Question 3.1

Saff_brox12

What is the iodine and Benedict’s practical for question 3.1

Reply 1

Kallisto

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Where is the question? you have not linked an attachement but perhaps I am too blind to see it.

Reply 2

TypicalNerd

Original post

by Saff_brox12

What is the iodine and Benedict’s practical for question 3.1

This sounds an awful lot like a GCSE biology practical that I only vaguely remember.

Iodine is used to test for starch as it forms a distinct, blue-black complex.

Benedict's solution is used to test for reducing sugars (glucose, in particular). It is a pale blue solution that upon heating with a reducing sugar gives a red solid (copper(I) oxide). That said, it really does depend on how much of the reducing sugar is present - lower concentrations may make the resulting mixture appear green or orange.

Reply 3

Kallisto

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Original post

by TypicalNerd

In terms of the iodine and the starch, I can confirm what you said. I remember that a leaf is used to get cooked first to wash the chlorophyll out in order to see how the blue-black-complex changes the colour of the leaf after iodine reacts with the remained starch.

Just an added explanation to your statement, nothing else. You had a good reminder. :smile:

Reply 4

TypicalNerd

Original post

by Kallisto

In terms of the iodine and the starch, I can confirm what you said. I remember that a leaf is used to get cooked first to wash the chlorophyll out in order to see how the blue-black-complex changes the colour of the leaf after iodine reacts with the remained starch.
Just an added explanation to your statement, nothing else. You had a good reminder. :smile:

I don’t think the iodine test is carried out on leaves at GCSE (though I didn’t do all the practicals then as COVID came along in the final year, so I definitely didn’t do every practical) - in the practical I remember doing, the tests were done as a way of identifying nutrients in food. That said, you are right that cooking a leaf to decompose chlorophyll is something you can do to make a starch test on leaves less ambiguous.

I do believe there were more tests we had to do for the nutrient tests practical, including using yet another copper(II) sulphate-based mixture called biuret to test for proteins (if memory serves, it goes from blue to purple when the copper(II) ions coordinate to the amino acids to form ammine complexes).