Food tests: identify the tested group and the observation
הוצג| English | משמעות |
|---|---|
| qualitative test/ˈkwɒlɪteɪtɪv test/ | מבחן איכותי |
| positive control/ˈpɒzɪtɪv kənˈtrəʊl/ | בקר חיובי |
What would explain this observation?
- A colour change supports a particular chemical test result. It does not identify every nutrient in the sample or measure its full nutritional value.
Build the model
- Benedict’s reagent, heated with a sample in an approved water bath, tests for reducing sugars: a positive result changes from blue toward green, yellow, orange or brick-red depending on conditions.
- Iodine solution changes from orange-brown to blue-black with starch.
- Biuret reagent changes from blue to purple with protein.
- A school-approved ethanol emulsion test gives a cloudy white emulsion when lipid is present.
- qualitative test 定性检验: A test identifying presence or a category rather than an exact amount.
- positive control 阳性对照: A known positive sample used to check that a test works.
Which observation is a positive starch test?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Starch turns iodine from orange-brown to blue-black. Purple is the Biuret result for protein, and a cloudy emulsion is the ethanol test for lipids - each test has its own reagent and positive result.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Keep the test and result paired.
- A negative iodine observation does not show that all carbohydrates are absent, because the test concerns starch.
- Qualitative colour observations can indicate presence under the test conditions; a precise concentration requires calibration and controlled measurements.
In the lab: For required practical 4, prepare comparable sample extracts and use positive and negative controls. Use the school’s approved reagents and volumes, eye protection and supervised heating. Keep ethanol away from flames. Use separate clean apparatus for tests so contamination does not transfer a positive result between samples.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: four model samples each undergo the iodine test; three turn blue-black and one remains orange-brown.
- The observed fraction positive is 3/4=75%.
- This describes the tested samples, not the starch concentration or the percentage of starch in each food.
Your turn: Five of eight model samples give a positive test. Calculate the percentage positive.
Five of eight model samples give a positive test. Calculate the percentage positive.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 62.5 %. Known: 5 of 8 samples positive. Use percentage = positive / total x 100. Percentage = 5/8 x 100 = 62.5%. This describes the tested samples, not the composition of any one food.
Check the conclusion and its limits
- Heating is part of Benedict’s test, not a reason to heat every reagent in the same way.
- A colour chart without matched time, temperature and amounts is not an exact sugar assay.
- A food can contain several tested groups at once.
Not true: A negative iodine test proves that a food contains no carbohydrate. Iodine tests only for starch. A negative iodine result means no starch was detected; sugars are carbohydrates too and need Benedict's test.
- Benedict’s reagent, heated with a sample in an approved water bath, tests for reducing sugars: a positive result changes from blue toward green, yellow, orange or brick-red depending on conditions.
- Keep the test and result paired.
- Watch for: heating is part of Benedict’s test, not a reason to heat every reagent in the same way.
A negative iodine test proves that a food contains no carbohydrate.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Iodine tests only for starch. A negative iodine result means no starch was detected; sugars are carbohydrates too and need Benedict's test.
A test identifying presence or a category rather than an exact amount: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
qualitative test means A test identifying presence or a category rather than an exact amount.