Why did I lose the mark?

The answer that scoresNitrogen makes up about 78% of the atmosphere and oxygen about 21%, with argon (nearly 1%), carbon dioxide (about 0.04%) and water vapour making up the rest. Nitrogen is the most abundant gas, not oxygen.

The answer that scoresWater evaporates from the oceans, lakes and rivers, and plants transpire, sending water vapour up into the air. The vapour rises, cools and condenses into clouds, then falls as precipitation. Evaporation and transpiration move water up; condensation forms the clouds; precipitation brings it down.

The answer that scoresThe ozone layer absorbs most of the Sun's ultraviolet radiation in the stratosphere, protecting the surface. The greenhouse effect is separate: the surface absorbs shortwave solar radiation and re-emits it as infrared, which greenhouse gases absorb and trap, keeping the Earth's ambient temperature warm enough for life.

The answer that scoresEnergy is lost at every step of a food chain through respiration (released as heat), waste products and uneaten material, so each higher trophic level contains less energy than the level below. This is why food chains are rarely more than four or five links long.

The answer that scoresn1 = 40, n2 = 50, m2 = 10, so N = (40 x 50) / 10 = 2000 / 10 = 200 woodlice. Check the answer is sensible: it must be larger than the first sample, not smaller.

The answer that scoresThe results support the hypothesis - they are consistent with its prediction and give no reason to reject it yet. Proof would require many independent, consistent investigations, at which point the hypothesis may become a theory, and even a theory remains open to revision.

The answer that scoresLay a transect from the path inland and place quadrats at regular intervals along it, so the samples are evenly distributed across the gradient and the vegetation change can be measured without personal bias. This is systematic sampling, and it suits a known environmental gradient.

The answer that scoresLight intensity is the independent variable, the one the student deliberately changes, and plant growth is the dependent variable, the one measured. Control variables such as water, temperature and soil type must be kept constant.

The answer that scoresPopulation density = population / area. For a country of 60 million people in 500 000 km^2, the density is 60 000 000 / 500 000 = 120 people per km^2. Always divide and attach the unit people per km^2.

The answer that scoresDependency ratio = [(young 0 to 14 + old 65+) / working age 15 to 64] x 100. Both the young and the elderly are dependents, the denominator is the working-age population only, and the answer is read as dependents per 100 working-age people.

The answer that scoresA wide base means a high birth rate and a young, growing population. A country with an ageing population shows a narrow base and a broad top, because the birth rate is low and life expectancy is high. The youngest ages are always at the bottom of the pyramid.

The answer that scoresMigration changes both size and composition. Because most migrants are young working-age adults, net in-migration adds to the 15 to 64 band and lowers the average age, while net out-migration of workers can age a population and shrink its workforce.

The answer that scoresEnergy flows in one direction through a food web and is lost at every level through respiration and heat, and in uneaten parts and waste, so only about 10% is passed on. Food chains are short because there is not enough energy left to sustain many levels.

The answer that scoresGPP is the total rate at which producers convert light energy into chemical energy. NPP is the energy left after the producers have used some in respiration, NPP = GPP - respiration, and it is NPP, not GPP, that is available to the consumers.

The answer that scoresA native species occurs naturally in an area, while an invasive species has been introduced by humans and then spreads and causes harm. The cane toad is introduced and invasive in Australia, not native, because humans brought it there.

The answer that scoresA pyramid of biomass is usually upright but can be inverted, for example in a pond where fast-reproducing phytoplankton are eaten so quickly by zooplankton that the zooplankton outweigh them at any instant. Only the pyramid of energy, which is always lost at each level, can never be inverted.

The answer that scoresEnergy security is the reliable availability of energy sources at an affordable price, with consideration of the environmental impacts. A country can be energy secure on renewables, and a country with large oil reserves can still be insecure if its supply is disrupted or its energy is unaffordable.

The answer that scoresBiomass is renewable because the plants it comes from regrow, so the supply replaces itself, while uranium is non-renewable because it exists in a fixed amount and is used faster than it forms. Renewable means replenished on a human timescale, not clean.

The answer that scoresBioaccumulation is the build-up of a toxin within one organism because it takes the toxin in faster than it can excrete it. Biomagnification is the increasing concentration at each successive trophic level, because each predator eats many contaminated prey and retains the toxin, so top predators carry the highest concentrations.

The answer that scoresThe waste hierarchy is reduce, reuse, recycle, in that order of preference. Reducing waste at source is best because there is less to dispose of at all, reuse keeps items out of the waste stream without reprocessing, and recycling comes third because it still costs energy and only works for some materials.

The answer that scoresAlmost all of the planet's water is salt water. Only about 2.5% is fresh, and most of that is frozen in ice sheets and glaciers, so fresh water is scarce and unevenly spread. Water insecurity arises wherever people cannot access clean water in sufficient quantity, which can happen in a wet country if the supply is polluted, distant or too expensive.

The answer that scoresWater security is the ability to access sufficient quantities of clean water to maintain adequate standards of food and of manufacturing of goods, adequate sanitation and sustainable health care. It depends on quality, quantity and access, and rainfall alone does not guarantee any of the three.

The answer that scoresA dam and reservoir store river water from the wet season for release in the dry season, securing a reliable supply for towns and farms and often generating hydroelectric power. Against this, the dam floods land and displaces people, evaporative losses from the open surface are high in hot climates, and sediment can fill the reservoir, so the strategy has real costs as well as benefits.

The answer that scoresSalinisation is the build-up of salt in the soil caused by poor irrigation and drainage. As the irrigation water evaporates it leaves dissolved salts behind, the soil becomes too salty for most crops, and crop yields fall or fail altogether, which is why salinisation is a cause of water insecurity rather than a benefit.

The answer that scoresAcid deposition is a mix of air pollutants deposited as acidic wet deposition, with a pH below 5.6, or acidic dry deposition. Wet deposition falls as snow, rain, hail or fog, while dry deposition settles directly as dust and gases. Acid rain is only the wet half of this.

The answer that scoresOzone depletion is caused by chlorofluorocarbons (CFCs), which are unreactive in the troposphere, move into the stratosphere, and break down under ultraviolet light to release chlorine atoms. These chlorine atoms convert ozone (O3) to oxygen (O2), thinning the ozone layer. Carbon dioxide is a greenhouse gas involved in climate change, a separate issue.

The answer that scoresThe ozone hole is an area of the atmosphere where the average concentration of ozone is below 100 Dobson Units. It is a region of severe thinning, measured in Dobson Units, and it still contains some ozone, just far less than normal.

The answer that scoresFlue gas desulfurisation removes sulfur dioxide from the waste gases of a power station, and fuel desulfurisation removes sulfur from the fuel before it is burned. Particulates are removed separately, by electrostatic precipitators that charge the particles and pull them onto collecting plates.

The answer that scoresThe natural greenhouse effect traps infrared radiation and keeps the Earth warm enough for life. The enhanced greenhouse effect is the additional warming caused by the higher concentrations of greenhouse gases, mainly carbon dioxide and methane, released by human activities such as burning fossil fuels. It is the enhancement, not the effect itself, that drives global warming.

The answer that scoresBurning fossil fuels releases carbon dioxide and water vapour. Methane is released by rice fields, by the digestive systems of livestock, and by landfill sites, because anaerobic bacteria produce it when they decompose organic matter without oxygen.

The answer that scoresSRM, through albedo enhancement, space reflectors or stratospheric aerosols, reflects a little more incoming sunlight so less warms the surface. It does not remove greenhouse gases, so it does not address ocean acidification, and the warming would return if the intervention stopped. It treats a symptom, not the cause.

The answer that scoresClimate change affects human populations through more frequent and severe extreme weather, damage to property and loss of life, forced migration, falling crop yields with more pests, and threats to food, energy and water security. Managing it therefore needs both mitigation, to cut emissions, and adaptation, to cope with the warming that is already locked in.