Practical 1 - Food tests

Question 1

What do we use these food tests for?

Answer 1

Investigating the chemical content of some foods or unknown substances.

Question 2

How can we use these food tests?

Answer 2

Can use on pure chemicals to demonstrate positive results then use it to demonstrate the composition in different foods

Question 3

As these tests don’t tell us the exact concentration of different chemicals in the samples, what does that make them?

Answer 3

Qualitative tests

Question 4

What are the 5 different foods we tested for?

Answer 4

-Reducing sugars
-Non-reducing sugars
-Proteins
-Starch
-Fats and oils

Question 5

Which test solution gave us our reducing sugar?

Answer 5

Glucose

Question 6

Which test solution gave us our non-reducing sugar?

Answer 6

Sucrose

Question 7

Which test solution gave us our proteins?

Answer 7

Albumen solution

Question 8

Which test solution gave us our starch?

Answer 8

Starch solution

Question 9

What’s the absolute alcohol that we used? What property does this have that stands out?

Answer 9

Ethanol
Highly flammable

Question 10

What was the colour change with the reducing sugar?

Answer 10

Blue through to brick red (through the rainbow)

Question 11

What was the colour change with the non-reducing sugar?

Answer 11

Blue to red

Question 12

What was the colour change with the proteins?

Answer 12

Blue to violet

Question 13

What was the colour change with the starch?

Answer 13

Orange to black

Question 14

What was the colour change with the fats and oils?

Answer 14

Clear to cloudy/milky

Question 15

Reducing sugars test

Answer 15

-Mix 2cm3 of the test solution (glucose) with an equal volume of Benedict’s reagent
-Heat the mixture in a water bath at between 70-90 degrees for 5 minutes

Question 16

Non-reducing sugars test

Answer 16

-Mix 2cm3 of the test sample with an equal volume of Benedict’s reagent
-Heat in a water bath at between 70-90 degrees for 5 minutes
-Observe any colour changes
-Add 2 drops of hydrochloric acid to another 2cm3 of the test solution to a boiling tube and heat in a water bath between 70 and 90 degrees for 2 minutes
-Add two drops of Sodium Hydroxide/a small spatula of Sodium Bicarbonate
-Add 2cm3 Benedict’s reagent
-Heat the mixture in a water bath to between 70 and 90 degrees for 5 minutes

Question 17

Test for Proteins

Answer 17

-Add 2cm3 of the test solution (albumen) to 2cm3 of Biuret reagent in a boiling rube
-Cover the top and invert once

Question 18

Test for starch

Answer 18

-Add 2cm3 of test solution (starch solution) and 2 drops of iodine in potassium-iodide solution

Question 19

Test for fats and oils

Answer 19

-Mix the fat or oil with 5cm3 of absolute alcohol (ethanol) in a boiling tube
-Shake the tube
-Pour the mixture into another boiling tube with an equal volume of cold water

Question 20

How does the reducing sugars test work scientifically speaking?

Answer 20

The reducing sugar donates an electron to reduce copper (II) ions in copper sulphate (blue) to copper (I) ions in copper oxide (red)

Question 21

Why do we need hydrochloric acid in the non-reducing sugars test?

Answer 21

To break the test solution down into constituent monosaccharides

Question 22

Why do we add sodium hydroxide/sodium bicarbonate during the non-reducing sugars test?

Answer 22

Benedict’s reagent needs alkaline to work
+ neutralise excess acid

Question 23

What does Benedict’s reagent need to work?

Answer 23

Alkaline

Question 24

What do you call the cloudiness created during the fats and oils test?

Answer 24

Emulsion

Question 25

What type of test is Benedict’s reagent?

Answer 25

Semi quantitative

Question 26

Which type of food will not change colour with just Benedict’s reagent?

Answer 26

Non-reducing sugars

Question 27

Why cant we use the Benedict’s Reagent test to estimate the concentration of glucose in a food sample?

Answer 27

There could be more than one reducing sugar in a food- all would produce a positive result in the Benedict’s reagent

Question 28

What’s the hazard with ethanol?

Answer 28

Highly flammable

Question 29

What’s semi-quantitative?

Answer 29

Gives an approximation but does not give actual values

Question 30

Difference between qualitative and quantitative?

Answer 30

Quantitative- numbers based
Qualitative - interpretation based

Question 31

What factors should be controlled during the food tests experiments?

Answer 31

The volume of test solution and reagents added, the concentration of them and the temperature

Question 32

What would be a neutral result in the reducing sugars food test?

Answer 32

Blue precipitate

Question 33

Which problem could arise using very low concentrations of the polymers in these food tests?

Answer 33

Little colour change

Question 34

What is sucrose?

Answer 34

A non reducing sugar