A laboratory technician set up the apparatus in Fig. 1.1 to carry out the Benedict's test on a sample of fruit juice thought to contain reducing sugar. A bo...
Question 1 Report
A laboratory technician set up the apparatus in Fig. 1.1 to carry out the Benedict's test on a sample of fruit juice thought to contain reducing sugar. A boiling tube holding 2 cm3 of the juice and an equal volume of Benedict's solution stands in a water bath heated by a Bunsen burner, with a thermometer to check the temperature. The technician heated the tube for four minutes and watched the colour develop. As the amount of reducing sugar rises, the colour passes from blue, through green, yellow and orange, to brick-red.
Fig. 1.1
(a) State the colour of Benedict's solution before it is heated [1]
(b) State the temperature to which the water bath should be raised for this test [1]
(c) Describe the colour change you would expect if the juice contained a high concentration of reducing sugar [2]
(d) Suggest why the tube is heated in a water bath rather than directly in the Bunsen flame [2]
Answer Details
This tests the food test for reducing sugar and safe heating in a water bath.
(a) Before heating, Benedict's solution is blue. [1]
(b) The water bath should be raised to about 90 to 100 degrees C (boiling). [1]
(c) With a high concentration of reducing sugar the blue colour changes through green, yellow and orange [1], and finishes brick-red (orange-red) [1] [2]. The further along this sequence, the more reducing sugar is present, so a full change to brick-red indicates a high concentration.
(d) A water bath is used rather than the naked flame because it heats the tube evenly to a controlled temperature [1], whereas direct heating overheats one spot so the contents may spurt (bump) out of the tube [1] [2], which is unsafe.
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