Tube Glucose concentration / % Final colour A 0.0 blue B 0.5 green C 1.0 ? D 2.0 orange E 5.0 brick-red A technician prepared five test tubes, labelled A to...
Question 1 Report
| Tube | Glucose concentration / % | Final colour |
|---|---|---|
| A | 0.0 | blue |
| B | 0.5 | green |
| C | 1.0 | ? |
| D | 2.0 | orange |
| E | 5.0 | brick-red |
A technician prepared five test tubes, labelled A to E, and added 2 cm3 of a different glucose solution to each. Tube A held distilled water only, and the glucose concentration increased from tube B to tube E. She added 2 cm3 of blue Benedict's solution to every tube and stood the tubes in a boiling water bath for four minutes. The final colours are recorded in Table 1.1.
Fig. 1.1 shows the tubes of glucose solution and Benedict's solution heated in a boiling water bath with a thermometer.
(a) State the colour of Benedict's solution before it is heated. [1]
(b) Before heating, a thermometer in the water bath read 19 °C; when the water boiled it read 92 °C. Measure the increase in temperature of the water bath. [1]
(c) The technician used a syringe to add the Benedict's solution. The plunger moved from the 10 cm3 mark to the 8 cm3 mark as she filled one tube. Measure the volume of Benedict's solution she added. [1]
(d) Complete Table 1.1 by stating the colour you would expect in tube C. [1]
(e) Describe the trend shown in Table 1.1 between the glucose concentration and the final colour. [2]
(f) Name the type of sugar that gives a positive result with Benedict's solution. [1]
(g) Explain why tube A stayed blue. [2]
(h) The test is described as semi-quantitative. Explain what this means. [2]
(i) State two variables the technician kept constant so the colours could be compared fairly. [2]
(j) A food sample gave an orange colour in this test. Use Table 1.1 to estimate its glucose concentration and give a reason for your answer. [2]
(k) Describe how to carry out the Benedict's test on a new food solution, from adding the reagent to reading the result. [3]
(l) State one hazard of this method and a precaution against it. [2]
Answer Details
This question tests the Benedict's test for reducing sugar, reading measuring instruments, the colour scale, and safe method design.
(a) Before heating, Benedict's solution is blue. [1]
(b) Increase in temperature: \(92 - 19 = 73\) °C. [1]
(c) Volume added is the change in the syringe reading: \(10 - 8 = 2\) cm3. [1]
(d) Tube C (1.0 % glucose) lies between tube B (0.5 %, green) and tube D (2.0 %, orange) on the colour scale, so it would be yellow (accept green-yellow). Completed table: [1]
| Tube | Glucose concentration / % | Final colour |
|---|---|---|
| A | 0.0 | blue |
| B | 0.5 | green |
| C | 1.0 | yellow |
| D | 2.0 | orange |
| E | 5.0 | brick-red |
(e) As the glucose concentration increases, the final colour changes in order from blue through green, yellow and orange to brick-red [1]; the more reducing sugar present, the further along the scale the colour moves (a redder colour) [1]. [2]
(f) Benedict's solution gives a positive result with a reducing sugar such as glucose. [1]
(g) Tube A stayed blue because it contained only distilled water, with no glucose / no reducing sugar [1], so there was nothing to reduce the blue copper(II) ions and the solution was not changed [1]. [2]
(h) "Semi-quantitative" means the test gives an estimate of the amount / concentration of reducing sugar from the colour produced [1], rather than an exact measured value; the colour is matched against a scale to judge roughly how much is present [1]. [2]
(i) Any two variables kept constant so colours can be compared fairly: the volume of Benedict's solution, the volume of glucose solution, the heating time, and the temperature of the water bath. [2]
(j) An orange colour matches tube D, so the food's glucose concentration is about 2.0 % [1], because tube D was 2.0 % and also gave an orange colour [1]. [2]
(k) Carrying out the test on a new food solution: [3]
- add an equal volume of blue Benedict's solution to the food solution [1];
- heat the tube in a boiling water bath for a few minutes [1];
- observe and record the final colour, reading it against the scale from blue (negative) through to brick-red (high concentration) [1].
(l) Hazard: the hot water or steam can scald, and boiling tubes can spit [1]; precaution: use a test-tube holder, wear eye protection, and point the tube away from people [1]. [2]
Exam tip: the Benedict's colour is a scale, not a yes/no result, so read the exact shade to judge how much reducing sugar is present.
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