Food tests: identify the tested group and the observation
| English | Français |
|---|---|
| qualitative test/ˈkwɒlɪteɪtɪv test/ | qualitative test |
| positive control/ˈpɒzɪtɪv kənˈtrəʊl/ | positive control |
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.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
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?
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.
Match each technical term to its precise meaning.
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.
- 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?
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.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- 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.
Five of eight model samples give a positive test. Calculate the percentage positive. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Five of eight model samples give a positive test. Calculate the percentage positive.
The result is 62.5 %. 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.
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.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A negative iodine test proves that a food contains no carbohydrate. This claim is false: 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.
Food tests: identify the tested group and the observation: 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.
A negative iodine test proves that a food contains no carbohydrate.
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.
A test identifying presence or a category rather than an exact amount: write the technical term.
qualitative test means A test identifying presence or a category rather than an exact amount.