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Classification of Carbohydrates

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.

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Student-friendly explanation

Carbohydrates are classified mainly by the number of sugar units produced on hydrolysis. Monosaccharides cannot be hydrolysed further into simpler carbohydrates; glucose and fructose are common examples. Disaccharides give two monosaccharide units on hydrolysis; sucrose, lactose, and maltose belong here. Polysaccharides give many monosaccharide units and include starch, glycogen, and cellulose. This classification is chemical because it depends on hydrolysis behaviour, not just sweetness or solubility.

How to write this in exams

  1. 1

    Start with the exact idea

    Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.

  2. 2

    Then show how to use it

    First identify whether hydrolysis occurs. Next count the number of monosaccharide units produced. Then write the class with one correct example.

  3. 3

    Add one concrete example

    Glucose is a monosaccharide, sucrose is a disaccharide, and starch is a polysaccharide made from many glucose units.

  4. 4

    Avoid this incomplete answer

    Calling starch a disaccharide because it is made of glucose units is wrong; it contains many glucose units and is a polysaccharide.

Definition

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.

Example

Glucose is a monosaccharide, sucrose is a disaccharide, and starch is a polysaccharide made from many glucose units.

Rule to remember

Rule: no hydrolysis means monosaccharide; hydrolysis to two monosaccharides means disaccharide; hydrolysis to many monosaccharides means polysaccharide. General carbohydrate formula is often written as Cx(H2O)y, but this is not a reliable definition for every carbohydrate.

Memory hook

Mono means one, di means two, poly means many sugar units after hydrolysis.

Examples and method

Worked example

Classify glucose, maltose, and cellulose. Glucose cannot be hydrolysed further, so it is a monosaccharide. Maltose gives two glucose units, so it is a disaccharide. Cellulose gives many glucose units, so it is a polysaccharide.

Method to apply

First identify whether hydrolysis occurs. Next count the number of monosaccharide units produced. Then write the class with one correct example.

Diagram support

A flowchart is useful: carbohydrates branch into monosaccharides, disaccharides, and polysaccharides, each with examples.

How CBSE asks it

Usually asked as definition, classification with examples, or a comparison between mono-, di-, and polysaccharides.

Avoid common mistakes

Common confusion

Students often classify all sweet substances as carbohydrates or assume all carbohydrates are sweet. Cellulose and starch are carbohydrates but are not sweet like glucose or sucrose.

Common wrong answer

Calling starch a disaccharide because it is made of glucose units is wrong; it contains many glucose units and is a polysaccharide.

Exam tip

When asked to classify, check the number of monosaccharide units obtained on hydrolysis before writing the class.

Quick check

What class of carbohydrate is sucrose, and why?

Sucrose is a disaccharide because one molecule gives two monosaccharide units on hydrolysis.

Answer writing and exam use

1-mark answer

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.

2-mark answer

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. Rule: no hydrolysis means monosaccharide; hydrolysis to two monosaccharides means disaccharide; hydrolysis to many monosaccharides means polysaccharide. General carbohydrate formula is often written as Cx(H2O)y, but this is not a reliable definition for every carbohydrate. Glucose is a monosaccharide, sucrose is a disaccharide, and starch is a polysaccharide made from many glucose units.

3-mark answer

Carbohydrates are classified mainly by the number of sugar units produced on hydrolysis. Monosaccharides cannot be hydrolysed further into simpler carbohydrates; glucose and fructose are common examples. Disaccharides give two monosaccharide units on hydrolysis; sucrose, lactose, and maltose belong here. Polysaccharides give many monosaccharide units and include starch, glycogen, and cellulose. This classification is chemical because it depends on hydrolysis behaviour, not just sweetness or solubility. Rule: no hydrolysis means monosaccharide; hydrolysis to two monosaccharides means disaccharide; hydrolysis to many monosaccharides means polysaccharide. General carbohydrate formula is often written as Cx(H2O)y, but this is not a reliable definition for every carbohydrate. Classify glucose, maltose, and cellulose. Glucose cannot be hydrolysed further, so it is a monosaccharide. Maltose gives two glucose units, so it is a disaccharide. Cellulose gives many glucose units, so it is a polysaccharide. Usually asked as definition, classification with examples, or a comparison between mono-, di-, and polysaccharides. Calling starch a disaccharide because it is made of glucose units is wrong; it contains many glucose units and is a polysaccharide.
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