Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Which substance chars when heated with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) protein (b) hydrocarbon (c) fat (d) carbohydrate

Step-by-Step Solution

Answer

Carbohydrate (d) chars when heated with conc. H2SO4 .

Step 1: Understand the Reaction

Concentrated sulfuric acid ( H2SO4 ) acts as a strong dehydrating agent. It removes water (H2O) from compounds.

Step 2: Identify the Charring Substance

When a compound such as carbohydrate is heated with concentrated H2SO4 , the acid removes water (dehydrates it), leaving behind carbon. This carbon forms a charred mass.

Step 3: Recognize Relevant Compounds

Carbohydrates, which are composed of carbon, hydrogen, and oxygen, can easily lose these elements as water (H2O). Other substances like proteins, hydrocarbons, and fats do not char in the same way when treated with concentrated H2SO4 .

Step 4: Conclusion

Thus, carbohydrates are the compounds that easily form a charred mass when heated with concentrated H2SO4 due to the dehydration process.

Key Concepts

Concentrated Sulfuric Acid

Concentrated sulfuric acid is a powerful chemical with several applications, including its use as a dehydrating agent. It is well-known for its ability to strip water from many compounds, a process known as dehydration. This strong acid has a remarkable affinity for water, which makes it a potent agent in reactions where removing water is essential.

When concentrated sulfuric acid comes into contact with a substance like a carbohydrate, it actively removes hydrogen and oxygen atoms in the form of water. This removal process can alter the chemical structure of the compound dramatically.

• Concentrated sulfuric acid acts intensely on carbon-based substances.
• The acid's dehydrating nature is particularly evident with compounds containing hydrogen and oxygen.
• Its ability to dehydrate directly leads to the phenomenon of charring.

This property of sulfuric acid is not only limited to carbohydrates but also extends to other reactions where dehydration is required for the transformation of compounds.

Carbohydrates

Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen. Their general formula is often expressed as \(Cn(H2O)n\), which signifies their close relation to water. This composition of carbohydrates makes them an ideal target for dehydration reactions.

The elements in carbohydrates can be easily separated by chemical means, such as using concentrated sulfuric acid. When sulfuric acid acts on carbohydrates, it removes water molecules (H2O) by detaching the hydrogen and oxygen atoms.

• Carbohydrates exist in various forms, including sugars, starches, and cellulose.
• The structured removal of water leaves behind primarily carbon, causing charring.
• Due to their composition, carbohydrates serve as an informative example for studying dehydration reactions.

Understanding this property highlights the sensitivity and specificity of chemical reactions with sulfuric acid and explains why they char compared to other organic substances.

Charring Process

The charring process is a chemical transformation where a substance, upon losing water, turns into carbon or a carbon-rich residue. This is distinctive of what happens when carbohydrates encounter concentrated sulfuric acid.

During charring, sulfuric acid's dehydrating power comes into play by removing the components of water, causing significant changes in the material’s structure. This leaves behind a blackened, carbon-filled mass, which is termed as "char".

• The charring of carbohydrates vividly demonstrates their structural change upon dehydration.
• Carbon is predominantly left as the water content is expelled.
• This transformation is visual and serves as an effective demonstration of dehydration in educational settings.

The charring process not only shows the practical application of sulfuric acid's properties but also demonstrates the chemical resilience of carbon, which remains after the volatile elements have been removed.