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Biology- Testing for starch and sugars questions:

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    As you can tell when reading these, I'm not sure whether I have answered these right so I would be really grateful for some help please. Thank you.

    Q. What do the results show about the tests for starch and glucose?
    My answer: Is it just about the colour change?

    Q. Why test with distilled water?
    My answer: because it determines whether the test reagent is responsible colour change or an external factor is.

    Q. Explain why (a) starch and iodine; and (b) glucose and Benedict's react as they do.
    My answer: I'm not sure about this, so please can you help me?

    Q. Could you determine the amount of starch and glucose using these tests?
    My answer: Is the answer yes because for the test for glucose, the more glucose present the colour change goes form green, yellow then red.


    Q. What is the purpose of heating the sucrose with hydrochloric acid at the start of the test? Explain what happened during this part of the experiment.
    My answer: Is it because it speeds up the chemical reaction?

    Q. Why is sodium hydrogencarbonate added to the tube before testing with Benedict's solution?
    My answer: Does this also speed up the reaction?

    Q. What did the Benedict's control test, carried out on the sucrose which had no pre-treatment with acid and sodium hydrogencarbonate, show you?
    My answer: A colour change from blue to green/yellow/ brick-red
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    Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.


    Just quoting in Fox Corner so she can move the thread if needed :wizard:
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    (Original post by leo_147)
    As you can tell when reading these, I'm not sure whether I have answered these right so I would be really grateful for some help please. Thank you.

    Q. What do the results show about the tests for starch and glucose?
    My answer: Is it just about the colour change?

    I think so.

    Q. Why test with distilled water?
    My answer: because it determines whether the test reagent is responsible colour change or an external factor is.

    I would essentially agree, but the question might want you to comment on the lack of ions in distilled water which could react.

    Q. Explain why (a) starch and iodine; and (b) glucose and Benedict's react as they do.
    My answer: I'm not sure about this, so please can you help me?

    Starch and Iodine - I don't know how much you need to know but as far as I am aware the iodine (in KI or similar) produces a I3- ion which forms a coloured complex with the starch.
    Glucose and Benedict's - Glucose is a reducing sugar, so it can be oxidised and reduce something else. Benedict's reagent is reduced by the glucose by reacting with the glucose aldehyde group (look at the straight form of glucose).

    Q. Could you determine the amount of starch and glucose using these tests?
    My answer: Is the answer yes because for the test for glucose, the more glucose present the colour change goes form green, yellow then red.

    I would disagree - the tests are qualitative not quantitative.

    Q. What is the purpose of heating the sucrose with hydrochloric acid at the start of the test? Explain what happened during this part of the experiment.
    My answer: Is it because it speeds up the chemical reaction?

    Sucrose is not a reducing sugar, HCl can break the bond in sucrose between the glucose and fructose. (acid hydrolysis).

    Q. Why is sodium hydrogencarbonate added to the tube before testing with Benedict's solution?
    My answer: Does this also speed up the reaction?

    NaHCO3 is a base, HCl (from the previous question) is an acid. What might happen?

    Q. What did the Benedict's control test, carried out on the sucrose which had no pre-treatment with acid and sodium hydrogencarbonate, show you?
    My answer: A colour change from blue to green/yellow/ brick-red

    I don't think it should've, sucrose is not a reducing sugar.
    It's been a while since I've done these tests but I've had a crack at them. I'm only going to give hints if I think you haven't got them wrong, if you need any extra help with one just ask.
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    (Original post by ThatPerson2)
    It's been a while since I've done these tests but I've had a crack at them. I'm only going to give hints if I think you haven't got them wrong, if you need any extra help with one just ask.
    Thank you for your offer to help! But I've handed it in now. I should probably delete this post now x
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    (Original post by leo_147)
    Thank you for your offer to help! But I've handed it in now. I should probably delete this post now x
    Oh ok! Good luck!
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    (Original post by ThatPerson2)
    Oh ok! Good luck!
    I got them all right after all. Thanks for your time though!
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    (Original post by leo_147)
    I got them all right after all. Thanks for your time though!
    Congratulations!

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