Practical

Question 1

Test of Energy from Food

Answer 1

Weigh each crisp / use the same mass of crisp; [l mark]
• (Measure out a) stated volume/mass of water in a test tube/beaker/calorimeter; [l mark]
• Ignite/set fire to/light crisp; [1 mark]
• Heat the water (in the tube/beaker) using the burning crisp OR place the burning crisp under the test tube; [l mark]
• Burn (the crisp) until it has completely burnt / no longer relights; [1 mark]
• Measure the temperature of the water before and after burning / measure the change in temperature; [l mark]
• Calculate the energy content (per g) = 4.2 x mass of water x temp rise ÷ (mass of crisp); [1 mark]
• Repeat the method (to obtain reliable data); [1 mark]
• Insulate the tube/beaker OR move crisp under the tube quickly; [1 mark]

Question 2

Test for glucose

Answer 2

Add Benedict’s solution to the sample solution in a test tube
Heat in a boiling water bath for 5 minutes
Take the test tube out of the water bath and observe the colour
A positive test will show a colour change from blue to orange / brick red.

Question 3

Test for starch

Answer 3

We can use iodine to test for the presence or absence of starch in a food sample
Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-black

Question 4

Test for protein

Answer 4

Add drops of Biuret solution to the food sample
A positive test will show a colour change from blue to violet / purple

Question 5

Test for lipid

Answer 5

Mix the food sample with 4cm3 of ethanol and shake
Allow time for the sample to dissolve in the ethanol
Strain the ethanol solution into another test tube
Add the ethanol solution to an equal volume of cold distilled water (4cm3)
A positive test will show a cloudy emulsion forming

Question 6

Important hazards

Answer 6

Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm
Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles
Iodine is also an irritant to the eyes
Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately
Ethanol is highly flammable; keep it away from any Bunsen burner
The Bunsen burner itself is a hazard due to the open flame