5 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
LESSON B4 · EXPLAINER
Food Tests and a Balanced Diet
Your roti and curry chicken — what is actually in them, and how would you prove it?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Starch test
Iodine: brown → blue-black.
Reducing sugar
Benedict’s solution, heat in water bath: blue → green → yellow → brick-red.
Non-reducing sugar
Negative Benedict’s; boil with dilute HCl, neutralise, retest → brick-red.
Quote the COLOUR CHANGE (from … to …) and the reagent for each food test.
Benedict’s needs HEATING; biuret does not.
For dietary advice questions, give a change AND a consequence of ignoring it (e.g. reduce roti portions → otherwise blood glucose stays high → kidney/eye damage).
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Identifying unknown solutions
Four unlabelled solutions, P, Q, R and S, were tested.
Test
P
Q
R
S
Iodine solution
blue-black
yellow-brown
yellow-brown
yellow-brown
Benedict’s, heated
blue
brick-red
blue
blue
Boiled with dilute HCl, neutralised, then Benedict’s
—
—
brick-red
blue
Biuret
blue
blue
blue
purple
Identify the main food substance in each.
Try it in your notebook first, then reveal the solution one step at a time.
R: negative with Benedict’s at first, but positive after boiling with acid and neutralising → the acid hydrolysed it into reducing sugars, so R is a non-reducing sugar (e.g. sucrose).
S: purple with biuret → protein.
Answer. P = starch; Q = reducing sugar (glucose); R = non-reducing sugar (sucrose); S = protein.
Examiner’s tip. For the non-reducing sugar test, say you neutralise with sodium hydrogencarbonate before adding Benedict’s — Benedict’s does not work in acid. Missing this step is a common lost mark.
Worked example 2
Energy in a snack
A 30 g serving of crackers contains 20 g carbohydrate, 5 g fat and 3 g protein. Carbohydrate and protein each release about 17 kJ per gram and fat about 37 kJ per gram. Calculate (a) the energy in one serving and (b) the percentage of that energy from fat.
Try it in your notebook first, then reveal the solution one step at a time.
Carbohydrate: 20 g × 17 kJ/g = 340 kJ.
Fat: 5 g × 37 kJ/g = 185 kJ.
Protein: 3 g × 17 kJ/g = 51 kJ.
Total = 340 + 185 + 51 = 576 kJ.
Percentage from fat = 185 ÷ 576 × 100 ≈ 32%.
Answer. (a) 576 kJ. (b) About 32% of the energy comes from fat, although fat is only 5 g of the 30 g serving.
Examiner’s tip. Gram for gram, fat releases more than twice as much energy as carbohydrate or protein. That is why fatty foods are energy-dense and easy to over-eat.
On the board · The food tests at a glance
Food
Reagent / method
Positive result
Negative result
Starch
Few drops of iodine solution
Blue-black
Yellow-brown
Reducing sugar
Benedict’s solution; heat in a water bath
Green → yellow → orange → brick-red
Stays blue
Non-reducing sugar
Boil with dilute HCl, neutralise with NaHCO₃, then Benedict’s test
Brick-red (after hydrolysis)
Stays blue
Protein
Biuret reagent (NaOH + CuSO₄)
Purple / violet
Blue
Fat
Shake with ethanol, pour into water (emulsion test); or grease-spot test
Cloudy white emulsion; translucent spot
Stays clear
Learn each test as reagent → method → positive result. Always include a control (water) to compare colours.
Key terms
Balanced diet
All the nutrients, fibre and water in the right proportions for a person’s age, sex and activity.
Malnutrition
A diet with too much, too little or the wrong balance of nutrients.
Deficiency disease
Illness caused by a lack of a nutrient, e.g. scurvy (vitamin C), anaemia (iron).
Reducing sugar
A sugar such as glucose that turns Benedict’s solution brick-red.
Roughage (fibre)
Indigestible plant material that helps peristalsis and prevents constipation.
Obesity
Excess body fat that increases the risk of diabetes, hypertension and heart disease.
Common misconceptions
✗ Sucrose gives a positive Benedict’s test.✓ Sucrose is a non-reducing sugar. It must be hydrolysed with acid and neutralised first.
✗ All fats are bad for you.✓ Fats are needed for energy storage, insulation, cell membranes and to absorb vitamins A, D, E and K — in moderation.
✗ A balanced diet means equal amounts of every nutrient.✓ It means the right proportions — mostly carbohydrate, with smaller amounts of protein and fat, and tiny amounts of vitamins and minerals.
Memory aid
Benedict’s → Brick-red (sugar). Biuret → Purple for Protein. Iodine → Ink-black for starch.
Real-life link
Diabetes and hypertension are among the leading causes of illness in the Caribbean (PAHO). Diets high in sugary drinks, salt and fatty fried foods, and low in fruit, vegetables and fibre, are major risk factors.
Try this
Collect three food labels at home. For each, calculate the percentage of the mass that is sugar, and rank them from most to least sugary.
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (18 marks in total). Your answers are saved on this device.
Based on CSEC P2 January 2024, Question 1(a)–(c)
Question 1 [10 marks]
Roy, who is diabetic, eats two portions of roti (wheat flour, water, oil) and chicken curry every day.
(a) Name TWO substances produced when (i) roti and (ii) chicken are digested. [4]
MARK SCHEME — (a) (max 4)
(b) Suggest TWO dietary changes Roy should make and ONE consequence of not following EACH. [4]
MARK SCHEME — (b) (max 4)
(c) Explain why converting excess glucose to glycogen is NOT efficient in diabetic patients. [2]
MARK SCHEME — (c) (max 2)
Based on SO B2.6 practical
Question 2 [8 marks]
A student is given three unlabelled solutions: glucose, starch and egg albumen.
(a) Describe how the student could identify each solution. [6]
MARK SCHEME — (a) (max 6)
(b) State TWO safety precautions for these tests. [2]
MARK SCHEME — (b) (max 2)
Your progress
Lesson readiness
0% READY
Not started
Video watched (20%)0%
Multiple choice (40%)0%
Structured (40%)0%
Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.