A student used the biuret test to find out which of four liquid foods contained protein. To 2 cm3 of each food in a test tube she added a few drops of coppe...
Question 1 Report
A student used the biuret test to find out which of four liquid foods contained protein. To 2 cm3 of each food in a test tube she added a few drops of copper sulfate solution followed by an equal volume of dilute sodium hydroxide solution, mixed gently, and left the tubes in the rack shown in Fig. 3.1 for two minutes. Tubes that turned purple are shaded in the figure; the others stayed blue. The foods tested were soup, sugar solution, egg white and apple juice.
(a) State the colour a food turns in the biuret test when protein is present [1]
(b) State the colour seen when no protein is present [1]
(c) Name the two foods that contain protein [2]
(d) The two reagents are corrosive. Describe two precautions the student should take when using them [2]
(e) Explain why the biuret test is quicker to carry out than the Benedict's test [2]
Answer Details
(a) When protein is present the biuret test turns purple (lilac / violet). [1]
(b) When no protein is present the mixture stays blue (the colour of the copper sulfate), so there is no colour change. [1]
(c) The two shaded tubes that turned purple, and therefore contain protein, are soup [1] and egg white [1]. Sugar solution and apple juice stayed blue, so they contain little or no protein.
(d) Because the copper sulfate and sodium hydroxide are corrosive, take any two precautions: wear eye protection / safety goggles; wear gloves; wipe up any spills at once; avoid skin contact and wash off splashes with plenty of water. [1] [1]
(e) The biuret test is quicker because it works at room temperature with no heating [1], so there is no wait for a water bath to heat the sample and the purple colour appears almost at once [1], whereas Benedict's test needs several minutes of heating.
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