Science — Mixed challenge

One thing is deliberately being changed here — find it, then think about everything else.

A test is only 'fair' if a single factor differs between trials; otherwise you can't tell what caused the change.

Temperature is the independent variable, so it is the ONE thing allowed to differ.

The volume of water, the tablet's size, and the container — everything except temperature

The question she is really asking is a reaction-rate question: does raising temperature speed up the dissolving reaction? Temperature is therefore the variable she is testing (the independent variable), so it must change from cup to cup.

For the comparison to be fair, every OTHER factor that could affect the rate — water volume, tablet mass/surface area, stirring, the container — has to be identical, so temperature is the only possible cause of any difference.

That points to B.

Combine two ideas: how to read a leveling-off (plateau) on a graph, and what limits a population's size.

A flat line means the value has stopped changing — growth has stalled, not crashed.

Ecologists have a name for the maximum population an environment can sustain.

carrying capacity — the ceiling the environment's food and space impose

Reading the graph: a curve that climbs and then holds steady shows growth slowing to zero — the population has settled at a stable maximum, not crashed (that would fall to zero) and not errored.

In ecology, that stable maximum an environment's resources can support is the carrying capacity, so the plateau near 500 is exactly that.

The answer is A.

You measured two quantities; density is the property that connects them.

Density = mass ÷ volume.

Divide 54 g by 20 cm³, then compare with 2.7 g/cm³.

2.7 g/cm³

Density is a physical property that identifies a material, and it is found from two measurements: mass and volume.

Density = mass ÷ volume = 54 g ÷ 20 cm³ = 2.7 g/cm³.

Since that matches aluminum's 2.7 g/cm³, the block could indeed be aluminum: 2.7 g/cm³.

The numbers form a table — look at how position changes for each second that passes.

On a position–time graph, the steepness (slope) of the line IS the speed.

Speed = change in position ÷ change in time.

3 m/s

The data lie on a straight line: every 1 s, the position increases by 3 m, so the motion is steady.

Speed is the slope of position vs. time = (9 m − 0 m) ÷ (3 s − 0 s) = 9/3.

The car moves at 3 m/s.

First reduce 78 : 24 to a simple whole-number ratio.

A 3:1 ratio among offspring is the signature outcome of one specific kind of cross in a Punnett square.

Two purple parents producing white children means each parent must have hidden a white allele.

Both parents secretly carry one recessive white allele (they are heterozygous), and white is recessive — a cross of two carriers gives a 3:1 ratio

Interpreting the data: 78 : 24 simplifies to about 3 : 1.

In a Punnett square, crossing two heterozygotes (Pp × Pp) gives 3 dominant : 1 recessive. So both purple parents must be Pp — each carrying one hidden white allele — and white must be the recessive trait that appears only when two of those alleles meet.

The answer is B.

Two ideas have to meet: how natural selection removes a visible trait, and where a recessive allele can 'hide'.

Only green (bb) beetles are eaten — brown Bb beetles look identical to BB beetles.

Selection can only act on what it can see, and it cannot see a b that is paired with a B.

It becomes rarer but is not wiped out, because it survives hidden inside brown (Bb) carriers

Natural selection removes green (bb) beetles, so each generation fewer b alleles are passed on and b grows rarer — that is selection at work.

But b does not vanish: heterozygous brown (Bb) beetles look exactly like BB beetles, so birds can't remove them, and they keep carrying b forward hidden from selection.

So b declines but persists — C.

Compare which way the ions are being pushed with the direction things drift on their own.

Moving a substance from low concentration to high is 'uphill' — the opposite of diffusion.

Anything that runs uphill in a cell needs an energy source.

Active transport, which spends energy (ATP) to pump ions against their concentration gradient

Diffusion and osmosis only move things DOWN a gradient (high to low) and cost no energy, so they cannot build or hold a higher inside concentration.

Pushing potassium the 'wrong' way, from low to high, is active transport, and it is powered by the cell's energy currency, ATP.

The answer is C.

Trees are one arm of the carbon cycle; think about both the carbon locked in the wood and the job the living trees were doing.

Photosynthesis is the process that removes CO_2 from the air; burning is a process that releases it.

Two effects push the same direction here.

It rises: burning releases the carbon stored in the trees, and losing the trees also removes the photosynthesis that had been pulling CO_2 out of the air

Burning the wood is combustion: the carbon that photosynthesis had locked into the tree's sugars and wood is released back to the air as CO_2.

At the same time, removing the living forest deletes a major CO_2 sink — those trees had been pulling carbon out of the atmosphere by photosynthesis. Both changes raise atmospheric CO_2.

The answer is B.

Hold two facts together: every body cell got the same DNA by cell division, yet cells become specialized.

Genes act by directing the building of proteins — and proteins determine what a cell looks like and does.

A cell can choose which genes to use without changing the DNA itself.

They carry identical genes but switch on (express) different subsets of them, so each builds a different collection of proteins

Because all the body's cells descend by mitosis from one fertilized egg, they share one identical genome — the DNA is the same in skin and nerve cells.

Cells specialize by expressing different genes: each cell type transcribes and translates a different subset of its genes into proteins, and that different protein mix gives each cell its distinct structure and function.

The answer is B.

Count how many transfer steps separate producers from tertiary consumers.

Each step keeps only 10% (multiply by 0.1); the missing 90% mostly leaves as heat.

Apply the 10% factor once for each of the three steps up the chain.

20 kJ

From producers to tertiary consumers is three transfers, and at each one only 10% of the energy moves up while the rest is lost as heat.

So multiply by 0.1 three times: 20,000 × 0.1 × 0.1 × 0.1 = 20,000 × 0.001.

That leaves 20 kJ reaching the tertiary consumers.

Three moves are hiding here: grams of Al → moles of Al → moles of H_2 → litres of gas.

Moles of Al = 5.4 ÷ 27. Then use the equation's 2 : 3 ratio of Al to H_2.

Convert moles of H_2 to volume with 22.4 L per mole.

6.72 L

Moles of Al = 5.4 g ÷ 27 g/mol = 0.20 mol.

The balanced equation gives 3 mol H_2 for every 2 mol Al, so moles of H_2 = 0.20 × (3/2) = 0.30 mol.

At STP each mole of gas fills 22.4 L, so volume = 0.30 × 22.4 = 6.72 L.

The 890 kJ is 'per mole', but you're given grams — those aren't the same amount.

First find how many moles are in 8.0 g of CH_4.

Multiply the moles by 890 kJ/mol.

445 kJ

Convert mass to moles: 8.0 g ÷ 16 g/mol = 0.50 mol CH_4.

Each mole releases 890 kJ, and this reaction is exothermic, so heat released = 0.50 mol × 890 kJ/mol.

That gives 445 kJ.

Start from what you can compute directly: moles of acid = molarity × volume in litres.

Look hard at the equation — each H_2SO_4 needs TWO NaOH, not one.

Once you have moles of NaOH, turn them back into a volume using its 0.50 M concentration.

20 mL

Moles of H_2SO_4 = 0.20 mol/L × 0.025 L = 0.0050 mol.

The balanced equation's 1 : 2 ratio means each mole of acid consumes 2 mol NaOH, so moles NaOH = 2 × 0.0050 = 0.010 mol.

Volume = moles ÷ molarity = 0.010 mol ÷ 0.50 mol/L = 0.020 L = 20 mL.

Use each element's group to decide whether it wants to lose or gain an electron.

A metal that gives up an electron plus a nonmetal that grabs one makes charged ions.

Ask what has to be true — free-moving charges — for a solution to conduct.

An ionic bond, and it conducts when dissolved because it splits into free-moving ions

Group 1 is a metal with 1 valence electron it readily loses; Group 17 is a nonmetal one electron short of a full shell. The metal transfers its electron to the nonmetal, giving oppositely charged ions held by an ionic bond.

Dissolved in water, an ionic compound dissociates into free-moving ions, and moving charges carry a current — so the solution conducts.

The answer is B.

Percent yield compares what you actually got with the most you could have gotten.

Find the theoretical mass of MgO first: grams Mg → moles Mg → moles MgO → grams MgO.

Percent yield = (actual ÷ theoretical) × 100.

75 %

Theoretical yield: 48 g Mg ÷ 24 g/mol = 2.0 mol Mg; the 2 : 2 ratio gives 2.0 mol MgO; MgO's molar mass is 24 + 16 = 40 g/mol, so 2.0 × 40 = 80 g MgO possible.

Percent yield = actual ÷ theoretical × 100 = 60 g ÷ 80 g × 100.

That is a 75 % yield.

The force acts over a distance — that is the clue to use work and energy, not just F = ma.

Work done = force × distance, and on a frictionless floor all of it becomes kinetic energy.

Set that work equal to ½mv² and solve for v.

4 m/s

The net force does work W = F × d = 4.0 N × 4.0 m = 16 J as the cart moves.

Starting from rest, all that work becomes kinetic energy: ½mv² = 16 J, so ½(2.0)v² = 16 → v² = 16.

Therefore v = √16 = 4 m/s.

Kinetic energy needs a speed, but you're given an acceleration and a time — find the speed first.

With steady acceleration from rest, v = a × t.

Then KE = ½mv².

75000 J

First get the final speed from the motion: starting at rest, v = a × t = 2.0 m/s² × 5.0 s = 10 m/s.

Now the energy: KE = ½mv² = ½ × 1500 kg × (10 m/s)² = ½ × 1500 × 100.

That is 75000 J (75 kJ).

Power needs current, but you're only given voltage and resistance — get the current first.

Ohm's law: current I = V ÷ R.

Then electrical power P = V × I.

36 W

Find the current with Ohm's law: I = V ÷ R = 12 V ÷ 4.0 Ω = 3.0 A.

Electrical power delivered is P = V × I = 12 V × 3.0 A.

So the resistor dissipates 36 W.

The sound is a wave moving at a known speed, so distance = speed × time.

Careful: the 4.0 s covers the trip DOWN and back UP.

The depth is only half of the total distance the sound travels.

3000 m

The pulse travels at the wave's speed, so the total path = speed × time = 1500 m/s × 4.0 s = 6000 m.

But that round trip is down to the floor and back, so the depth is half of it: 6000 ÷ 2.

The ocean floor is 3000 m deep.

In a collision with no outside push, the total momentum before equals the total momentum after.

Momentum = mass × velocity; only the first cart is moving beforehand.

After impact the two masses move together as a combined 3.0 kg.

2 m/s

Momentum is conserved: total before = total after. Before, only the 2.0 kg cart moves, giving 2.0 × 3.0 = 6.0 kg·m/s (the 1.0 kg cart contributes 0).

After, the stuck-together carts have combined mass 3.0 kg moving at one speed v: 3.0 × v = 6.0.

So v = 6.0 ÷ 3.0 = 2 m/s.

Link the setting — a convergent boundary building mountains — to the conditions rock experiences there.

The key words are high pressure and heat, but crucially the rock does NOT melt.

Which of the three rock types is defined by solid rock being changed by heat and pressure?

Metamorphic rock, as heat and pressure transform rock that stays solid

Where two continental plates converge, rock gets buried deep and subjected to intense pressure and heat as the crust thickens into mountains.

Because the rock is changed while staying solid (no melting, so not igneous; no sediment settling, so not sedimentary), this is metamorphism — it produces metamorphic rock.

The answer is C.

Weather here is really about heat moving air — think about which surface warms faster in the sun.

Warm air is less dense and rises (convection); something has to flow in underneath to replace it.

Compare how quickly land and water change temperature.

Land heats faster than water; the warm air over the land rises, and cooler, denser air from over the sea flows in to take its place

Land warms faster than water under the same sunshine, so by afternoon the air over the land is hotter, less dense, and rises — a convection current driven by uneven heating.

That rising air leaves lower pressure over the land, and cooler, denser air from over the sea flows in beneath it: a sea breeze blowing from sea to land.

The answer is B.

Distance to the Sun is nearly constant, so warmth must come from HOW the sunlight arrives, not how far it travels.

Think about sunlight hitting the ground straight-on versus at a low slant — which delivers more energy per square metre?

Earth's tilt is the cause of the seasons.

Because the Northern Hemisphere is tilted toward the Sun, so sunlight strikes more directly — concentrating energy on each patch of ground — and days are longer

The seasons come from Earth's axial tilt, not its distance. In Northern summer the Northern Hemisphere leans toward the Sun.

That tilt makes sunlight strike the surface more directly, so the same beam of solar energy is concentrated on a smaller area (more energy per square metre), and it also lengthens the day — both warm the hemisphere.

The answer is B.

Two dating rules are in play: which came first among stacked layers, and what it means for something to slice through them.

By superposition, lower layers are older — so among the three layers the top one is youngest.

But the lava didn't settle as a layer; it cut through layers that were already there.

The hardened lava filling the crack, because a feature that cuts across the layers must be younger than every layer it cuts

Superposition tells us that in undisturbed layers the deepest is oldest and the top layer (sandstone) is the youngest of the three sedimentary beds.

The lava-filled crack, though, cuts across all three layers — and by cross-cutting relationships a feature must be younger than anything it slices through. Since it cuts even the top layer, the intrusion is the youngest feature of all.

The answer is C.

This is a rate: distance moved away from the ridge divided by the time it took.

The numbers are in kilometres and years, but the answer is in centimetres per year — convert 60 km to centimetres.

60 km = 6,000,000 cm; divide by 3,000,000 years.

2 cm/year

Spreading rate = distance ÷ time. The crust moved 60 km away from the ridge over 3 million years.

Convert units: 60 km = 60 × 100,000 cm = 6,000,000 cm, and the time is 3,000,000 years.

Rate = 6,000,000 cm ÷ 3,000,000 years = 2 cm/year.

Follow the carbon: it had been locked out of the cycle underground, and burning puts it back into the fast part of the cycle.

Compare the speed of release by burning with the speed nature removes CO_2.

Extra CO_2 in the air is linked to the greenhouse effect.

It releases long-buried carbon as CO_2 faster than photosynthesis and the oceans can reabsorb it, raising atmospheric CO_2 and strengthening the greenhouse effect

Fossil fuels are a long-term carbon store. Burning them injects that carbon into the atmosphere as CO_2 much faster than natural sinks — photosynthesis and the oceans — can take it back out.

So atmospheric CO_2 climbs, and because CO_2 is a greenhouse gas, the greenhouse effect strengthens and the planet warms.

The answer is C.

Sort each energy source into renewable or nonrenewable first.

Then trace the energy transformations: what turns into what in each device?

Only one of them burns a fuel and puts out carbon dioxide.

The solar panel transforms an endlessly renewed energy flow (sunlight) straight into electrical energy with no fuel burned, while the coal plant burns a nonrenewable fuel, releasing stored chemical energy as heat and emitting CO_2

Sunlight is a renewable flow that is constantly resupplied; coal is a finite, nonrenewable fuel. That difference alone rules out the other options.

Tracing the energy: a solar panel converts radiant energy directly into electrical energy with no combustion, while a coal plant releases the chemical energy stored in coal as heat, then converts that to electricity — and emits CO_2 in the process.

The answer is B.

Ask of each one: can the ecosystem get it back and reuse it, or does it have to keep coming from outside?

Atoms are not created or destroyed — carbon can pass from air to plant to animal to soil and back to air.

Energy degrades to heat at every step and radiates away.

Matter cycles — the same atoms are reused over and over between organisms and the environment — while energy flows one way, entering as sunlight and leaving as heat, so it must be constantly resupplied

Matter is conserved and reused: the same carbon, nitrogen, and water atoms cycle repeatedly between organisms and the physical environment — that's what a biogeochemical cycle is.

Energy behaves differently: it enters as sunlight, moves one way up the food chain, and is steadily lost as heat, so it cannot be recycled and must be resupplied by the Sun.

The answer is C.

Trace the knock-on effects: wolves gone → deer → plants → other species.

This is a human action rippling through a web of feeding relationships.

Think about how the stability of an ecosystem depends on its species staying in balance.

Species are interconnected, so removing a top predator can cascade through the food web and destabilize the whole ecosystem

Removing the wolves is a human impact that doesn't stop with the wolves: with their predator gone, deer numbers surge and overgraze, which harms plants and every species tied to them.

Because the organisms are linked in a food web, taking out a top predator cascades through the whole system and destabilizes it — showing how interconnected and vulnerable ecosystem balance is.

The answer is C.

You can't jump straight to CO_2 — first work out how much coal is burned to supply the energy.

Coal needed = energy required ÷ energy per kilogram.

Then multiply the kilograms of coal by the CO_2 released per kilogram.

6000 kg

First find the fuel used: 60,000 MJ ÷ 30 MJ per kg = 2000 kg of coal per day.

Each kilogram of coal emits about 3 kg of CO_2, so emissions = 2000 kg × 3 = 6000 kg.

The town releases about 6000 kg of CO_2 per day.