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ecosystems."},{"text":"The production of food by plants using light energy.","correct":false,"explanation":"This describes photosynthesis, not decomposition. Decomposition is the breakdown of dead organisms and waste by microorganisms, which process organic material."},{"text":"The movement of water and mineral ions through a plant.","correct":false,"explanation":"This describes transport in plants, not decomposition. The correct definition is the breakdown of dead organisms and waste by microorganisms."},{"text":"The removal of carbon dioxide from the atmosphere by animals.","correct":false,"explanation":"Animals do not remove carbon dioxide in this way, and this is not decomposition. Decomposition means microorganisms break down dead organisms and waste."}]},{"id":"a51fb9f5-7a9b-4d0d-b7af-87e92fe943f8","cardType":"basic","points":1,"front":"Which two groups of microorganisms mainly carry out decomposition?","back":"Bacteria and fungi.","mcqOptions":[{"text":"Bacteria and fungi.","correct":true,"explanation":"Bacteria and fungi are the main decomposers. They secrete enzymes onto dead material and absorb the soluble products of digestion."},{"text":"Plants and animals.","correct":false,"explanation":"Plants and animals are producers and consumers rather than the main decomposers. The principal decomposing groups are bacteria and fungi."},{"text":"Viruses and protoctists.","correct":false,"explanation":"Viruses cannot carry out independent decomposition, and protoctists are not the main GCSE examples. Bacteria and fungi mainly decompose dead material."},{"text":"Algae and mosses.","correct":false,"explanation":"Algae and mosses are photosynthetic organisms, not the main decomposer groups. Bacteria and fungi release enzymes to digest dead material."}]},{"id":"0063905c-5375-4189-85db-f2fa410e1524","cardType":"basic","points":2,"front":"How do decomposers digest dead material?","back":"They release enzymes onto it, then absorb the digested products.","mcqOptions":[{"text":"They release enzymes onto it, then absorb the digested products.","correct":true,"explanation":"Decomposers digest externally by secreting enzymes onto dead material. The enzymes break large insoluble molecules into smaller soluble products that the decomposers absorb."},{"text":"They absorb whole pieces of it, then digest the products internally.","correct":false,"explanation":"Decomposers do not usually ingest whole pieces and digest them internally. They release enzymes onto material first, then absorb the digested products."},{"text":"They release hormones onto it, then absorb the undigested products.","correct":false,"explanation":"Hormones do not digest dead material, and undigested substances cannot be absorbed in this process. Decomposers secrete enzymes and absorb soluble products."},{"text":"They release antibodies onto it, then absorb the mineral products.","correct":false,"explanation":"Antibodies defend against specific pathogens and do not digest dead material. Decomposers release enzymes, which produce soluble digested substances for absorption."}]},{"id":"8874e8bb-e569-42ac-a28b-7d463d50b7a4","cardType":"basic","points":1,"front":"Which nutrients are returned to the soil and air during decomposition?","back":"Carbon, nitrogen and mineral ions.","mcqOptions":[{"text":"Carbon, nitrogen and mineral ions.","correct":true,"explanation":"Decomposition returns carbon compounds, nitrogen compounds and mineral ions to the environment. These substances can then be reused by plants and other organisms."},{"text":"Oxygen, glucose and protein.","correct":false,"explanation":"Glucose and protein are organic substances rather than the main nutrients listed as being returned. The correct set is carbon, nitrogen and mineral ions."},{"text":"Water, oxygen and starch.","correct":false,"explanation":"These substances may be involved in biological processes, but they are not the specified nutrients returned by decomposition. The answer is carbon, nitrogen and mineral ions."},{"text":"Chlorophyll, lipase and carbon dioxide.","correct":false,"explanation":"Chlorophyll and lipase are biological molecules, not nutrients returned to soil and air by decomposition. Carbon, nitrogen and mineral ions are returned."}]},{"id":"a4b418dd-9718-4b5f-b15f-74fbe654843f","cardType":"basic","points":2,"front":"Why does warmth usually speed up decomposition?","back":"Warmth increases the activity of the decomposers' enzymes.","mcqOptions":[{"text":"Warmth increases the activity of the decomposers' enzymes.","correct":true,"explanation":"Increasing temperature within a suitable range gives decomposer enzymes more kinetic energy, causing more frequent successful collisions and increasing decomposition activity."},{"text":"Warmth increases the activity of the decomposers' antibodies.","correct":false,"explanation":"Antibodies are immune proteins, not the molecules that digest dead material. Warmth usually speeds decomposition because it increases the activity of decomposers' enzymes."},{"text":"Warmth decreases the activity of the decomposers' enzymes.","correct":false,"explanation":"A suitable rise in temperature increases enzyme activity rather than decreasing it. Decomposition slows only when temperatures become too high and enzymes denature."},{"text":"Warmth increases the activity of the decomposers' chlorophyll.","correct":false,"explanation":"Chlorophyll absorbs light for photosynthesis and is not responsible for digesting dead material. Decomposition speeds up because decomposer enzymes become more active."}]},{"id":"079f787e-16f1-48ec-a3db-26f826589770","cardType":"basic","points":2,"front":"Why does decomposition slow down or stop at too high a temperature?","back":"The decomposers' enzymes denature.","mcqOptions":[{"text":"The decomposers' enzymes denature.","correct":true,"explanation":"Excessive heat changes the shape of decomposer enzymes. Their active sites no longer work effectively, so digestion slows down or stops."},{"text":"The decomposers' enzymes become permanently more active.","correct":false,"explanation":"Very high temperatures do not make enzymes permanently more active. They denature the enzymes, changing their active sites and reducing decomposition."},{"text":"The decomposers' cells begin photosynthesising.","correct":false,"explanation":"Decomposer cells do not stop decay because they begin photosynthesising. Excessive temperature denatures their enzymes, preventing effective digestion."},{"text":"The decomposers' DNA dissolves immediately.","correct":false,"explanation":"DNA does not simply dissolve immediately at high temperature in this practical context. The key cause of stopped decomposition is denaturation of decomposer enzymes."}]},{"id":"be8e64ae-845f-48d4-8310-182bd1c128c2","cardType":"basic","points":2,"front":"Why do moist conditions speed up decomposition?","back":"Decomposers need water to live and digest the material.","mcqOptions":[{"text":"Decomposers need water to live and digest the material.","correct":true,"explanation":"Water is needed for decomposer cells to survive and for their enzyme-controlled digestion reactions. Moist conditions therefore usually increase the rate of decomposition."},{"text":"Decomposers need light to photosynthesise the material.","correct":false,"explanation":"Decomposers do not need light to photosynthesise dead material. Moisture helps because decomposers need water to live and digest the material."},{"text":"Decomposers need carbon dioxide to produce the material.","correct":false,"explanation":"Carbon dioxide is used in photosynthesis, not to produce or digest dead material by decomposers. Water supports decomposer life and digestion."},{"text":"Decomposers need salt to preserve the material.","correct":false,"explanation":"Salt generally inhibits microorganisms by drawing water out of their cells, so it does not speed decomposition. Decomposers require water to live and digest material."}]},{"id":"99072ae5-82e1-4d12-bc9a-0f03e91a75fd","cardType":"basic","points":2,"front":"Why does a good oxygen supply speed up decomposition?","back":"Most decomposers respire aerobically.","mcqOptions":[{"text":"Most decomposers respire aerobically.","correct":true,"explanation":"Most decomposers use aerobic respiration when oxygen is available. This releases energy efficiently to support growth, enzyme production and decomposition."},{"text":"Most decomposers respire anaerobically.","correct":false,"explanation":"Although some microorganisms can respire without oxygen, most decomposers respire aerobically. A good oxygen supply therefore usually speeds decomposition."},{"text":"Most decomposers respire photosynthetically.","correct":false,"explanation":"Photosynthesis is not a type of respiration and does not provide the relevant energy pathway here. Most decomposers respire aerobically."},{"text":"Most decomposers respire without releasing energy.","correct":false,"explanation":"Respiration releases energy for cell processes, including growth and enzyme production. Most decomposers respire aerobically when oxygen is available."}]},{"id":"3509a2d1-fcb4-43a2-b9c4-c78d26266276","cardType":"basic","points":1,"front":"Under what conditions is decay fastest?","back":"It is fastest when conditions are warm, moist and well oxygenated.","mcqOptions":[{"text":"It is fastest when conditions are warm, moist and well oxygenated.","correct":true,"explanation":"Decomposition is fastest when temperature supports enzyme activity, water supports living cells and digestion, and oxygen allows efficient aerobic respiration."},{"text":"It is fastest when conditions are cold, dry and lacking oxygen.","correct":false,"explanation":"Cold, dry, oxygen-poor conditions reduce enzyme activity, limit water availability and restrict aerobic respiration. Warm, moist and well oxygenated conditions are fastest."},{"text":"It is fastest when conditions are hot, dry and highly acidic.","correct":false,"explanation":"Excessive heat can denature enzymes, while dryness limits microbial activity. The fastest decay occurs in warm, moist and well oxygenated conditions."},{"text":"It is fastest when conditions are cool, flooded and sterile.","correct":false,"explanation":"Cool temperatures slow enzymes, flooding can reduce oxygen, and sterile conditions contain no decomposers. Decay is fastest when conditions are warm, moist and well oxygenated."}]},{"id":"5ef733c0-a30c-4e11-8f63-f3d004d9c202","cardType":"basic","points":2,"front":"How is the rate of decay calculated?","back":"$$$\\text{rate} = \\dfrac{\\text{change in measurement}}{\\text{time taken}}$$","mcqOptions":[{"text":"$$$\\text{rate} = \\dfrac{\\text{change in measurement}}{\\text{time taken}}$$","correct":true,"explanation":"Rate measures how much a quantity changes during a given time. Dividing the change in measurement by the time taken gives the decay rate."},{"text":"$$$\\text{rate} = \\dfrac{\\text{time taken}}{\\text{change in measurement}}$$","correct":false,"explanation":"This reverses the rate calculation and gives time per unit change. Decay rate is calculated by dividing the change in measurement by the time taken."},{"text":"$$$\\text{rate} = \\text{change in measurement} \\times \\text{time taken}$$","correct":false,"explanation":"Multiplying the change by time gives the wrong units for a rate. The correct calculation divides the change in measurement by the time taken."},{"text":"$$$\\text{rate} = \\text{change in measurement} + \\text{time taken}$$","correct":false,"explanation":"Adding the two quantities does not calculate a rate and combines incompatible units. Rate requires change in measurement divided by time taken."}]},{"id":"0daec5bf-00f2-4c94-af8f-b49ae41bda95","cardType":"basic","points":2,"front":"A compost heap loses 60 g in 30 days. What is its rate of decay?","back":"$$$\\dfrac{60}{30} = \\mathbf{2\\text{ g per day}}$$","mcqOptions":[{"text":"$$$\\dfrac{60}{30} = \\mathbf{2\\text{ g per day}}$$","correct":true,"explanation":"The rate is the mass lost divided by the time: 60 g divided by 30 days equals 2 g per day."},{"text":"$$$\\dfrac{30}{60} = \\mathbf{0.5\\text{ g per day}}$$","correct":false,"explanation":"This reverses the mass change and time, calculating days per gram rather than grams per day. The correct rate is 60 divided by 30, giving 2 g per day."},{"text":"$$$\\dfrac{60}{30} = \\mathbf{30\\text{ g per day}}$$","correct":false,"explanation":"The division is correct, but the result has been copied as the denominator rather than calculated. 60 divided by 30 equals 2 g per day."},{"text":"$$$\\dfrac{60}{30} = \\mathbf{90\\text{ g per day}}$$","correct":false,"explanation":"Adding the two given numbers instead of dividing them produces 90, which is not a decay rate. The required calculation is 60 divided by 30, or 2 g per day."}]},{"id":"b7b9e523-b625-4f63-97a5-6425097945e8","cardType":"basic","points":1,"front":"In the milk practical, what does a shorter colour-change time indicate?","back":"A faster rate of decay.","mcqOptions":[{"text":"A faster rate of decay.","correct":true,"explanation":"A shorter time means the same indicator colour change was reached more quickly. Since the change is caused by decay products, this indicates a faster rate of decay."},{"text":"A slower rate of decay.","correct":false,"explanation":"A slower decay rate would take longer to produce the colour change. A shorter time indicates that decay reached the endpoint more quickly, so the rate is faster."},{"text":"A higher final pH.","correct":false,"explanation":"The indicator turns colourless when pH falls to the endpoint, not when the final pH is higher. A shorter time indicates a faster rate of decay."},{"text":"A larger volume of milk.","correct":false,"explanation":"The colour-change time does not directly measure milk volume. It measures how quickly decay causes the pH to reach the indicator endpoint, indicating decay rate."}]},{"id":"48b6dc29-b279-4b01-8601-0967c16df97a","cardType":"basic","points":1,"front":"What is the dependent variable in the milk decay practical?","back":"The time taken for the indicator to turn colourless.","mcqOptions":[{"text":"The time taken for the indicator to turn colourless.","correct":true,"explanation":"The dependent variable is what is measured in the experiment. Here, the measured outcome is the time taken for the indicator to turn colourless."},{"text":"The temperature of the water bath.","correct":false,"explanation":"The water-bath temperature is the independent variable because it is changed by the investigator. The dependent variable is the time taken for the indicator to turn colourless."},{"text":"The volume of lipase added.","correct":false,"explanation":"The volume of lipase is a control variable if kept constant, rather than the dependent variable. The measured dependent variable is the colour-change time."},{"text":"The type of milk used.","correct":false,"explanation":"Milk type should normally be controlled so it does not affect the comparison. The dependent variable is the time taken for the indicator to turn colourless."}]},{"id":"367c9021-7ce4-4e7f-a50e-c15d785e9d0c","cardType":"basic","points":2,"front":"Why is phenolphthalein used in the milk decay practical?","back":"It turns colourless when the falling pH reaches a set drop in pH.","mcqOptions":[{"text":"It turns colourless when the falling pH reaches a set drop in pH.","correct":true,"explanation":"Phenolphthalein provides a consistent endpoint: it changes from pink to colourless when the pH falls through its transition range after a set drop in pH."},{"text":"It turns colourless when the falling pH reaches a random drop in pH.","correct":false,"explanation":"A random endpoint would make results unreliable and comparisons unfair. Phenolphthalein changes colour at a defined pH range, representing a set drop in pH."},{"text":"It turns colourless when the rising pH reaches a set increase in pH.","correct":false,"explanation":"Decay produces fatty acids, causing pH to fall rather than rise. Phenolphthalein turns colourless when this falling pH reaches a set drop."},{"text":"It turns colourless when the falling pH reaches a complete loss of acidity.","correct":false,"explanation":"The indicator does not require all acidity to disappear. It changes colour when the pH reaches its transition point, representing a set drop in pH."}]},{"id":"bc439ade-23a2-4b5c-b363-47a7110f0508","cardType":"basic","points":2,"front":"Why does adding lipase make the indicator colourless?","back":"Lipase breaks milk fats into fatty acids, which lower the pH.","mcqOptions":[{"text":"Lipase breaks milk fats into fatty acids, which lower the pH.","correct":true,"explanation":"Lipase hydrolyses fats into fatty acids and glycerol. The fatty acids increase acidity, lowering the pH until the phenolphthalein becomes colourless."},{"text":"Lipase breaks milk fats into amino acids, which lower the pH.","correct":false,"explanation":"Amino acids are produced when proteins are digested by proteases, not when fats are broken down by lipase. Lipase produces fatty acids, which lower pH."},{"text":"Lipase breaks milk fats into glucose molecules, which lower the pH.","correct":false,"explanation":"Glucose is produced from carbohydrate digestion by carbohydrases, not fat digestion by lipase. Lipase breaks fats into fatty acids and glycerol."},{"text":"Lipase breaks milk fats into alkalis, which lower the pH.","correct":false,"explanation":"Alkalis raise pH rather than lower it. Lipase breaks fats into fatty acids, and these acids cause the pH to fall and the indicator to turn colourless."}]},{"id":"8a20ad91-c41f-499f-a821-2f45f1835708","cardType":"basic","points":2,"front":"How can results from the milk practical be made more reliable?","back":"Repeat each temperature and calculate a mean.","mcqOptions":[{"text":"Repeat each temperature and calculate a mean.","correct":true,"explanation":"Repeating each temperature provides replicate measurements, allowing anomalous results to be identified and a mean to be calculated for greater reliability."},{"text":"Test one temperature and calculate a mean.","correct":false,"explanation":"A mean from one temperature does not improve reliability across the investigation. Each temperature should be repeated so results can be compared and averaged."},{"text":"Change several variables and calculate a mean.","correct":false,"explanation":"Changing several variables prevents a fair test because the effect of temperature cannot be isolated. Reliability is improved by repeating each temperature and calculating a mean."},{"text":"Record one result and ignore anomalies.","correct":false,"explanation":"A single result cannot reveal random variation or identify anomalies. Repeating each temperature and calculating a mean makes the findings more reliable."}]},{"id":"9be522e7-ef06-4396-a8c9-a17807b90e98","cardType":"basic","points":2,"front":"Why are the volumes of milk, alkali and lipase kept the same in the practical?","back":"They are control variables, so only temperature affects the result.","mcqOptions":[{"text":"They are control variables, so only temperature affects the result.","correct":true,"explanation":"Keeping these volumes constant makes the test fair, because temperature is the only factor deliberately changed and any difference in the result can be attributed to it."},{"text":"They are dependent variables, so temperature affects the result.","correct":false,"explanation":"The volumes are controlled rather than measured as the outcome. Calling them dependent variables confuses the quantities kept constant with the result being observed, while temperature is the independent variable."},{"text":"They are independent variables, so all factors affect the result.","correct":false,"explanation":"The independent variable is the factor deliberately changed, which is temperature in this practical. Milk, alkali and lipase volumes are kept constant to prevent them affecting the comparison."},{"text":"They are validity variables, so the result is always accurate.","correct":false,"explanation":"Validity is improved by controlling these volumes, but “validity variables” is not the correct term. They are control variables because they must remain constant during the investigation."}]},{"id":"8566efcd-a441-4130-9113-4d17f5d24ddf","cardType":"basic","points":2,"front":"Why is compost a natural fertiliser?","back":"It returns mineral ions to the soil for crops and garden plants.","mcqOptions":[{"text":"It returns mineral ions to the soil for crops and garden plants.","correct":true,"explanation":"Decomposed organic material contains mineral ions released from dead organisms and waste. Adding compost therefore replenishes soil nutrients needed for healthy crop and plant growth."},{"text":"It returns carbon dioxide to the soil for crops and garden plants.","correct":false,"explanation":"Carbon dioxide is a gas used in photosynthesis, not the main soil nutrient supplied by compost. Compost returns mineral ions to the soil, replacing nutrients removed by growing plants."},{"text":"It returns oxygen molecules to the soil for crops and garden plants.","correct":false,"explanation":"Oxygen may be present in air spaces within soil, but it is not the fertilising material supplied by compost. Compost improves nutrient availability by returning mineral ions."},{"text":"It returns chlorophyll molecules to the soil for crops and garden plants.","correct":false,"explanation":"Chlorophyll is a pigment made inside plant cells and is not absorbed from compost as a fertiliser. The useful nutrients released during decomposition are mineral ions."}]},{"id":"918e71d8-7737-40a7-8d16-6561cb0e00e3","cardType":"basic","points":1,"front":"What gas is produced by anaerobic decay?","back":"Methane.","mcqOptions":[{"text":"Methane.","correct":true,"explanation":"Anaerobic decay occurs without oxygen and produces biogas, whose main useful component is methane. Methane is combustible and can be collected as a renewable fuel."},{"text":"Oxygen.","correct":false,"explanation":"Oxygen is required for aerobic respiration and is absent during anaerobic decay. The characteristic gas produced in anaerobic decay is methane, formed by microorganisms breaking down waste."},{"text":"Nitrogen.","correct":false,"explanation":"Nitrogen makes up most of the air but is not the gas produced by anaerobic decay. Anaerobic microorganisms produce methane as organic waste is decomposed without oxygen."},{"text":"Hydrogen.","correct":false,"explanation":"Hydrogen can occur in some biological processes but is not the expected gas from anaerobic decay at GCSE level. The relevant product is methane, a combustible component of biogas."}]},{"id":"f6c20ac0-6052-403c-b4b4-6d3c905f21a6","cardType":"basic","points":2,"front":"How is methane from biogas generators used?","back":"It is burned as a fuel to provide renewable energy.","mcqOptions":[{"text":"It is burned as a fuel to provide renewable energy.","correct":true,"explanation":"Methane releases energy when combusted, allowing biogas generators to produce useful heat or electricity. The methane is renewable because it comes from recently produced biological waste."},{"text":"It is stored as a fertiliser to provide mineral ions.","correct":false,"explanation":"Methane is a gas and does not act as a fertiliser supplying mineral ions. It is burned as a fuel, while the digested solid material may be used as fertiliser."},{"text":"It is released into the atmosphere to increase photosynthesis.","correct":false,"explanation":"Releasing methane wastes its energy and increases greenhouse-gas emissions. Biogas generators collect methane and burn it as a fuel to provide useful renewable energy."},{"text":"It is converted into oxygen to support aerobic decay.","correct":false,"explanation":"Burning methane uses oxygen rather than producing it, and the generator is intended to recover energy from the gas. Methane is burned as a fuel for heat or electricity."}]},{"id":"4ab814e8-7ab8-4f88-9802-f89599525e1c","cardType":"cloze","points":null,"front":"Decomposition is carried out mainly by [ ].","back":"Decomposition is carried out mainly by bacteria and fungi.","mcqOptions":[{"text":"bacteria and fungi","correct":true,"explanation":"Bacteria and fungi are decomposers. They secrete enzymes onto dead material, digest large molecules into smaller soluble substances, and absorb the products as nutrients."},{"text":"viruses and algae","correct":false,"explanation":"Viruses depend on living host cells and are not the main organisms responsible for decomposition. Algae mainly photosynthesise; bacteria and fungi carry out most decay."},{"text":"plants and protoctists","correct":false,"explanation":"Plants generally make organic substances by photosynthesis rather than decomposing dead material, and protoctists are not the main GCSE decomposers. The principal decomposers are bacteria and fungi."},{"text":"animals and mosses","correct":false,"explanation":"Animals can fragment dead material and mosses can grow on it, but neither group carries out most chemical decomposition. Bacteria and fungi digest the material using extracellular enzymes."}]},{"id":"88ffe9f0-c22c-4f32-aace-a63a6e682613","cardType":"cloze","points":null,"front":"Decay is fastest when conditions are [ ].","back":"Decay is fastest when conditions are warm, moist and well oxygenated.","mcqOptions":[{"text":"warm, moist and well oxygenated","correct":true,"explanation":"Decomposing microorganisms work efficiently when enzymes have a suitable warm temperature, water is available, and oxygen supports aerobic respiration. Together these conditions increase the rate of decay."},{"text":"cold, dry and well oxygenated","correct":false,"explanation":"Oxygen can support aerobic decay, but cold temperatures slow enzyme activity and dryness limits microorganism metabolism. Decay is fastest in warm, moist and well-oxygenated conditions."},{"text":"warm, dry and without oxygen","correct":false,"explanation":"Warmth can increase enzyme activity, but dryness restricts microorganism activity and anaerobic conditions usually reduce the rate of decay. Moisture and oxygen are also needed for fastest aerobic decay."},{"text":"cold, moist and without oxygen","correct":false,"explanation":"Moisture alone is insufficient for rapid decomposition. Cold conditions slow enzyme-controlled reactions, and the absence of oxygen prevents efficient aerobic respiration, so decay is fastest under warm, moist, oxygenated conditions."}]},{"id":"5e7caec0-4f45-4ae2-b66c-edca6f305798","cardType":"cloze","points":null,"front":"The rate of decay is calculated using $$\\text{rate} = \\dfrac{\\text{change in measurement}}{\\text{time taken}}$$, so a 40 g loss in 8 days gives $\\text{[...]}$.","back":"The rate of decay is calculated using $$\\text{rate} = \\dfrac{\\text{change in measurement}}{\\text{time taken}}$$, so a 40 g loss in 8 days gives $\\boxed{5\\text{ g per day}}$.","mcqOptions":[{"text":"$$5\\text{ g per day}$","correct":true,"explanation":"Rate is calculated by dividing the change in measurement by the time taken. Dividing 40 g by 8 days gives 5 g per day."},{"text":"$$32\\text{ g per day}$","correct":false,"explanation":"Multiplying 40 g by 8 days gives 320 rather than a rate, while subtracting gives 32 with the wrong units. The required calculation is 40 divided by 8, giving 5 g per day."},{"text":"$$0.2\\text{ g per day}$","correct":false,"explanation":"This value comes from dividing 8 by 40, reversing the rate formula. Rate requires change in measurement divided by time, so 40 g divided by 8 days equals 5 g per day."},{"text":"$$48\\text{ g per day}$","correct":false,"explanation":"Adding the mass loss and time is not a rate calculation and also combines unlike quantities. Dividing the 40 g change by 8 days gives the correct rate of 5 g per day."}]},{"id":"11feaeae-2883-4625-8187-54e3f136844d","cardType":"cloze","points":null,"front":"In the milk practical, the independent variable is [ ].","back":"In the milk practical, the independent variable is temperature.","mcqOptions":[{"text":"temperature","correct":true,"explanation":"Temperature is deliberately changed between samples in the milk practical to investigate its effect on the rate of lipase activity. It is therefore the independent variable."},{"text":"pH","correct":false,"explanation":"pH may affect lipase activity, but it is not the factor deliberately changed in this practical when temperature is being investigated. The independent variable is temperature."},{"text":"time taken for the colour change","correct":false,"explanation":"The time taken is measured as an outcome of the investigation, so it is the dependent variable. Temperature is deliberately varied and is therefore the independent variable."},{"text":"volume of lipase","correct":false,"explanation":"The volume of lipase is kept constant to make the comparison fair, so it is a control variable. Temperature is the factor deliberately changed and is the independent variable."}]},{"id":"3de0f607-fdd4-46bb-b978-91a7ecdf6b9c","cardType":"cloze","points":null,"front":"Biogas is produced by the anaerobic decay of waste and is mainly [ ].","back":"Biogas is produced by the anaerobic decay of waste and is mainly methane.","mcqOptions":[{"text":"methane","correct":true,"explanation":"Biogas is a mixture of gases made during anaerobic decay, and methane is its main component. Methane can be burned to release energy for heating or generating electricity."},{"text":"oxygen","correct":false,"explanation":"Anaerobic decay takes place without oxygen, so oxygen is not the main gas produced. Biogas is mainly methane, which is generated by microorganisms decomposing waste anaerobically."},{"text":"carbon dioxide","correct":false,"explanation":"Carbon dioxide may be present in biogas, but it is not its main component in this GCSE context. Methane is the principal gas and the useful combustible fuel."},{"text":"nitrogen","correct":false,"explanation":"Nitrogen is abundant in air but is not the main product of anaerobic waste decay. The main component of biogas is methane, which provides its fuel value."}]}]}],"examCountdown":{"examDate":"2027-05-11","paperLabel":"Paper 1"},"heading":"7.2.3 Decomposition","title":"7.2.3 Decomposition Flashcards","topbar":{"backHref":"/gcse/biology/aqa/flashcards"},"topicSidebar":"$L38"}]}],"topicId":"74c982f9-ad79-57a4-b6a8-05ce55f6a766","walled":"$L39"}],"$L3a","$L3b"]