Physical Science: Matter, Forces & Energy — Grade 8 Science — Midterm worked answers
Student exam paperOriginal cumulative physical-science paper with evidence, model, graph and calculation reasoning. All records are supplied illustrative cases. It is not an official New York State test.
20 questions · 60 points · suggested 90 minutes
Student instructions
- Work independently before opening the separate answer key.
- Use only the supplied reference relationships and a basic calculator; show methods and units.
- Explain observations, mechanisms and model assumptions in sentences or labeled drawings.
- Each question has three criterion marks; equivalent supported explanations are accepted.
- Written responses remain awaiting human review until the parent or teacher assigns valid marks.
Permitted reference relationships
| Quantity | Relationship / condition |
|---|---|
| Density | mass / volume |
| Average speed; velocity | distance / time; displacement / time |
| Acceleration | net force / mass |
| Weight | mass × supplied gravitational field |
| Pressure; force | force / area; pressure × area |
| Aligned-force work | constant force × displacement |
| Kinetic energy | ½ × mass × speed² |
| Power | energy transferred / elapsed time |
| Efficiency | useful output / input × 100% |
| Wave relation | speed = frequency × wavelength |
| Frequency | complete cycles / time |
| Simple resistor current | voltage / resistance |
| Mean; range | sum / count; maximum − minimum |
Coverage and marks
| Chapter | Questions | Points |
|---|---|---|
| Properties of Matter | 1, 2, 3, 4, 5 | 15 |
| Phases of Matter | 6, 7, 8, 9, 10 | 15 |
| Chemistry | 11, 12, 13, 14, 15 | 15 |
| Forces | 16, 17, 18, 19, 20 | 15 |
Parent / teacher marking guide
- Assign each criterion independently; accept equivalent valid reasoning and accessible representations.
- A model response is not the only acceptable wording. Check the supplied conditions and units.
- Use method marks for a correct approach and avoid penalizing the same arithmetic slip repeatedly when follow-through is sound.
- Discuss unsupported generalizations, then assign a changed case to verify correction.
Question 1 · 3 points
Two neutral atoms each have 6 protons but have 6 and 8 neutrons. Give their mass numbers and electron counts; explain why they are isotopes.
Worked answer
Mass numbers are 12 and 14; each neutral atom has 6 electrons. They are isotopes because proton count is unchanged while neutron count differs.
Use proton count 6 for shared identity.
Calculate mass numbers 12 and 14.
Use neutrality for both 6-electron counts and explain the isotope distinction.
Marking criteria
- 1 point: Use proton count 6 for shared identity.
- 1 point: Calculate mass numbers 12 and 14.
- 1 point: Use neutrality for both 6-electron counts and explain the isotope distinction.
Question 2 · 3 points
Cutting a metal sheet changes its shape; a supplied reaction changes that metal into a different compound. Compare classification and evidence, without using “looks different” as your only reason.
Worked answer
Cutting retains substance identity and is physical. The supplied new compound establishes a chemical change. Appearance alone is insufficient to decide identity.
Identify retained identity in cutting.
Identify the different substance in the reaction.
Distinguish observation from sufficient composition evidence.
Marking criteria
- 1 point: Identify retained identity in cutting.
- 1 point: Identify the different substance in the reaction.
- 1 point: Distinguish observation from sufficient composition evidence.
Question 3 · 3 points
A sample of mass 54 g occupies 20 cm³. A matched material sample has twice both values. Find both densities and explain why the larger sample need not sink more merely because it is heavier.
Worked answer
Both have density 2.7 g/cm³. Doubling mass and volume leaves their ratio unchanged; sinking or floating needs comparison with fluid and other relevant support conditions.
Calculate 54/20=2.7 g/cm³.
Calculate 108/40=2.7 g/cm³.
Explain the ratio and the fluid/support conditions needed.
Marking criteria
- 1 point: Calculate 54/20=2.7 g/cm³.
- 1 point: Calculate 108/40=2.7 g/cm³.
- 1 point: Explain the ratio and the fluid/support conditions needed.
Question 4 · 3 points
An element has 10 protons and another 11. State atomic-number order and explain whether electron gain would move either to the next element. Include a limit of a simplified electron-shell picture.
Worked answer
10 precedes 11. Electron gain changes charge, not proton-defined element identity. Shell diagrams simplify electron structure and are not literal planet-like tracks.
Order by proton count.
Distinguish electron gain from changed identity.
State a shell-model limit.
Marking criteria
- 1 point: Order by proton count.
- 1 point: Distinguish electron gain from changed identity.
- 1 point: State a shell-model limit.
Question 5 · 3 points
Material P survives 5 model uses and Q 30, but Q has an unknown leak rate. Explain why longer life alone does not settle a wet-use container choice and name two missing measurements.
Worked answer
Q has longer tested lifetime, but leak resistance remains unmeasured and can be a hard criterion. Compare matched leak tests plus cost, mass or relevant production/disposal information for the stated purpose.
Use the tested reuse comparison.
Identify unmeasured water resistance as relevant.
Specify two evidence needs tied to a purpose rather than a universal winner.
Marking criteria
- 1 point: Use the tested reuse comparison.
- 1 point: Identify unmeasured water resistance as relevant.
- 1 point: Specify two evidence needs tied to a purpose rather than a universal winner.
Question 6 · 3 points
A 60 g sealed pure-water sample freezes from liquid to ice. Explain mass, identity and particle arrangement, and predict the effect of later melting on the classification.
Worked answer
Mass remains 60 g and identity remains H₂O. The arrangement and allowed motion change; ice particles still vibrate. Later melting is another physical state change, not atom destruction.
Account for retained mass and identity.
Describe changed arrangement while retaining particle motion.
Classify later melting by identity.
Marking criteria
- 1 point: Account for retained mass and identity.
- 1 point: Describe changed arrangement while retaining particle motion.
- 1 point: Classify later melting by identity.
Question 7 · 3 points
Two same-phase water samples at 20 °C have different masses. A student says they must contain identical total internal energy. Correct the claim and explain the distinction between heat and temperature.
Worked answer
Equal temperature does not determine equal total internal energy because amount and interactions matter. Heat is energy transferred because of temperature difference; temperature is not the amount transferred.
Distinguish equal temperature from equal amount.
Connect amount with internal energy.
Define heat as transfer rather than stored material.
Marking criteria
- 1 point: Distinguish equal temperature from equal amount.
- 1 point: Connect amount with internal energy.
- 1 point: Define heat as transfer rather than stored material.
Question 8 · 3 points
A fixed gas at matched temperature is modeled at 110 kPa in 40 mL and 220 kPa in 20 mL. Check the relation and explain what changes between particles.
Worked answer
Products are both 4,400 kPa·mL. Average spacing decreases as volume halves; individual particle sizes remain unchanged. The result assumes fixed amount and temperature.
Calculate both products.
Explain spacing and wall-impact mechanism.
State fixed-amount and temperature assumptions.
Marking criteria
- 1 point: Calculate both products.
- 1 point: Explain spacing and wall-impact mechanism.
- 1 point: State fixed-amount and temperature assumptions.
Question 9 · 3 points
Supplied air has dew point 10 °C. Cup surfaces are 6 and 16 °C. Predict the condensation comparison and explain why a puddle can evaporate in the same room without boiling.
Worked answer
The 6 °C surface is below the supplied dew point and more likely to collect condensation; the 16 °C surface is above it. Surface evaporation can occur below boiling, transferring liquid water into air as vapor.
Compare each surface with the 10 °C dew point.
Explain condensation source from existing air water.
Explain evaporation below boiling and account for water.
Marking criteria
- 1 point: Compare each surface with the 10 °C dew point.
- 1 point: Explain condensation source from existing air water.
- 1 point: Explain evaporation below boiling and account for water.
Question 10 · 3 points
A modeled heater supplies 60 J/min while a pure-water melting plateau lasts 6 min. Find the input, explain the flat temperature and name a condition that would change the real graph.
Worked answer
Input is 360 J. Energy can change phase fraction and particle interactions while temperature is nearly steady. Impurities, pressure, heat losses or measuring conditions can change the graph.
Calculate 60×6=360 J.
Explain phase-change energy without stopped input.
State a relevant purity, pressure or apparatus limit.
Marking criteria
- 1 point: Calculate 60×6=360 J.
- 1 point: Explain phase-change energy without stopped input.
- 1 point: State a relevant purity, pressure or apparatus limit.
Question 11 · 3 points
A student observes cloudiness after mixing and calls it proof of a reaction. Explain a supported next step and why a visible clue alone is not enough.
Worked answer
Record the cloudiness, then compare composition or characteristic properties to determine whether a different substance formed. Suspended pre-existing material can also appear cloudy; the observation alone does not establish its cause.
Separate observed cloudiness from reaction inference.
Name evidence relevant to new identity.
Identify an alternative or appropriate limitation.
Marking criteria
- 1 point: Separate observed cloudiness from reaction inference.
- 1 point: Name evidence relevant to new identity.
- 1 point: Identify an alternative or appropriate limitation.
Question 12 · 3 points
An open apparatus falls from 83.0 to 80.5 g while a gas leaves. Account for the change and explain what a sealed comparison would need to weigh.
Worked answer
The measured decrease is 2.5 g, which can be accounted for as escaped matter under the supplied conditions. Weigh the entire sealed apparatus and contents, not only the remaining liquid; rearrangement does not destroy atoms.
Calculate the 2.5 g difference.
Place escaped gas outside the measured open boundary.
Specify complete sealed measurement and conserved atoms.
Marking criteria
- 1 point: Calculate the 2.5 g difference.
- 1 point: Place escaped gas outside the measured open boundary.
- 1 point: Specify complete sealed measurement and conserved atoms.
Question 13 · 3 points
Balance C + O₂ → CO₂ and compare it with a proposed product CO. Explain why balancing cannot silently replace one product formula with another.
Worked answer
The given CO₂ reaction is already balanced with one carbon and two oxygen atoms on each side. CO is a different composition, not a balancing correction. Coefficients, not intended formulas, are adjusted.
Count carbon on both sides.
Count oxygen on both sides.
Distinguish a formula change from coefficient balancing.
Marking criteria
- 1 point: Count carbon on both sides.
- 1 point: Count oxygen on both sides.
- 1 point: Distinguish a formula change from coefficient balancing.
Question 14 · 3 points
A simplified reaction requires 95 units for breaking bonds and releases 65 from forming bonds. Explain the net energy direction, classification and why bond breaking is not the release step.
Worked answer
Net 30 units must be supplied, making the process endothermic under the model. Breaking requires energy; formation releases it, but the released amount here is smaller.
Compare 95 required with 65 released.
Calculate the net 30-unit input.
Explain separate contributions and endothermic net result.
Marking criteria
- 1 point: Compare 95 required with 65 released.
- 1 point: Calculate the net 30-unit input.
- 1 point: Explain separate contributions and endothermic net result.
Question 15 · 3 points
Near-room-temperature dilute aqueous samples have pH 2 and 9. Classify them and explain why neither taste nor pH alone is an appropriate way to identify an unknown household chemical.
Worked answer
pH 2 is acidic and 9 basic under the stated reference conditions. Never taste unknowns. Use suitable approved indicators or supplied records; pH does not uniquely identify chemical composition or safety.
Apply the stated pH classifications.
Reject taste and select a safe evidence route.
Separate pH character from identity and safety.
Marking criteria
- 1 point: Apply the stated pH classifications.
- 1 point: Reject taste and select a safe evidence route.
- 1 point: Separate pH character from identity and safety.
Question 16 · 3 points
A cart has horizontal forces 12 N left and 12 N right and is already moving steadily. Explain net force and whether “it must stop” follows.
Worked answer
Net force is zero. Under the stated simple model its velocity can remain constant and nonzero; stopping requires a change in motion caused by a net force or another changed condition.
Combine opposing forces to zero.
Relate zero net force to no acceleration.
Distinguish constant velocity from rest.
Marking criteria
- 1 point: Combine opposing forces to zero.
- 1 point: Relate zero net force to no acceleration.
- 1 point: Distinguish constant velocity from rest.
Question 17 · 3 points
A book rests on a table. Its weight and support force are equal. Explain why these are not a third-law pair and name the receivers of their actual partners.
Worked answer
Weight and support both act on the book and can balance. Their interaction partners act on Earth and on the table respectively. A third-law pair acts on separate objects in one interaction.
Identify both supplied forces as acting on the book.
Name Earth and table as partner-force receivers.
Distinguish balance on one object from an interaction pair.
Marking criteria
- 1 point: Identify both supplied forces as acting on the book.
- 1 point: Name Earth and table as partner-force receivers.
- 1 point: Distinguish balance on one object from an interaction pair.
Question 18 · 3 points
A 4 kg model object is in a 9 N/kg gravitational field. Calculate weight and explain how a friction comparison could isolate surface type.
Worked answer
Weight is 36 N. Match object load, path, starting/steady-motion procedure and measuring method, changing the intended surface; repeated results are needed. Mass and gravitational force are different quantities.
Calculate 4×9=36 N.
State the mass–weight distinction.
Specify a matched surface comparison with repeated measurements.
Marking criteria
- 1 point: Calculate 4×9=36 N.
- 1 point: State the mass–weight distinction.
- 1 point: Specify a matched surface comparison with repeated measurements.
Question 19 · 3 points
A clip moves near a magnet across a paper gap. The same clip does not move farther away. Explain what this supports and which other factor could affect the motion threshold.
Worked answer
The supplied behavior supports an interaction without direct contact and a distance-dependent response in this setup. Friction, placement or orientation can affect movement; counts alone do not measure force or prove a universal field equation.
Identify across-gap interaction evidence.
Describe the bounded distance comparison.
Name a threshold/control factor and avoid a universal force claim.
Marking criteria
- 1 point: Identify across-gap interaction evidence.
- 1 point: Describe the bounded distance comparison.
- 1 point: Name a threshold/control factor and avoid a universal force claim.
Question 20 · 3 points
An ideal fluid transmits 1,500 Pa to areas 0.02 and 0.08 m². Find forces and explain why the larger force does not generate free work.
Worked answer
Forces are 30 and 120 N. The larger-area piston has one-quarter the displacement for matching displaced volume, keeping ideal work equal. Real losses require an expanded energy account.
Use pressure×area for both forces.
Calculate 30 and 120 N.
Connect equal displaced volume to shorter output distance and conserved ideal work.
Marking criteria
- 1 point: Use pressure×area for both forces.
- 1 point: Calculate 30 and 120 N.
- 1 point: Connect equal displaced volume to shorter output distance and conserved ideal work.