Topic 5 — Food Tests and Enzyme Experiments

1. Food Tests in ATP

Food tests are one of the MOST repeated ATP topics.

You must know:

• reagent

• method

• positive result

• negative result

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2. Test for Starch

Reagent

iodine solution

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Method

• add iodine drops to sample

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Positive Result

blue-black

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Negative Result

yellow-brown

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3. Test for Reducing Sugars

Reagent

Benedict’s solution

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Method

1. add Benedict’s solution

2. heat in water bath

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Positive Result

colour changes:

• green

• yellow

• orange

• brick-red

(depending on sugar amount)

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Negative Result

remains blue

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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4. Test for Protein

Reagent

Biuret solution

OR

• sodium hydroxide

• copper sulfate

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Positive Result

purple / lilac

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Negative Result

blue

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5. Test for Lipids (Fats)

Reagent

ethanol + water

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Method

1. add ethanol

2. shake

3. add water

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Positive Result

white cloudy emulsion

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Negative Result

solution remains clear

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6. Common ATP Food Test Questions

They may ask:

• identify food substance

• explain colour change

• compare food samples

• complete results table

• suggest control

• explain need for heating

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7. Water Bath

Used instead of direct flame.

Reasons

• safer

• prevents overheating

• provides constant temperature

Especially used with:

• Benedict’s test

• enzyme experiments

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8. Safety Precautions

Ethanol

• flammable

• keep away from flame

Benedict’s Solution

• heated carefully

• use test-tube holder

Scalpel

• cut away from body

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9. Enzymes

Enzymes are:

• proteins

• biological catalysts

Function:
speed up reactions

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10. Lock and Key Model

Idea

substrate fits enzyme active site exactly.

Incorrect shape →
reaction cannot occur.

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11. Enzyme Experiments Commonly Tested

• catalase + hydrogen peroxide

• amylase + starch

• effect of temperature

• effect of pH

• effect of concentration

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12. Catalase Experiment

Reaction

hydrogen peroxide → water + oxygen

Catalase speeds reaction.

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Measuring Activity

Common methods:

• foam height

• oxygen bubbles

• gas volume

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13. Why Foam Forms

Oxygen gas produced becomes trapped as bubbles.

Greater foam height =
greater enzyme activity

Very common ATP question.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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14. Effect of Temperature on Enzymes

Low Temperature

• particles move slowly

• fewer collisions

• slower reaction

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Optimum Temperature

• maximum activity

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High Temperature

enzyme denatures

Active site changes shape.

Substrate no longer fits.

Reaction decreases sharply.

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15. Effect of pH on Enzymes

Each enzyme has:

• optimum pH

Wrong pH:

• changes active site

• enzyme denatures

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16. Effect of Concentration

More Substrate

Usually:
reaction rate increases

Until:
all active sites occupied

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17. Controlled Variables in Enzyme Experiments

Common controlled variables:

• temperature

• pH

• enzyme concentration

• substrate volume

• reaction time

ATP LOVES asking this.

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18. Reliability in Enzyme Experiments

Improve reliability by:

• repeating experiment

• averaging results

• using same apparatus

• controlling variables

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19. Common Sources of Error

• unequal sample sizes

• timing delay

• temperature changes

• inaccurate measuring

• contamination

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20. Improvements

Examples

• use water bath

• use electronic timer

• repeat trials

• use syringe for accurate volume

• cut samples same size

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21. ATP Observation vs Explanation

Observation

“Solution turned blue-black.”

Explanation

“Starch present.”

Students often mix these.

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22. Common ATP Enzyme Graphs

Usually:

• increase to optimum

• then sudden decrease

Reason:
enzyme denaturation

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23. ATP Experimental Design Examples

Example

Investigating effect of temperature on catalase.

Independent Variable

temperature

Dependent Variable

foam height

Controlled Variables

• volume of hydrogen peroxide

• time

• amount of catalase

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24. Typical ATP Conclusion Style

Correct:
“Increasing temperature increased enzyme activity up to the optimum temperature, after which activity decreased.”

Weak:
“Temperature affected enzyme.”

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25. Common Examiner Complaints

• missing control

• vague conclusions

• confusing observation with explanation

• forgetting heating step

• incorrect food test colours

• not mentioning denaturation

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26. Quick ATP Food Test Summary

Food Substance	Reagent	Positive Result
Starch	Iodine	Blue-black
Reducing sugar	Benedict’s + heat	Brick-red/orange
Protein	Biuret	Purple
Lipid	Ethanol emulsion	White cloudy

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.