why starch gives blue colour with iodine
cellulose is derived from D-glucose units, which condensed through beta(1- 4)-glycosidic bond. This give cellulose to be a straight polymer therefore, it can't coil around iodine to produce blue color as starch does. To approximately 2 cm of starch solution two drops of iodine/potassium iodide solution were added. A blue-black colour indicates the presence of starch as a starch-polyiodide complex is formed. Starch is only slightly soluble in water, but the test works well in a suspension or as a solid.
In this ar]ti]cle, we will use a 5% al]co]hol so]lu]tion of which is used in medicine, and the ma]jor]i]ty of re]ac]tions car]ried out inPlab]o]ra]to]ries. Starch in]ter]acts with io]dine, form]ing in]clu]sion com]pounds, i. e. clathrates.
This chem]i]cal process was dis]cov]ered back in 1814 by the sci]en]tists Jean Jacques Col]in and Hen]ri-Franois Gaulti]er dePClaubry In]clu]sion com]pounds are spe]cial com]pounds in which the mol]e]cules of one sub]stance en]ter the molec]u]lar struc]ture of an]oth]erPsub]stance. In this case, amy]lose mol]e]cules (one of the main polysac]cha]rides of starch) will be the hosts and the mol]e]cules will be the guests. Click to see more stun]ning ex]per]i]ments with io]dine. This is quite a sim]ple chem]i]cal ex]per]i]ment which can be car]ried out at home and shown to chil]dren, to in]spire them with a love forPchem]istry. stir]ringProd. Pour wa]ter into the test tube and add 4-5 drops of io]dine.
Add a pinch of starch and mix well with a rod. The re]sult will be a dark bluePso]lu]tion. In]ci]den]tal]ly, you can also re]peat this ex]per]i]ment in oth]er ways, for ex]am]ple add one drop of io]dine to a small mound of starch, and a dark blue patch will ap]pear. You can also drip io]dine on to half a pota]to, as it has a high starch con]tent: if you im]merse a peeled pota]to in cold wa]ter, starch par]ti]cles will ap]pear in the wa]ter af]ter a cer]tain pe]ri]od of time. If you hold a peeled pota]to in your hands, they will also be]come coat]ed withPstarch. If you heat a test tube con]tain]ing a so]lu]tion of starch, io]dine and wa]ter over a spe]cial chem]i]cal burn]er for 10 sec]onds, the so]lu]tion will turn a white, trans]par]ent col]or.
This is be]cause the com]pound of io]dine and starch is un]sta]ble, but if you put the test tube in cold wa]ter, a dark blue sed]i]ment will form oncePmore. When starch is heat]ed to boil]ing point, it be]gins to break down, and the chains of amy]los]es break, thus form]ing short chains of dex]trins, so the col]or starts to change. Sep]a]rate com]pounds of glu]cose do not give any col]or in a re]ac]tion withPio]dine. An in]ter]est]ing fact: Amy]lopectin, a polysac]cha]ride of starch, gives a pur]ple-red col]or]ing when re]act]ed with io]dine. There is sig]nif]i]cant]ly more amy]lopectin in starch than amy]lose, which gives a blue col]or, but the blue col]or over]rides the red-pur]plePcol]or.
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