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PreparationFood tests

Biology · Related practical

Food tests

Test samples for nutrients, record colour changes and compare your conclusions.

Combined Science 4003/3, June 2023 · Question 1

Use this practical to study the skills in the linked examination question. The apparatus and samples practise the same ideas; they do not reproduce the examination setup exactly.

O Level · Form 3 to 4 · 25 minutes

Learning objectives

  • Measure food samples and use clean apparatus for each test
  • Carry out iodine, Benedict’s, biuret and emulsion tests
  • Record colour changes or cloudiness before identifying a nutrient
  • Compare results from all four tests for the same sample
Practical mode

Guided gives instructions as you work. Practice lets you choose your method.

Source paper

Original examination paper

Combined Science · Paper 3

4003/3 · JUNE · 2023 · 1 hour 30 minutes

Read the complete original paper below. The related question is identified in this practical’s introduction.

Candidates answer on the question paper.

Additional materials:
As listed in instructions to Supervisors
Calculator (optional)

INSTRUCTIONS TO CANDIDATES

Write your name, centre number and candidate number in the spaces at the top of this page.

Answer both questions.

Write your answers in the spaces provided on the question paper.

Use a sharp pencil for your drawings. Coloured pencils or crayons should not be used.

You should show the essential steps in any calculation and record all experimental results in the spaces provided in the question paper.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets [ ]
at the end of each question or part question.

This question paper consists of 7 printed pages and 1 blank page.

1[20 marks]

You are required to identify the nutritional value in a nutrient solution A.
You are provided with a test tube labelled T₁, a syringe, access to nutrient solution A, access to solution S, access to solution P, access to iodine solution, two droppers and white paper.

(a)[13 marks]

(i)[11 marks]

Carry out the tests which are described in Table 1.1.
Record the observations in Table 1.1.
Write conclusions in Table 1.1.

Table 1.1

testobservation(s)conclusion(s)
1. Add, using a syringe, 2.0 cm³ of the nutrient solution A into test tube T₁.<br>Thoroughly rinse the syringe.<br>Draw 2.0 cm³ of solution S and add it to test tube T₁.<br>Use a dropper to add one drop of solution P to the mixture in test tube T₁ and shake.<br>Continue to add one drop of Solution P and shaking until a change is noticed.<br>Throw away the contents of test tube T₁ and thoroughly rinse the test tube.
observation(s)conclusion(s)
2. Add, using a syringe, 2.0 cm³ of the nutrient solution A into test tube T₁.<br>Thoroughly rinse the syringe.<br>Then add about 1 cm³ of iodine solution and shake.
3. Use a dropper to add one drop of the nutrient solution A to a white sheet of paper.<br>Gently wave the paper in the air.

(ii)[1 marks]

Identify solution S.

Dotted response line.

(iii)[1 marks]

Suggest the identity of solution P.

Dotted response line.

(b)[2 marks]

State, giving a reason, one precaution that should be taken during the experiment.

Three dotted response lines.

(c)[5 marks]

(i)[2 marks]

Explain any one nutritional deficiency of the nutrient solution A.

Two dotted response lines.

(ii)[2 marks]

Explain any one nutritional advantage of the nutrient solution A.

Two dotted response lines.

(iii)[1 marks]

State a deficiency disease in children that may be controlled through drinking the nutrient solution A regularly.

Dotted response line.

2[20 marks]

You are required to determine the mechanical advantage (MA), velocity ratio (VR) and efficiency of a simple machine.
You are provided with a flat bar with marked positions of the pivot, A and B. You are also provided with masses and a pivot.

(a)[11 marks]

(i)[3 marks]

Set up the apparatus as shown in Fig.2.1.
The centre of the load should be 10 cm from the end of the flat bar.
Measure and record the distances A and B from the pivot
Apparatus diagram of a flat bar, effort, load and pivot, with distances A and B indicated.
Apparatus diagram of a flat bar, effort, load and pivot, with distances A and B indicated.
Lines provided for answers.

(ii)[6 marks]

Place a load of 100 g on the position marked A as shown in Fig. 2.1
Place masses (effort) at the position marked B (as shown in Fig.2.1) until the load is just lifted.
Record, in Table 2.1, the total mass that just lifted the load.
Repeat the experiment two more times, recording the masses in Table 2.1.
Complete Table 2.1 by converting the load and effort to newtons.

Table 2.1

experimentload/gload/Neffort/geffort/N
1
2
3

Table 2.1 response cells.

(iii)[2 marks]

Calculate the average effort applied in newtons.

Space provided for answer.

(b)[9 marks]

(i)[2 marks]

Calculate the mechanical advantage (MA) of the machine.

Space provided for answer.

(ii)[2 marks]

Calculate the velocity ratio (VR) of the machine.

Space provided for answer.

(iii)[2 marks]

Calculate the efficiency of the machine.

Space provided for answer.

(iv)[2 marks]

Comment on the value of the efficiency obtained in (b)(iii).

(v)[1 marks]

State one way of increasing the efficiency of the machine.
4003/3 J2023

Study notes

Overview

Find out whether a food sample contains starch, reducing sugar, protein or fat. Carry out a separate test for each nutrient, record the colour or cloudiness you see and use that evidence to reach a conclusion.

Method

1. Choose a sample. Guided begins with Juice X. Move a clean tube from the rack into the
holder. Pour the sample into the 25 cm³ measuring cylinder, read the bottom of the
meniscus, and stop at 2 cm³. Move the cylinder above the tube and transfer your measured
sample. A used tube carries the previous reagent, so use a clean tube for each test.
2. Choose a test. For starch, collect iodine with the dropper and deliver five individual
drops. For reducing sugar, measure 2 cm³ of Benedict's solution with the cylinder and
transfer it. For protein, measure and transfer 2 cm³ of sodium hydroxide, then deliver
five drops of dilute copper sulphate. For fat, measure and transfer 2 cm³ of ethanol,
shake, then measure and transfer 2 cm³ of water. Use clean instruments between liquids.
3. For the Benedict's test only, light the burner and stand the tube in the water bath. The
colour develops over about a minute once the water is near boiling. Use the speed control
to shorten the wait.
4. Look at the colour of the tube, and at whether it has turned cloudy, then record the
observation and the conclusion: present or absent, and for reducing sugar low, medium or
high.
5. Record one result for each of the four tests of the same sample. These four distinct
records unlock that sample's composition comparison. Compare your conclusions, then
export CSV data or a PDF report. Other samples remain available for another investigation.

Safety and measurement

Benedict's solution is heated in a water bath, never directly over the flame: a tube held in a
naked flame boils suddenly and spits hot reagent out of the mouth of the tube. Ethanol is
flammable, so it is kept away from the burner and the burner is off during the emulsion test.
Sodium hydroxide burns the skin and eyes; a splash is rinsed at once with plenty of water.
Two errors can affect these tests. A tube used for one test and not rinsed carries that reagent into
the next test and gives a false colour. A Benedict's test that is not heated shows no change
whatever the sample holds.

Analysis

There is no calculation. Each colour is read against the expected result of the test.

Iodine solution is brown. It turns blue-black with starch and stays brown without it.

Benedict's solution is blue. Heated with a reducing sugar it turns green when there is a
little, orange with more, and brick red with a lot; without a reducing sugar it stays blue.
The colour only develops on heating, so an unheated tube always stays blue.

The biuret test uses sodium hydroxide then dilute copper sulphate. The mixture is blue; with
protein it turns purple or lilac.

The emulsion test uses ethanol then water. Fat dissolves in the ethanol and comes out of
solution when the water is added, giving a cloudy white emulsion; without fat the mixture
stays clear.

Practice quiz

Answer all questions, then check your answers and read the explanations.

1
Which reagent is used to test a food sample for starch?
2
A sample turns blue-black when iodine solution is added. What does this show?
3
Benedict's solution is added to a sample and heated in a water bath. The mixture turns brick red. What does this show?
4
Why is the Benedict's test heated in a water bath rather than directly in the Bunsen flame?
5
A student adds sodium hydroxide solution and then dilute copper sulphate solution to egg white. What colour is seen?
6
Ethanol is added to cooking oil, then water. What is observed?
7
A student adds Benedict's solution to glucose solution and records blue, no change. What is the most likely reason?
8
A tube used for the iodine test on starch solution is used again, without rinsing, for the iodine test on juice X. What is the likely result?
9
Sample P turns green and sample Q turns brick red when heated with Benedict's solution. What can be concluded?
10
Sucrose solution gives no colour change when heated with Benedict's solution. Why?