Final answer:
The mixture turned yellow due to the enzymatic breakdown of starch in bread by saliva into reducing sugars, which then react with Benedict's solution during heating to form a yellow precipitate.
Step-by-step explanation:
When bread is mixed with saliva and Benedict's solution, then heated, the Benedict's solution changes to a yellow color due to the presence of reducing sugars that result from the enzymatic digestion of starch in the bread by the enzyme amylase found in saliva. Benedict's reagent contains copper(II) sulfate, which reacts with these reducing sugars to form a precipitate that ranges in color from green to yellow to orange to brick red, depending on the concentration of the reducing sugars.
The enzyme amylase in saliva breaks down starch in the bread into shorter chains of sugars called maltose, which are then further broken down into glucose. These glucose molecules are reducing sugars, which means they can donate electrons to other molecules, including the cupric ions from copper(II) sulfate in the Benedict's reagent. In this specific test, a yellow precipitate indicates a moderate amount of reducing sugars present. The initial blue color of Benedict's reagent is due to the copper(II) ions in an alkaline solution. When reduced to copper(I) by glucose, it forms cuprous oxide, which is less soluble and precipitates out, changing the color of the solution.