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Why does starch change to blue black when iodine solution is added then heated yet glucose has no observable reaction?
Vibesangel
This is because starch is composed of polymers of glucose. Long linear chains are amylose. Amylopectin is similar but contains a branch point about every 25th glucose or so. Amylose coils into a helical secondary structure resembling a tube with a hollow core. Certain molecules including fatty acids and iodine can lodge inside the core as already mentioned. The complex of iodine stuck inside the amylose coil produces a characteristic blue-black colour. The starch itself is not altered. Starch-iodine complex becomes unstable at temperatures above 35 °C. This complex in presence of an oxidizing agent the solution turns blue, in the presence of reducing agent, the blue color disappears because triiodide (I3−) ions break up into three iodide ions, disassembling the complex. So starch turns into glucose molecules. Therefore the blue black colour disappears. However, when it cools down again, then the glucose macromolecules bonded up together again in a long chain, becoming starch. That is why it tested positive for starch and turns back into blue-black colour.
Wiki User
∙ 15y ago
Wiki User
∙ 9y agoWhen starch and iodine are mixed, the color becomes deep blue due to the amylose content in starch. When heat is applied, the association of amylose and iodine is disrupted, thus loses its color.
Wiki User
∙ 15y agoglucose reduces the copper (ii) ions Cu2+ in benedicts reagent to Copper (i) oxide. yellow-brown(actaully red brown). the color is due to Cu+ in the copper (i) oxide. starch cannot reduce the Copper ions in copper (ii) oxide.
Wiki User
∙ 11y agobecause starch is present in the solution
Probable a complex beta amylose (a compound from starch)-iodine is formed.
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What is the only sure sign of a chemical change?
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Why did glucose test strip change colour after saliva was added?
chemical reaction
What is the only sure sign of a chemical change?
Chemical change would cause change in physical quality or thermal value, the most observable quality is colour. It may observed a colour change of reaction but not all reaction had observable change of colour. In that case it will need to add indicator to observe a chemical change. Type of indicator may be different base on the concerned reaction. Indicator chosen may be the type that change colour due to pH or the type that react with contents of product to change colour.
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