= Solution
\b[<Glucose> can be obtained by <hydrolysis> of <starch> and detected as a <reducing sugar> by a heated alkaline <copper(II)> test. A positive copper test alone does not uniquely identify <glucose>.]
Suspend <starch> in <water>, warm it to make a paste, and heat with a dilute <acid>, such as dilute <sulfuric acid>, to cause <hydrolysis> of the <glycosidic bonds>. The <chemical reaction> passes through shorter <carbohydrates>; sufficient <hydrolysis> ultimately gives <glucose>. The usual repeat-unit accounting is
$$
\boxed{\mathrm{(C_6H_{10}O_5)_n+nH_2O\longrightarrow nC_6H_{12}O_6}.}
$$
This idealized <polymer> formula omits end-group details. The <acid> catalyses cleavage and is not consumed in the displayed equation. Merely using <amylase> from malt should not be described as necessarily producing pure <glucose>; it can leave <maltose> and shorter <polysaccharides>.
When <sulfuric acid> is used, neutralization with <calcium carbonate> removes the acidity and much of the <sulfate> as <calcium sulfate>. Filter out the insoluble residue, then concentrate the <chemical solution> gently and crystallize the <glucose>; further <recrystallization> is required to distinguish a reasonably pure <chemical substance> from a crude syrup. Overheating and unnecessarily prolonged strong <acid> treatment degrade the <sugar>. <Hydrolysis> of <sucrose> is a less selective preparative answer because it gives <fructose> alongside <glucose>.
For the historical copper test, use freshly mixed <Fehling's reagent>: an aqueous <copper(II) sulfate> portion and an alkaline <tartrate> portion. The <tartrate> keeps <copper(II)> in <coordination complexes> instead of allowing immediate bulk precipitation of <copper(II) hydroxide>. Add a suitably prepared sample to a small test portion and warm in a hot <water bath>. A yellow-orange to brick-red <precipitate> of <copper(I) oxide> indicates <reduction> of the <copper(II)> reagent. Run a known <glucose> sample and a <water> blank alongside it. In a simple sample, a visible difference between these controls helps distinguish a true positive from a deteriorated reagent or contamination.<Estimation of Glucose by Amrita Vishwa Vidyapeetham>[Theory, uses of Fehling's solution and formation of red copper(I) oxide]{ref}
The key <redox> accounting on the <copper> side is
$$
\mathrm{2Cu^{2+}+2OH^-+2e^-\longrightarrow Cu_2O+H_2O}.
$$
The required <electrons> come from <oxidation> of the <sugar>. This half-equation deliberately omits the changing <tartrate> <ligands> and does not assign a unique organic product. The often printed claim that hot alkaline <glucose> simply becomes <gluconate> in this test is too simple: competing <isomerization>, <oxidation> and fragmentation make the organic chemistry more complicated.<The Iodine Test for Reducing Sugars by Holger Fleischer>[§2.3, interpretation of copper tests and product complexity]{ref}
<Fructose>, <lactose>, <maltose> and various other <reducing substances> can also give a positive <Fehling's test>. In particular, <fructose> can form <aldoses> by <tautomerization> in <alkaline solution>; an initial <ketone> functional group does not make it reliably negative.<Organic Chemistry by John McMurry/25.6 Reactions of Monosaccharides>[Oxidation of Monosaccharides; fructose rearrangement]{ref} Fermentation by suitable <yeast>, with <carbon dioxide> formation, and <optical rotation> can supply further historical evidence, but neither is individually specific to <glucose>. These are anticipated diagnostic observations, not measurements supplied by the paper. In present laboratory practice an appropriately validated <enzymatic glucose assay> can establish <glucose> more selectively; the old reducing-sugar test is not by itself a diagnosis of <diabetes>.<Diagnosis of Diabetes in PDB-101>[Monitoring: Diagnosis, blood tests and glucose-oxidase assay explanation]{ref}
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