Amylum starch is starch, a natural carbohydrate polymer stored by plants. In general chemistry, it is treated as a polysaccharide made from many glucose-derived repeating units. Its usual formula representation is \( (\text{C}_6\text{H}_{10}\text{O}_5)_n \), where \(n\) is a large whole number that varies with the size of the starch molecule.
Chemical identity
Amylum is the traditional or pharmacopeial name for starch. Chemically, starch is not a single small molecule with one fixed molecular mass. It is a mixture of large polysaccharide chains built from glucose units joined by glycosidic bonds. The two main structural fractions are amylose, which is mostly linear, and amylopectin, which is highly branched.
The repeating unit of starch is derived from glucose, \( \text{C}_6\text{H}_{12}\text{O}_6 \). During condensation polymerization, water is removed when glucose units join. Each incorporated glucose residue is therefore represented as \( \text{C}_6\text{H}_{10}\text{O}_5 \), not \( \text{C}_6\text{H}_{12}\text{O}_6 \).
For a more exact chain-growth representation, a chain of \(n\) glucose units forms \(n-1\) glycosidic bonds and releases \(n-1\) water molecules:
The compact formula \( (\text{C}_6\text{H}_{10}\text{O}_5)_n \) remains the standard classroom representation because it emphasizes the repeating anhydroglucose unit in the polymer.
Formula mass of the repeating unit
The formula mass of one anhydroglucose repeating unit, \( \text{C}_6\text{H}_{10}\text{O}_5 \), is calculated from atomic masses of carbon, hydrogen, and oxygen:
A starch molecule containing \(n\) repeating units has an approximate polymer molar mass of:
This expression is approximate because real starch samples contain chains with different lengths, branching patterns, and molecular masses.
Amylose and amylopectin
| Starch fraction | Structural feature | Bonding pattern | General chemistry importance |
|---|---|---|---|
| Amylose | Mostly linear chain | Primarily α(1→4) glycosidic bonds | Forms helical regions that interact strongly with iodine |
| Amylopectin | Highly branched chain | α(1→4) main-chain bonds with α(1→6) branch points | Explains branching, high molecular size, and different physical behavior |
| Amylum starch sample | Mixture of amylose and amylopectin | Many glycosidic bonds across many glucose residues | Has no single exact molecular mass like a small compound |
Iodine test behavior
Amylum starch gives a characteristic blue-black color with iodine solution. The color is not a simple precipitation reaction. It arises because iodine-containing species fit inside helical amylose regions, producing a charge-transfer complex that absorbs visible light differently from free iodine.
The iodine test is a qualitative analysis method. A positive blue-black color indicates starch or amylose-rich material, while the absence of that color suggests little or no detectable starch under the test conditions.
Pure compound versus polymer sample
Starch has a recognizable repeating formula, but an amylum starch sample is not one molecule of fixed size. The value of \(n\) varies from chain to chain, so the sample contains a distribution of polymer molecules. This is why starch is represented by a repeating formula rather than by a single small-molecule formula.
Final answer: amylum starch is a plant polysaccharide composed of glucose-derived repeating units, commonly written as \( (\text{C}_6\text{H}_{10}\text{O}_5)_n \). It contains amylose and amylopectin, forms glycosidic bonds through condensation, and gives a blue-black iodine test due to iodine interaction with helical amylose regions.