Read the information and use Table 1. A food company is testing a new breakfast cereal made from oats, dried fruit and seeds. Four cereal samples were given...
Question 1 Report
Read the information and use Table 1. A food company is testing a new breakfast cereal made from oats, dried fruit and seeds. Four cereal samples were given the same mass of dry cereal. Each sample was tested for starch using iodine solution, for reducing sugar using Benedict's solution, and for protein using biuret solution. The company also measured the energy released when each cereal sample was burned in a simple calorimeter.
| Sample | Iodine test | Benedict's test after heating | Biuret test | Energy released / kJ g^-1 |
|---|---|---|---|---|
| W | blue-black | blue | lilac | 15 |
| X | brown-orange | brick-red | lilac | 14 |
| Y | blue-black | brick-red | purple | 18 |
| Z | brown-orange | blue | purple | 20 |
(a) Complete the conclusion for sample W: it contains starch but no detectable ______ sugar. [1]
(b) Name the food substance indicated by a purple biuret result. [1]
(c) State which sample contains starch, reducing sugar and protein. [1]
(d) Explain why Benedict's test is heated. [1]
(e) Describe the test results that show sample Z does not contain starch. [2]
(f) Calculate the energy released by 35 g of sample Y. Show your working. [3]
(g) Explain why results from a simple food calorimeter may be lower than the true energy value. [2]
(h) Design one improvement that would make the comparison of cereal samples fair. [2]
Answer Details
(a) Sample W contains starch but no detectable reducing sugar. [1]
(b) A purple Biuret result indicates protein. [1]
(c) The sample with a blue-black iodine result, brick-red Benedict's result and purple Biuret result contains starch, reducing sugar and protein. [1]
(d) Benedict's test is heated because heat is needed for Benedict's solution to react with reducing sugar and produce the colour change. [1]
(e) For sample Z, iodine remains brown-orange; it does not change to blue-black. This is a negative test for starch. [2]
(f) \[18\text{ kJ g}^{-1}\times35\text{ g}=630\text{ kJ}\] The energy released is 630 kJ. [3]
(g) A simple calorimeter can give a low value because heat is lost to the surroundings rather than heating the water. Also, combustion may be incomplete, so not all energy in the cereal is released. [2]
(h) For a fair comparison, use the same mass of each cereal and the same volume of water. Starting at the same water temperature, keeping the flame-to-container distance constant, and repeating to calculate means are further valid controls. [2]
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