Required practical 10: conditions and decay rate
הוצג| English | משמעות |
|---|---|
| decomposition/ˌdiːkɒmpəˈzɪʃn/ | פירוק |
| biogas/ˌbaɪəʊˈɡæs/ | גז ביולוגי |
What would explain this observation?
- A faster pH change in milk can indicate faster decay under a defined method. Compare rates over the same stated endpoint rather than only final colour.
Build the model
- Microorganisms decompose biological material.
- Suitable temperature and water support their reactions, while oxygen supports aerobic respiration; excessive temperature can inhibit or kill organisms rather than continually increasing rate.
- Composting aims to provide conditions for rapid decomposition 腐解 and produces material used as a natural fertiliser.
- Anaerobic decay can produce methane, which biogas 生物气 generators collect as fuel.
- decomposition: Breakdown of biological material by organisms and their enzymes.
- biogas: Fuel gas including methane produced by anaerobic decay.
Which comparison measures a rate in the defined pH method?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
A rate needs a change over time: pH change divided by elapsed time. A final colour without time, or a volume without an endpoint, contains no rate information.
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
- Required practical 10 investigates the effect of temperature on fresh-milk decay rate by measuring pH change.
- Use teacher-approved fresh milk, reagents, controlled water baths and a consistent endpoint.
- The AQA handbook models faster decay with lipase acting on fresh milk or cream made alkaline using sodium carbonate, with Cresol red changing from purple to yellow.
- Lipase produces fatty acids; natural microbial decay instead lowers pH through processes including lactic-acid production.
- These are different mechanisms.
In the lab: In the handbook model, equilibrate separate milk and lipase tubes in each water bath, add a fixed lipase amount to the milk mixture, start timing immediately and stir until the same yellow endpoint. Keep milk volume, starting pH and reagent concentrations comparable; repeat at several measured temperatures. Record temperature and time or a pH-time series, then use the defined rate measure. Do not taste material, culture unknown spoilage organisms or heat sealed gas-producing containers. Complete actual supervised laboratory work rather than replacing it with an invented written practical.
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: a supplied pH record falls from 8.0 to 7.0 in 5 min.
- Mean magnitude of pH change = 1.0/5 = 0.20 pH units/min. pH is a logarithmic measure, so this is a method-defined comparison, not a direct mass production rate.
- If using time to a fixed endpoint, compare 1/time instead.
Your turn: A supplied pH falls by 1.2 units in 6 min. Find mean magnitude of pH-change rate.
A supplied pH falls by 1.2 units in 6 min. Find mean magnitude of pH-change rate.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 0.2 pH units/min. Known: pH falls by 1.2 units in 6 min. Mean rate magnitude = 1.2/6 = 0.2 pH units per minute. pH is logarithmic, so this is a method-defined comparison, not a mass rate.
Check the conclusion and its limits
- A lower endpoint pH without timing is not a rate.
- Oxygen is not required for every kind of decomposition; distinguish aerobic compost processes from anaerobic biogas production.
- A temperature optimum should be inferred from supplied data rather than assumed universal.
Not true: Every increase in temperature must increase decay rate without limit. Microbial enzymes have an optimum temperature; beyond it they denature and decay slows. "Higher temperature always faster" fails once the optimum is passed.
- Microorganisms decompose biological material.
- Required practical 10 investigates the effect of temperature on fresh-milk decay rate by measuring pH change.
- Watch for: A lower endpoint pH without timing is not a rate.
Every increase in temperature must increase decay rate without limit.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Microbial enzymes have an optimum temperature; beyond it they denature and decay slows. "Higher temperature always faster" fails once the optimum is passed.
Breakdown of biological material by organisms and their enzymes: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
decomposition means Breakdown of biological material by organisms and their enzymes.