Energy, Water & Material Resources — Chapter Practice, Grade 12

Evaluate resources through conservation, replenishment, life-cycle boundaries, and equal service.

Problem 1

Which of the following is a fossil fuel?
  1. Wind
  2. Coal
  3. Uranium
  4. Sunlight
Answer and explanation

Coal

Coal, oil, and natural gas are the three fossil fuels. Uranium is a nuclear fuel, and sunlight and wind are renewable.

Problem 2

Which fossil fuel typically releases the LEAST carbon dioxide per unit of energy when burned?
  1. Coal
  2. Oil (petroleum)
  3. Natural gas
  4. Peat
Answer and explanation

Natural gas

Natural gas emits roughly half the CO2 of coal per kilowatt-hour, which is why it is often called a 'cleaner' fossil fuel.

Problem 3

Coal emits about 2.2 lb of CO2 per kWh and natural gas about 0.9 lb per kWh. How many pounds of CO2 are avoided by generating 1,000 kWh with natural gas instead of coal?
Answer and explanation

1300 lb

Coal: 2.2 x 1,000 = 2,200 lb. Gas: 0.9 x 1,000 = 900 lb. Avoided: 2,200 - 900 = 1,300 lb.

Problem 4

Nuclear power plants generate energy mainly through which process?
  1. Nuclear fusion of hydrogen nuclei
  2. Ordinary radioactive decay of coal
  3. Combustion of uranium in air
  4. Nuclear fission — splitting heavy nuclei like uranium-235
Answer and explanation

Nuclear fission — splitting heavy nuclei like uranium-235

A nuclear plant splits heavy uranium-235 nuclei (fission), releasing heat that boils water to spin a turbine — no burning is involved.

Problem 5

Which statement about nuclear power is TRUE?
  1. It emits almost no CO2 while operating but produces long-lived radioactive waste
  2. It is a renewable energy source like solar
  3. It releases more CO2 per kWh than coal
  4. It cannot provide steady, around-the-clock electricity
Answer and explanation

It emits almost no CO2 while operating but produces long-lived radioactive waste

Nuclear power is nearly carbon-free during operation and runs reliably as baseload power, but it produces radioactive waste that stays hazardous for thousands of years, and uranium is nonrenewable.

Problem 6

Natural gas is often called a 'cleaner' fossil fuel than coal, yet scientists warn that its climate benefit can be undercut. Explain why, using the term methane.
Answer and explanation

Burning natural gas releases only about half the CO2 of coal per unit of energy, which is the source of its 'cleaner' reputation. But natural gas is mostly methane, a powerful greenhouse gas, and some of it leaks unburned into the atmosphere during drilling, storage, and transport. Because methane traps far more heat than CO2 (especially in the short term), those leaks can cancel out much of the CO2 advantage.

Problem 7

A plant converts 200 MJ fuel to 80 MJ electricity. Find efficiency. Show the reasoning, not just the conclusion.
Hints

Compare energy at the same system boundary.

Answer and explanation

40%.

Problem 8

Why is a fuel considered nonrenewable on human timescales?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Its formation/replenishment is much slower than extraction and use.

Problem 9

Trace upstream and downstream effects instead of using one labelTrace upstream and downstream effects instead of using one labelExtractionLeaks / land / waterProcessing /transportEnergy / lossesCombustion /serviceCO₂ / pollutantsLife-cycle boundary includes more than the smokestack.Different pollutants require their own quantities and impact models.
Original pathway inventory; no current plant emission factor or universal fuel ranking is asserted.
Interpret the supplied visual. Why can a lower combustion CO₂ factor be insufficient for a full climate ranking?
Answer and explanation

Upstream methane, conversion efficiency and other life-cycle emissions can change the result under the stated horizon/boundary.

A fuel life cycle includes extraction, processing, transport, conversion and residual management. Direct CO₂, methane leakage, particulates, sulfur compounds and land/water effects are different pathways that need separate evidence. “Cleaner” is incomplete without naming the pollutant, service and boundary. A climate comparison using CO₂-equivalent values also needs a stated time horizon and conversion method; it is not simply a count of molecules emitted.

Upstream methane, conversion efficiency and other life-cycle emissions can change the result under the stated horizon/boundary.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

Plant A delivers 40 MJ electricity from 100 MJ fuel; B delivers 50 MJ from 100 MJ. Direct fuel factors are 80 and 70 g CO₂/MJ input. Compare direct emissions per delivered MJ, then name an omitted term.
Answer and explanation

A:8000/40=200 g/MJ delivered. B:7000/50=140 g/MJ. Upstream leakage, transport or construction is omitted; this direct comparison is not a complete life-cycle ranking.

A:8000/40=200 g/MJ delivered.

B:7000/50=140 g/MJ.

Upstream leakage, transport or construction is omitted; this direct comparison is not a complete life-cycle ranking.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

Which device converts sunlight directly into electricity?
  1. A wind turbine
  2. A photovoltaic (solar) cell
  3. A steam turbine
  4. A geothermal well
Answer and explanation

A photovoltaic (solar) cell

A photovoltaic cell turns sunlight straight into electricity with no moving parts.

Problem 12

Which renewable energy source uses the kinetic energy of moving air?
  1. Hydroelectric
  2. Geothermal
  3. Wind
  4. Biomass
Answer and explanation

Wind

Wind turbines capture the kinetic energy of moving air and convert it into electricity.

Problem 13

A solar farm has a nameplate capacity of 80 MW but averages only 20 MW of output over the year. What is its capacity factor, as a percent?
Answer and explanation

25 %

Capacity factor = average output / maximum output = 20 / 80 = 0.25 = 25%.

Problem 14

Which of these is a genuine drawback of large hydroelectric dams?
  1. They produce no electricity at all during the day
  2. They cannot be turned on or off to match demand
  3. They release more CO2 than a coal plant
  4. They can flood large areas of land and block fish migration
Answer and explanation

They can flood large areas of land and block fish migration

Dams flood land behind them and block migrating fish, and they trap sediment and reshape rivers — real ecological trade-offs even though the power itself is clean.

Problem 15

'Intermittency' means that a power source...
  1. only produces power when its natural input, like sun or wind, is available
  2. produces exactly the same output every hour of the day
  3. can never be stored in any form
  4. releases greenhouse gases as it runs
Answer and explanation

only produces power when its natural input, like sun or wind, is available

Intermittent sources such as solar and wind generate power only when the sun shines or the wind blows, not on demand.

Problem 16

Biomass is sometimes described as 'carbon-neutral.' Explain the reasoning behind that claim and one important condition that must be met for it to actually hold true.
Answer and explanation

Burning biomass releases CO2, but the plants pulled that same CO2 out of the air through photosynthesis while they were growing, so in principle the net addition to the atmosphere can be close to zero. For this to truly hold, the plants must actually be regrown at the same rate they are burned, so the released carbon is reabsorbed. If forests are cleared faster than they regrow (or if lots of fossil fuel is used to grow, harvest, and transport the biomass), it is not really carbon-neutral.

Problem 17

A wind generator supplies average 2 kW over 10 h. Find energy. Show the reasoning, not just the conclusion.
Hints

Rated power and actual time-integrated energy differ.

Answer and explanation

20 kWh.

Problem 18

Why might renewable deployment still need storage, transmission or demand management?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Supply varies in time and location; useful service requires matching production to demand and grid constraints.

Problem 19

Energy timing and storage losses can change adequacyEnergy timing and storage losses can change adequacyHour blockDemandGenerationStorage change beforelossesDaylight10 kWh16 kWh+6 kWhNight8 kWh0 kWh−8 kWh neededDaily total18 kWh16 kWhDeficit before lossesA daily energy total does not guarantee hourly adequacy.
Illustrative two-block supply/demand account; it is not an optimized real grid design.
Interpret the supplied visual. Can loss-free storage alone eliminate the displayed net daily 2 kWh deficit indefinitely?
Answer and explanation

No. It can shift timing but cannot replace a persistent energy deficit without another input.

A source can produce substantial daily energy while failing to meet demand at certain hours. Storage shifts energy through time but has capacity, power and round-trip-loss limits; it does not create the missing energy in a daily deficit. The displayed 16 kWh generation is already below 18 kWh demand before losses. Compare time-resolved supply and demand, initial storage state and allowable shortfalls rather than assuming an annual energy total guarantees continuous service.

No. It can shift timing but cannot replace a persistent energy deficit without another input.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 20

A 5 kW system has 30% capacity factor for 2000 h. Find generated energy. If storage returns 80% of an allocated 500 kWh, how much useful stored energy returns?
Answer and explanation

Generation=5×0.30×2000=3000 kWh. The allocated storage input returns 400 kWh and loses/transfers 100 kWh under the stated boundary; do not apply the storage loss to energy that bypasses it.

Generation=5×0.30×2000=3000 kWh.

The allocated storage input returns 400 kWh and loses/transfers 100 kWh under the stated boundary; do not apply the storage loss to energy that bypasses it.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 21

About what fraction of Earth's water is freshwater?
  1. About 97%
  2. About 3%
  3. About 30%
  4. About 50%
Answer and explanation

About 3%

Roughly 97% of Earth's water is salty ocean, leaving only about 3% as freshwater — and most of that is frozen or deep underground.

Problem 22

A watershed is best described as...
  1. a type of hydroelectric dam
  2. a machine that purifies drinking water
  3. the area of land that drains to a common body of water
  4. a large underground cave full of water
Answer and explanation

the area of land that drains to a common body of water

A watershed is all the land from which water drains into one shared river, lake, or bay.

Problem 23

For this supplied allocation model, agriculture uses about 70% of freshwater and industry about 20%, with homes and cities using the rest. What percent is used by homes and cities?
Answer and explanation

10 %

The three uses must total 100%: 100 - 70 - 20 = 10%.

Problem 24

Which is the LARGEST single use of freshwater worldwide?
  1. Bottling drinking water
  2. Household drinking and bathing
  3. Industrial cooling
  4. Agricultural irrigation
Answer and explanation

Agricultural irrigation

Agriculture, mostly through irrigation, accounts for roughly 70% of global freshwater use — far more than industry or homes.

Problem 25

Fertilizer running off farmland and down a watershed into a lake can cause...
  1. algal blooms and low-oxygen 'dead zones'
  2. cooler water temperatures
  3. an increase in drinking-water supply
  4. stronger bedrock beneath the lake
Answer and explanation

algal blooms and low-oxygen 'dead zones'

Fertilizer nutrients feed algal blooms; when the algae die and decompose they use up the oxygen, creating dead zones where fish cannot survive.

Problem 26

New York City gets much of its drinking water from the Catskill watershed and avoids building an expensive filtration plant. Explain how protecting the LAND in a watershed helps protect the WATER supply.
Answer and explanation

A watershed drains all the rain and runoff from its land into the reservoirs, so whatever happens on that land — pollution, sewage, erosion, or development — ends up in the water. By keeping the watershed forested and undeveloped, the soil and plants naturally filter and clean the water before it reaches the reservoir. Protecting the land therefore keeps the water clean without the cost of an artificial filtration plant.

Problem 27

Supply is 90 and demand 75 units/day. Find surplus in this simple account. Show the reasoning, not just the conclusion.
Hints

Efficiency improvements need to meet the full service balance.

Answer and explanation

15 units/day.

Problem 28

Why can water be abundant but still unavailable for drinking?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Quality, treatment, access, infrastructure and seasonal distribution can constrain usable supply.

Problem 29

Land cover changes water pathways through physical mechanismsLand cover changes water pathways through physical mechanismsLand-cover changePotential mechanismEvidence neededMore impermeable areaLess infiltration / fasterrunoffMatched rain/runoffrecordsVegetation restorationChanged interception /uptakeSoil, flow and habitatrecordsSoil compactionChanged pore connectivityInfiltration andstructure testsMechanisms depend on soil, slope, rainfall and design.
Original land-cover mechanism comparison, not a universal numerical effect size.
Interpret the supplied visual. Does a lower runoff peak always mean less total runoff volume?
Answer and explanation

No. Flow can be delayed or spread out while event volume remains similar; measure both quantities.

Infiltration is water entering soil; runoff is flow over the land surface. Impermeable cover, compaction, vegetation and drainage design can alter partitioning among infiltration, storage, evapotranspiration and rapid runoff. The effect depends on rainfall intensity, soil and slope, so a single before/after photograph is not enough to quantify it. Compare matched events or an explicit model and distinguish peak flow from total event volume.

No. Flow can be delayed or spread out while event volume remains similar; measure both quantities.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 30

An aquifer receives 8 million m³/year recharge, loses 2 million to streams and is pumped at 9 million. Find annual storage change and a limitation of predicting well depth from it.
Answer and explanation

Δstorage=8−2−9=−3 million m³/year. Exact water-level change also needs aquifer geometry, storage properties and spatial pumping/flow patterns.

Δstorage=8−2−9=−3 million m³/year.

Exact water-level change also needs aquifer geometry, storage properties and spatial pumping/flow patterns.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 31

An ore is best defined as...
  1. any rock at all, no matter what is in it
  2. a rock or deposit rich enough in a valuable mineral to be worth mining
  3. a type of fertile soil
  4. a renewable energy source
Answer and explanation

a rock or deposit rich enough in a valuable mineral to be worth mining

An ore is a rock or deposit that holds enough of a valuable metal or mineral that it is worth the cost of mining it.

Problem 32

Which soil layer is the fertile topsoil where most crops grow?
  1. The C horizon (weathered parent rock)
  2. The bedrock at the bottom
  3. The A horizon (topsoil)
  4. The deepest layer of the profile
Answer and explanation

The A horizon (topsoil)

The A horizon is the dark, fertile topsoil rich in humus where most plant roots grow.

Problem 33

Recycling aluminum uses only about 5% of the energy needed to make it from raw ore. If making new aluminum from ore takes 100 units of energy, how many units are SAVED by recycling the same amount of aluminum instead?
Answer and explanation

95 units

Recycling uses 5% of 100 = 5 units, so it saves 100 - 5 = 95 units of energy.

Problem 34

The 1930 s Dust Bowl showed how quickly soil can be lost when...
  1. farmers plant too many cover crops
  2. land is left as undisturbed native prairie
  3. metals are recycled instead of mined
  4. native grasses are plowed up, exposing bare topsoil to wind erosion
Answer and explanation

native grasses are plowed up, exposing bare topsoil to wind erosion

Plowing up the deep-rooted prairie grasses left bare topsoil that blew away in huge dust storms once drought hit.

Problem 35

Which farming practice best conserves soil?
  1. Planting cover crops and using no-till methods
  2. Plowing straight up and down steep slopes
  3. Removing all crop residue and leaving the soil bare
  4. Growing the same crop every year until the nutrients run out
Answer and explanation

Planting cover crops and using no-till methods

Cover crops and no-till farming keep roots in the ground and the surface covered, holding soil in place and protecting its nutrients.

Problem 36

Soil is technically a renewable resource, yet environmental scientists often treat it like a nonrenewable one. Explain this apparent contradiction.
Answer and explanation

Soil does form naturally from weathering rock and decaying organisms, so it is technically renewable. But it forms extraordinarily slowly — often hundreds of years to build a single inch — while erosion from plowing, deforestation, or overgrazing can strip that same inch away in only a few decades. Because we can lose it far faster than nature rebuilds it, in human terms soil behaves like a nonrenewable resource that must be conserved.

Problem 37

A mine recovers 3 tonnes of metal from 100 tonnes ore. Find recovered mass fraction. Show the reasoning, not just the conclusion.
Hints

Distinguish a rate from total soil inventory.

Answer and explanation

3/100=0.03, or 3%.

Problem 38

Why does recycling not completely remove the need for resource accounting?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Collection losses, quality, energy requirements, growing demand and unrecovered material remain relevant.

Problem 39

Soil function depends on structure, organisms and formationSoil function depends on structure, organisms and formationOrganic-rich surface: roots / organismsMinerals + pores holding air and waterParent material / deeper horizonsSoil is a structured living system, not only loose mineral grains.
Original generic soil-horizon cross-section; horizon depths and colors are illustrative.
Interpret the supplied visual. Why can compaction matter even if the soil’s total mineral mass hardly changes?
Answer and explanation

It can alter pore space and connectivity, affecting infiltration, aeration and root growth.

Soil contains mineral particles, organic matter, water, air and organisms arranged in a structure. Pores allow storage and movement of air/water; roots and microbial processes interact with aggregation and nutrients. Horizons develop through weathering, transport and biological activity over time. Rapid erosion can remove a productive surface layer much faster than it forms. A schematic layer drawing is not a guarantee that every local soil has the same depths or composition.

It can alter pore space and connectivity, affecting infiltration, aeration and root growth.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 40

A 2000 kg ore batch has 3% target metal and 75% recovery. Find contained, recovered and unrecovered target metal, then name a separate soil-quality measure.
Answer and explanation

Contained=60 kg; recovered=45 kg; unrecovered target=15 kg. Infiltration, organic matter, pore structure or measured erosion can characterize soil quality separately from mineral yield.

Contained=60 kg; recovered=45 kg; unrecovered target=15 kg.

Infiltration, organic matter, pore structure or measured erosion can characterize soil quality separately from mineral yield.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 41

Delivered energy 300MJ, invested 30MJ. Find modeled EROI.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

10

300/30=10.

Problem 42

Why specify a functional unit?
Answer and explanation

It ensures the alternatives provide comparable service.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 43

A 4 kW unit produces 24 kWh in 12 h. Find capacity factor as a percent.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

50 %

24/(4×12)×100=50.

Problem 44

How does capacity factor differ from efficiency?
Answer and explanation

Capacity factor compares output with rated output over time; efficiency compares useful output energy with input energy.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 45

Find total A energy over 15 years. Show the reasoning, not just the conclusion.
Hints

A service life shorter than 40 years may prevent break-even.

Answer and explanation

600+20×15=900 MJ.

Problem 46

What could change the estimated break-even result?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Different actual use rates, lifetimes, replacements, maintenance, production methods and omitted life-cycle stages.

Problem 47

Break-even compares cumulative accounts over repeated serviceBreak-even compares cumulative accounts over repeated serviceCumulative energy (illustrative units)0100200300400020406080Number of equivalent usesReusable:120+2nDisposable:5n
Calculated hypothetical equal-service energy lines; this is not empirical product LCA data.
Interpret the supplied visual. At60 uses, how much smaller is the reusable energy account?
Answer and explanation

Disposable=300; reusable=240; difference=60 illustrative energy units.

A break-even point is where two cumulative accounts are equal. Here reusable energy is 120+2n while disposable energy is 5n, giving n=40 uses. Before 40 uses the reusable account is larger; after 40 it is smaller under these assumptions. Equality at 40 does not prove every environmental impact also breaks even there. Actual loss, replacement, washing and service differences can shift or remove the threshold.

Disposable=300; reusable=240; difference=60 illustrative energy units.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 48

Reusable production energy is 180 units with 3 units/use; disposable energy is 9 units/use. Find break-even and energy difference after 50 uses.
Answer and explanation

180+3n=9n gives n=30 uses. At50, reusable=330 and disposable=450, a 120-unit advantage in this hypothetical energy category; other impacts need separate accounts.

180+3n=9n gives n=30 uses.

At50, reusable=330 and disposable=450, a 120-unit advantage in this hypothetical energy category; other impacts need separate accounts.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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