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Biology A Level > food tests > Flashcards

food tests Flashcards

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1
Q

what type of sugars are known as reducing sugars?

A

all monosaccharides and many disaccharide sugars are known as reducing sugars.

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2
Q

what is a reducing sugar?

A

a reducing sugar is a molecule that can react with other molecules by giving electrons to them (reducing agent)

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3
Q

what is the name of the test for reducing sugars?

A

Benedict’s test

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4
Q

what is the method for a Benedict’s test?

A
  1. place about 1cm3 of a solution in a test tube
  2. add an equal volume about 1cm3 of Benedict’s solution (usually blue)
  3. boil or heat to 80oC in a water bath for 2 mins
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5
Q

what will a positive result show for the Benedict’s test?

A

the Benedict’s reagent will gradually turn red through a series of colours
BLUE –> GREEN –> YELLOW –> ORANGE –> RED

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6
Q

what will a negative result show for the Benedict’s test?

A

no change to the blue colour

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7
Q

what factor affects the extent of the colour change of the Benedict’s reagent?

A

The higher the concentration of reducing sugar, the further the colour change goes

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8
Q

why is there a test for non-reducing sugars?

A

there is a test for non-reducing sugars because some sugars do not react with Benedict’s solution at all, so a reducing sugar test would show up as negative - no colour change

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9
Q

name a common non-reducing sugar?

A

Sucrose is the only common non reducing sugar

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10
Q

How is sucrose formed?

A

sucrose is a disaccharide and is formed by a condensation reaction between a glucose molecule and a fructose molecule

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11
Q

what is the method to be followed if a Benedict’s test proves to be negative?

A
  1. Add several drops of dilute hydrochloric acid –> 2cm3 of the test solution in a test tube and boil in a water bath for several minutes.
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12
Q

why is hydrochloric acid added to 2cm3 of test solution and left to boil in a water bath?

A

hydrochloric acid breaks down sucrose into its monosaccharides

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13
Q

what should be done after adding the acid?

A
  1. cool the solution and neutralise it by adding some sodium hydrogencarbonate solution
  2. carry out the Benedcict’s test again
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14
Q

why is sodium hydrogencarbonate added to the cooled solution?

A

the alkali neutralises the acid, which is important as the acid inhibits Benedict’s reagent

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15
Q

what will a positive result of the non-reducing sugar test look like?

A

The benedict’s reagent will now give a positive result because the monosaccharide’s glucose and fructose are present

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16
Q

what will a negative result of the non-reducing sugar test look like?

A

no change - which indicates no reducing or non-reducing sugars present

17
Q

what is the test for lipids known as?

A

the test for lipids is known as the Ethanol emulsion test

18
Q

what is the method for a lipid emulsion test?

A
  1. mix and crush the material to be tested with a glass rod in some ethanol in a test tube - shake for 1 minute to help it mix
  2. then filter, or carefully decant, the ethanol into a second test tube containing cold water (do not mix)
  3. discard the solid residue - avoid any solid getting in the water
19
Q

what does a positive result for the ethanol emulsion test look like?

A

a milky emulsion forming in the water indicates lipid in the ethanol

20
Q

what does a negative ethanol emulsion test look like?

A

no milky emulsion

21
Q

what is the test for polypeptides known as?

A

the Biuret test

22
Q

what is the method for the Biuret test?

A
  1. Place about 1cm3 of a solution or suspension of the test substance in a test tube
  2. Add an equal volume (about 1cm3) of biuret solution into the test tube
  3. mix the biuret solution and test solution by swirling
23
Q

What is biuret solution?

A

Biuret solution is a blue solution which contains copper sulphate and sodium hydroxide

24
Q

what is a positive result for the biuret test?

A

VIOLET/PURPLE or LILAC colour

25
Q

What is a negative result for the biuret test?

A

No change to the blue colour of copper sulphate

26
Q

what is the test for carbohydrates and polysaccharides known as?

A

the iodine test

27
Q

what is the method for the iodine test?

A
  1. Test substance may be solid or liquid
  2. place a small amount on a white tile/in a test tube
  3. add iodine solution
28
Q

what is iodine solution?

A

iodine solution is iodine dissolved in potassium iodide solution - It is yellow orange

29
Q

what does a positive iodine test result look like?

A

blue black colour

30
Q

what does a negative iodine test result look like?

A

a yellow orange colour - does not change

Biology A Level (36 decks)

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