reduced to Cu₂O. The reddish-brown precipitate observed confirms the presence of a non-reducing sugar.

Statement 1 is true, Statement 2 is false

Galactose, fructose, and lactose gave positive results in Fehling’s test. What chemical property do these sugars share that explains this result?

They have free anomeric carbons with hydroxyl groups that enable electron donation.

What might be a reasonable hypothesis if starch unexpectedly gave a positive result in the Fehling’s test during another trial?

I. The starch sample was contaminated with reducing sugars.

II. Partial hydrolysis of starch occurred, releasing glucose.

III. The Fehling’s solution was improperly prepared.

IV. The test tubes used were not washed and reused from a previous positive sample.

I, II, III, and IV

Which of the following best summarizes the purpose and chemical basis of Fehling’s test in experiment 2?

It distinguishes reducing from non-reducing sugars through a redox reaction resulting in a colored precipitate.

Which of the following statements best distinguishes glycogen from starch?

Glycogen is more highly branched than starch due to its frequent α(1→6) linkages.

Based on the results of the glycogen isolation and Molisch test, which of the following conclusions is most accurate?

Flocculation during isolation and a positive Molisch test both confirm the presence of glycogen as a carbohydrate.

What is the primary purpose of adding ice-cold ethanol during the extraction of glycogen from pork liver?

To precipitate glycogen from the liver extract.

Describe the general structure of monosaccharides in terms of carbon atom count.

Monosaccharides typically contain 3 to 9 carbon atoms.

Describe why glycogen is classified as a non-reducing sugar and its significance in biochemical processes.

Glycogen is classified as a non-reducing sugar because it lacks a free aldehyde or ketone group, which is important for its stability and energy storage.

The use of sand during the grinding of liver tissue primarily serves what purpose in the extraction process?

To physically break open liver cells and release glycogen.

During the glycogen isolation from chicken liver, Barfoed’s test showed no color change after heating. Which of the following best explains this observation in relation to the biochemical nature of the sample?

Glycogen does not react with Barfoed’s reagent because it is a polysaccharide and lacks free aldehyde groups necessary for reduction.

What is the primary purpose of using acetic acid when grinding the liver?

To break down cell structures and release glycogen

What is the role of sodium chloride if no precipitate forms?

To alter solubility and aid precipitation

Why is glycogen considered a short-term energy reservoir?

It is quickly broken down into glucose

What is the purpose of washing the sand before use?

To remove impurities that might contaminate the glycogen

What characteristic of glycogen makes it more effective than starch for rapid energy release in animals?

It is more highly branched due to frequent α(1→6) linkages.

Which observation most strongly indicates the presence of glycogen during an extraction from liver tissue?

A reddish-purple ring forms in the Molisch test and flocculation is seen.

In carbohydrate extraction, what is the specific reason ice-cold ethanol is added to the liver filtrate?

To cause glycogen to precipitate out of solution.

Why does glycogen not yield a positive result in Barfoed’s test?

It lacks free carbonyl groups required for reduction.

What would most likely happen if the liver extract is not properly filtered before adding ethanol?

The sample may contain impurities that affect glycogen precipitation.

Why is it important to keep the ethanol ice-cold during the extraction process?

To enhance the precipitation of glycogen

Why didn’t the Fehling’s test change color in the glycogen test from the liver extract?

Glycogen isn’t a reducing sugar

After the glycogen extraction, benedict’s test further analyzed the presence of reducing sugars in the sample. What color change was observed that signified a positive result for reducing sugars.

Blue to green

In determining if a glycogen is present in a solution, a student can use iodine’s test. What is the expected color reaction of the iodine test if glycogen is detected?

Reddish brown

While conducting the experiment of identifying the glycogen in chicken or pork liver, Fehling’s test was used. Why did the solution turn black?

Due to black precipitate of cupric oxide

 What is the correct process for isolating liver glycogen?

Grind the liver with washed sand—>Strain through a cheesecloth—>Add Ethanol—>Let the solution stand

How does one determine a successful isolation of liver glycogen?

By observing the formation of a white, fibrous precipitate after ethanol addition.

Which of the following tests is most appropriate for detecting the presence of glycogen in a liver extract?

Iodine’s Test because it binds to glycogen and produces a reddish-brown color, confirming its presence.

What does a high iodine value in the soap formulation suggest about the final product?

Softer bar with better lather and reduced shelf life

Which of the following best describes the role of palm oil in the soap formulation?

Contributes to hardness, creaminess, and conditioning

If coconut oil is replaced with olive oil in the same soap formulation, which of the following changes is most likely to occur in the final soap product?

The soap will become softer, with less lather but more moisturizing effects.

Babylonians were noted to produce soap using a mixture of ______ and ______.

animal fats and wood ash

After 3-4 weeks of curing, the cured bar of soap should have a pH of ____. (Choose which is applicable)

8

Among the list of oils used for the experiment, which is noted for its high cleaning and foaming properties?

coconut oil

What happens to the pH level of the soap when there is an insufficient amount of lye during saponification?

Lower pH due to an incomplete saponification.

Which of the following statements about Iodine Number Saponification (INS) is false?

The higher the INS value, the firmer yet less stable the soap.

The addition of corn oil in the formulation, which was high in __________ and __________, contributed to the soap’s conditioning properties, leaving the skin soft after use.

Linoleic acid; polyunsaturated fats

Coconut oil is the sole contributor to the soap’s lauric acid content. Palm oil is the largest contributor to the soap’s cleansing ability.

The first statement is true and the second statement is false.

During the saponification procedure, why is it important to weigh both sodium hydroxide and water precisely before mixing with oils?

To avoid excess lye or unreacted oil, which affects soap safety and quality.

Which fatty acid is 100% contributed by coconut oil in the soap sample?

Lauric acid

It is the process of reacting triglycerides with lye to produce glycerol and fatty acids.

Saponification

What is the recommended pH level for a soap?

All of the above

____ are organic compounds and membrane components that are grouped together due to their common characteristic of being hydrophobic.

Lipids

Which of the following contributes to the soap’s hardness?

Saturated Fatty Acids

The iodine value reflects the degree of saturation in the fatty acids present in the soap. The high iodine value indicates a greater proportion of saturated fatty acids

Both statements are false.

Which of the following contributes to the soap’s conditioning?

Unsaturated Fatty Acids

What parameter is referred to as the thickness and richness of the soap’s lather?

Creaminess

What type of chemical/lye is often used to create a soft or liquidized soap?

Potassium hydroxide (KOH)

Saponification is the process of making soap, which consists of potassium or sodium salts of long-chain fatty bases. During saponification, ester reacts with the organic base to produce alcohol and soap.

Both statements are false.

Saponification is an example of which type of chemical reaction?

Hydrolysis

A soap manufacturer wants to produce a softer soap. Which change in the saponification process would help achieve this?

Use potassium hydroxide instead of sodium hydroxide

In soap-making, why is it important to let the soap cure for several weeks after saponification?

To neutralize any remaining lye and let water evaporate

The following are the sample that was in the Experiment in DNA isolation Practical Class 12 EXCEPT the:

Carrot

What type of equipment has been used to Extract DNA from E.coli?

Digital Vortex Mixer