A student compared the reducing-sugar content of fruit juices. Equal volumes of Benedict's solution and juice were boiled together, then the tubes were left...

This question tests colorimetric estimation: the standards build a calibration relationship between a measurable quantity (depth of brick-red precipitate) and a known glucose concentration, and that relationship is then used to read off the unknown juices.

(a) When a reducing sugar is boiled with Benedict's solution the blue copper(II) ions are reduced to brick-red copper(I) oxide, so the colour changes from blue to brick-red, passing through green, yellow and orange as more precipitate forms. [1]

(b) The depth of settled precipitate is a length, so it is measured with a ruler. [1]

(c) The variable the experimenter deliberately sets when making the standards is the glucose concentration (0.0 to 4.0 %); everything else is kept the same. [1]

(d) Read the depths for the 1.0 % and 4.0 % standards and subtract: \( 16 - 4 = 12 \ \text{mm} \). [1]

(e) Grape gives 14 mm and pear gives 6 mm, so the difference is \( 14 - 6 = 8 \ \text{mm} \). [1]

(f) The standards show that \( 4 \ \text{mm} \) of precipitate corresponds to \( 1.0\% \) glucose, so each millimetre represents \( 0.25\% \). For the grape juice depth of 14 mm:

\[ \text{concentration} = \frac{14}{4} \times 1.0 = 3.5\% \]

Working [1], answer \( 3.5\% \) [1]. [2]

(g) Applying the same rule (divide the depth by 4) to each juice completes the table:

pear \( 6/4 = 1.5\% \), melon \( 10/4 = 2.5\% \), tomato \( 2/4 = 0.5\% \) (one mark each). [3]

(h) Reading the standards, as the glucose concentration increases the depth of precipitate increases [1]; because equal steps of 1.0 % give equal steps of 4 mm the two quantities are directly proportional (a straight-line, linear relationship through the origin) [1]. [2]

(i) The reduction of Benedict's solution is time-dependent, so the amount of precipitate depends on how long the reaction runs as well as on the sugar concentration [1]. Boiling and settling every tube for the same time keeps that factor constant, making the comparison fair [1], so any difference in depth is caused only by the different sugar concentrations [1]. [3]

(j) Any two controlled variables: the volume of Benedict's solution; the volume of juice; the boiling temperature or time; the diameter of the tube used (a wider tube spreads the same precipitate more thinly). [2]

(k) Sucrose is a non-reducing sugar: it does not reduce Benedict's solution and so gives no brick-red precipitate, meaning it could not produce a depth reading to calibrate against. Glucose is a reducing sugar and reacts. [1]

(l) Any two ways to improve reliability: repeat each test and take a mean; use tubes of the same diameter throughout; read the ruler at eye level to avoid parallax error; leave every tube to settle for the same fixed time before measuring. [2]

Exam tip: a calibration graph or rule only works if the measured effect (here precipitate depth) is proportional to the quantity of interest, so always keep reaction time and tube dimensions constant.