Chemical tests for biological molecules Flashcards
how do you carry the chemical tests to identify starch, sugars, proteins, fats and vitamin c?
first, important to carry out these tests on pure forms of these compounds. keep the results to compare with results from testing other materials. you should also do a test with water so you can see negative result as well
need to make extract from material you are testing. involves grinding up small amount with small water with a mortar. chemicals will be in solution in the extract
how do you test for starch?
.half-fill a test-tube with food extract
.add two or three drops of iodine solution (colour: yellow or light brown)
what will you see in a positive and negative result for starch?
a positive result for starch is if iodine solution turns blue-black.
a negative result (does not contain starch) is if the extract remains yellow or light brown colour,
how do you test for reducing sugars?
.put known volume of extract
.place a beaker on heat-proof mat
.half-fill beaker with boiling water from a kettle
.add same volume of Benedict’s solution (colour: bright blue) to test-tube containing food extract and put it into the hot water
you can use Benedict’s test to tell how much simple sugar is present
what will you see in a positive and negative result for reducing sugars?
a positive test for reducing sugars is when Benedict’s solution turns red or orange. if you leave test-tube to cool, you will also see a precipitate
a negative result (does not contain any reducing sugars) will be if the colour remains blue
you can use Benedict’s test to tell how much simple sugar is present. if colour changes to green, extract contains a little of reducing sugars. if it turns a deep orange, it contains a lot of reducing sugars
how do you test for protein?
.half-fill a test-tube with extract
.add five to six drops of biuret solution (colour: blue)
what will you see in a positive and negative result for protein?
a positive test for protein is if biuret solution turns purple, violet or lilac
. a negative test (does not contain protein) is if the colour remains blue
how do you test for fats?
fats will not dissolve in water but they will dissolve in ethanol
.chop up or grind a small amount of material (do not add water)
.put extract into a clean test-tube and add enough ethanol to cover it
.put stopper over test-tube ans shake up contents
.add some distilled water to make test-tube half ful
.shake contents once more
what will you see in a positive and negative result for fats?
a white emulsion that looks cloudy white or a milky colour is a positive test for fats
if this does not happen, the extract does not contain fat
how do you test for vitamin C?
.put known volume of DCPIP solution in test-tube (DCPIP is a blue liquid that loses its colour when it comes into contact with vitamin c)
.fill a syringe or dropping pipette with a solution of vitamin c
.add liquid one drop at a time
what will you see in a positive and negative result for vitamin c?
if colour of DCPIP disappears the test is positive for vitamin c
if blue colour persists then the test is negative
Where do fats dissolve and where not?
Fats will not dissolve in water but they in will ethanol
