Fehling's B is a chemical solution used in the Fehling's test, which helps identify reducing sugars. It is composed of a solution of copper(II) sulfate pentahydrate, typically mixed with Fehling's A, which contains sodium potassium tartrate and sodium hydroxide. When combined, these solutions create a deep blue color due to the presence of copper ions. In the presence of reducing sugars, such as glucose, Fehling's B undergoes a chemical reaction that reduces the copper ions to copper(I) oxide, resulting in a brick-red precipitate. This color change indicates the presence of reducing sugars in the tested sample.