1.1.3Unit 1: Lifestyle, Transport, Genes and Health
Core Practical 1: Estimating the Concentration of Sugars and Starch — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 10 of 29 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Qualitative Tests
Benedict’s test
- Benedict’s test Heat the sample with excess blue Benedict’s reagent;
- a reducing sugar converts copper(II) ions to red-brown copper(I) oxide precipitate.
Non-reducing sugar
- Non-reducing sugar Boil with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate,
- then repeat Benedict’s test to detect the sugars released.
Iodine test for starch
- Iodine test for starch Add iodine in potassium iodide solution; a blue-black colour shows that starch is present,
- while orange-brown means it is absent.
Colour scale
- Colour scale From blue through green, yellow and orange to brick-red, the colour reflects increasing concentration of reducing sugar.
Part 2 of 4Making It Quantitative
1
Prepare a set of glucose solutions of known concentrations using a serial dilution of a stock solution.
2
Add the same volume of excess Benedict’s reagent to each, heat for the same time in a water bath, then filter or centrifuge to remove the precipitate.
3
Measure the absorbance or transmission of each blue filtrate using a colorimeter with a red filter, after zeroing with distilled water.
4
Plot a calibration curve of colorimeter reading against concentration, noting that the paler the filtrate the more sugar was present.
5
Treat the unknown sample in exactly the same way and read its concentration from the calibration curve; repeat the iodine test for starch in a similar manner.
Part 3 of 4Reliability and Safety
Reliability and Safety
Control variables
- Control variables Use identical volumes, heating times, water bath temperature and reagent amounts for every sample to keep results comparable.
Repeats
- Repeats Repeating each reading and calculating a mean reduces the effect of random errors and shows whether results are reproducible.
Limitations
- Limitations Estimates by eye are subjective,
- and a colorimeter removes this but depends on reagent being in excess and consistent separation of precipitate.
Safety
- Safety Wear eye protection, use tongs or a test-tube holder for hot tubes, and take care with the hot water bath and dilute acid.
Part 4 of 4Exam Checklist & Tips
1.1.3 – Key Points for the Exam
- Calibration curve A graph of known concentrations against the measured reading, used to find unknown concentrations.
- Serial dilution A way of making a set of solutions of decreasing concentration by repeatedly diluting by the same factor.
- Colorimeter An instrument that measures how much light a coloured solution absorbs or transmits.
- Reducing sugar A sugar that can donate electrons and so reduce Benedict’s reagent.
Common Mistakes
- Using too little Benedict’s reagent, so that the reagent runs out before all the sugar has reacted.
- Forgetting to zero the colorimeter with a blank sample before taking readings.
- Reading the colorimeter without removing the precipitate, which scatters the light and affects the reading.
- Neutralising the acid insufficiently before the Benedict’s test on non-reducing sugars, so the test fails.
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