How do you test for glucose?
Reveal answer
Add Benedict’s solution and heat in a water bath; blue turns green, yellow, orange or brick red if glucose is present.
Key words: Benedict’s · water bath · brick red
2(c) BIOLOGICAL MOLECULES · RECALL
2.9 Food tests
20 recall questions. Answer from memory, reveal the answer, then rate yourself. Ratings stay on this device.
Ratings are saved in this browser on this device, under a KS4 key kept separate from International A Level recall. Written answers are temporary and cleared when you leave the page. Use Reset to clear the ratings on this device.
How do you test for starch?
Reveal answer
Add iodine solution; orange-brown turns blue-black if starch is present.
Key words: iodine · blue-black
How do you test for protein?
Reveal answer
Add biuret reagent; blue turns purple (lilac) if protein is present.
Key words: biuret · purple
How do you test for fat?
Reveal answer
Shake the food with ethanol, then pour into water; a milky white emulsion shows fat.
Key words: ethanol · emulsion
Why must Benedict’s solution be heated but iodine need not be?
Reveal answer
The Benedict’s reaction with reducing sugar only happens when heated; iodine reacts with starch at room temperature.
Key words: heat
Name the four reagents used in food tests and what each detects.
Reveal answer
Benedict’s (glucose / reducing sugar), iodine (starch), biuret (protein), ethanol (fat).
Key words: Benedict’s · iodine · biuret · ethanol
A Benedict’s test gives green. What does that tell you?
Reveal answer
A small amount of reducing sugar is present; brick red would show much more.
Key words: green · small amount
What is meant by the term reagent?
Reveal answer
A substance used to produce or detect a chemical reaction.
Key words: reagent
What is meant by the term positive control?
Reveal answer
A known positive sample used to confirm that the method and reagent work.
Key words: positive control
What is meant by the term negative control?
Reveal answer
A sample lacking the target, used to show the negative appearance and detect contamination.
Key words: negative control
What is meant by the term precipitate?
Reveal answer
An insoluble solid formed in a solution during a reaction.
Key words: precipitate
What is meant by the term emulsion?
Reveal answer
Tiny droplets of one liquid dispersed through another liquid.
Key words: emulsion
Food-test decision table: for Reducing sugar, give the method and the positive result.
Reveal answer
Method: Add Benedict’s reagent; heat in a hot-water bath. Positive result: Blue changes towards green/yellow/orange/brick-red precipitate.
Key words: Reducing sugar
Food-test decision table: for Starch, give the method and the positive result.
Reveal answer
Method: Add iodine solution at room temperature. Positive result: Orange-brown changes to blue-black.
Key words: Starch
Food-test decision table: for Protein, give the method and the positive result.
Reveal answer
Method: Add Biuret reagent and mix. Positive result: Blue changes to lilac or purple.
Key words: Protein
Food-test decision table: for Lipid, give the method and the positive result.
Reveal answer
Method: Shake with ethanol; add the extract to water. Positive result: Cloudy white emulsion forms.
Key words: Lipid
True or false, and why? “Benedict’s reagent tests only for glucose.”
Reveal answer
False. It detects reducing sugars; glucose is the common specification example.
True or false, and why? “A deeper colour always gives an exact concentration.”
Reveal answer
False. Visual colour comparison is only semi-quantitative unless calibrated instrumentally.
True or false, and why? “A negative test proves the molecule is absent.”
Reveal answer
False. It shows that the target was not detected above the method’s sensitivity under those conditions.
True or false, and why? “Ethanol can be heated over a flame to speed the lipid test.”
Reveal answer
False. Ethanol is highly flammable and must be kept away from flames and ignition sources.