Required practical 10: conditions and decay rate
| English | 中文 | Pinyin |
|---|---|---|
| decomposition/ˌdiːkɒmpəˈzɪʃn/ | 分解 | fēn jiě |
| biogas/ˌbaɪəʊˈɡæs/ | 生物气 | shēng wù qì |
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.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
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 分解 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?
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 acquired 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.
Match each technical term to its precise meaning.
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 acquired 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 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?
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.
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: 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.
A supplied pH falls by 1.2 units in 6 min. Find mean magnitude of pH-change rate. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A supplied pH falls by 1.2 units in 6 min. Find mean magnitude of pH-change rate.
The result is 0.2 pH units/min. 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.
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.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every increase in temperature must increase decay rate without limit. This claim is false: 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.
Required practical 10: conditions and decay rate: 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 acquired 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.
Every increase in temperature must increase decay rate without limit.
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.
Breakdown of biological material by organisms and their enzymes: write the technical term.
decomposition means Breakdown of biological material by organisms and their enzymes.