Justify Chemical Design Decisions
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Chemistry › Justify Chemical Design Decisions
A lab must store 6 M hydrochloric acid (HCl) for student experiments. The storage container must not react with the acid, must not contaminate it, and must remain safe if dropped. The lab chose a HDPE (high-density polyethylene) bottle instead of a glass bottle. Which reasoning best justifies this design decision using chemical properties and safety trade-offs?
Glass reacts strongly with hydrochloric acid and would quickly dissolve, so HDPE is the only material that can hold HCl.
HDPE is more transparent than glass, so students can see the acid level better, which prevents chemical reactions from happening too quickly.
HDPE was chosen because it is a conductor, and conducting materials prevent acids from corroding containers.
HDPE is chemically resistant to strong acids like HCl and does not leach metal ions into the acid; it is also less likely to shatter if dropped, improving safety even though glass can be more heat-resistant.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (HDPE bottle was chosen for 6M HCl storage), (2) EVIDENCE: Cite specific chemical properties of the chosen material (chemically resistant to strong acids, does not leach metal ions) and safety characteristics (less likely to shatter if dropped), (3) REASONING: Explain WHY each property matters for the application (acid resistance prevents container degradation and contamination; shatter resistance improves student safety in lab setting). Strong justifications also acknowledge trade-offs: "even though glass can be more heat-resistant" shows you've weighed alternatives and can defend your choice! The complete justification for HDPE balances chemical compatibility (resists HCl attack) with physical safety (won't create sharp fragments if dropped), recognizing that student labs prioritize safety over maximum heat resistance. Choice C provides complete justification by citing relevant chemical properties (acid resistance, no metal leaching), connecting them to application requirements (store HCl safely) with sound reasoning, and acknowledging the heat resistance trade-off while prioritizing safety. Choice A incorrectly claims HDPE is more transparent than glass; Choice B contains a major error (glass doesn't react with HCl); Choice D incorrectly states HDPE conducts electricity. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: must resist 6M HCl (strong acid), no contamination of stored acid, student safety if dropped. (2) List the material's relevant chemical properties: HDPE has excellent acid resistance, contains no metals to leach, impact-resistant polymer. (3) Match each requirement to a property: "Application needs HCl resistance → HDPE resists strong acids → HDPE suitable!" The safety factor (shatter resistance) provides additional justification for educational settings!
A reusable water bottle company chose a BPA-free copolyester plastic instead of polycarbonate after customer concerns about chemical leaching. In an internal test, bottles were filled with water and held at $60^\circ$C for 24 hours. The polycarbonate bottles showed measurable BPA in the water, while the copolyester bottles showed BPA below the detection limit. Which reasoning best justifies the material choice?
Copolyester was chosen because it can be colored more easily, and color additives prevent chemical leaching.
Polycarbonate was avoided because it is too light, and lighter materials always leach more chemicals than heavier materials.
Copolyester was chosen because it is BPA-free and more chemically stable against releasing BPA-related components into warm water; the leaching test data support improved consumer safety even if the copolyester costs slightly more.
Copolyester was chosen because all plastics are chemically identical, so any difference in leaching must be due to measurement error.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this water bottle material choice, a complete justification would claim copolyester was selected over polycarbonate due to its BPA-free stability, evidenced by no detectable leaching in warm water tests versus measurable BPA in polycarbonate, with reasoning that this enhances safety by preventing chemical release, acknowledging slight cost increase but prioritizing health. Choice B provides complete justification by citing relevant chemical properties like stability against leaching, connecting them to application requirements such as safety with sound reasoning, and using evidence like test data appropriately. Choice A fails by claiming all plastics are identical, ignoring specific chemical differences in leaching, so always highlight unique properties backed by evidence. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if water, need low leaching). What temperatures? (if 60°C, need stability). What safety needs? (if consumer use, need non-toxicity). What durability needed? (if reusable, need no degradation). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs low leaching (requirement) → Material is stable (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A wastewater facility needed a new tank liner for storing acidic waste (pH about 2) that also contains chloride ions. The team considered aluminum sheet, carbon steel, and fiberglass-reinforced plastic (FRP) with a vinyl ester resin. They selected FRP. After 2 years, the FRP liner showed no leaks and no visible corrosion, while a nearby carbon-steel bracket exposed to splashes had severe rust. Which option best justifies selecting FRP over the metals using chemical reasoning and evidence?
FRP was chosen because vinyl ester resins are chemically resistant to strong acids and chloride-containing solutions, so the liner does not undergo the oxidation/corrosion that metals do; the lack of leaks or visible degradation after 2 years supports the choice despite potentially higher repair complexity.
FRP was chosen because plastics are always environmentally harmless, so chemical compatibility does not need to be considered.
FRP was chosen because it is a better electrical conductor than metals, which prevents acid from reacting with the tank.
Aluminum should have been selected because it reacts with acid, and reacting quickly removes the acid from the tank.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this wastewater tank liner, a complete justification would claim FRP was selected over metals due to its resistance to acids and chlorides, evidenced by no leaks or degradation after 2 years versus rust in carbon steel, with reasoning that this avoids oxidation in low-pH environments, acknowledging repair complexity but favoring reliability. Choice B provides complete justification by citing relevant chemical properties like acid and chloride resistance, connecting them to application requirements such as corrosion prevention with sound reasoning, and using evidence like long-term observations appropriately. Choice A fails by claiming plastics are always harmless without addressing specific compatibility, so evaluate based on targeted chemical resistances. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if acids and chlorides, need resistance). What temperatures? (if ambient, need stability). What safety needs? (if waste storage, need no leaks). What durability needed? (if long-term, need no corrosion). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs acid resistance (requirement) → Material resists acids (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A school chemistry lab replaced glass reagent bottles with high-density polyethylene (HDPE) bottles for storing 1 M hydrochloric acid (HCl). The lab noted that over a semester, some glass stoppers became stuck and a few bottles showed etched markings near the neck, while the HDPE bottles did not show visible damage or leaks. Which reasoning best justifies switching to HDPE for this use?
HDPE was chosen because it is transparent like glass, allowing students to see the liquid level more easily, which prevents acid reactions.
HDPE was chosen because acids always dissolve glass quickly, so glass can never be used for any chemical storage.
HDPE was chosen because it is chemically resistant to dilute acids like HCl and does not react or corrode, reducing sticking/etching issues seen with some glass parts; the lack of leaks or visible damage over a semester supports the substitution despite HDPE being less heat-resistant than glass.
HDPE was chosen because it is softer than glass, so it cannot crack if dropped, and chemical compatibility is not important for storage containers.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this lab reagent bottle switch, a complete justification would claim HDPE was selected over glass due to its chemical resistance to dilute acids, evidenced by no damage or leaks over a semester versus etching and sticking in glass, with reasoning that this prevents reactions with HCl, acknowledging lower heat resistance but prioritizing safety in school settings. Choice B provides complete justification by citing relevant chemical properties like acid resistance, connecting them to application requirements such as non-reactivity with sound reasoning, and using evidence like visible damage comparisons appropriately. Choice A fails by prioritizing softness over chemical compatibility, which ignores the key issue of acid etching, so always link to specific chemical interactions. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if acids, need acid resistance). What temperatures? (if room temp, need stability). What safety needs? (if lab use, need non-toxicity). What durability needed? (if repeated use, need no etching). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs acid resistance (requirement) → Material is acid-resistant (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
An outdoor coastal lighting fixture needed a fastener material that would last with frequent salt spray. The design team compared (1) plain carbon-steel screws, (2) zinc-plated steel screws, and (3) 316 stainless-steel screws. In a 30-day salt-spray test, carbon steel showed heavy rust by day 5, zinc-plated steel showed white corrosion products and some rust by day 20, and 316 stainless showed no visible rust. The team selected 316 stainless despite the highest cost. Which justification best supports the decision?
316 stainless was selected because its corrosion resistance in chloride-rich environments prevents oxidation and metal loss; the salt-spray results show it maintained integrity far better than carbon steel or zinc-plated steel, justifying the added cost for reliability.
316 stainless was selected because it is the most expensive option, and higher price always means higher quality.
316 stainless was selected because it has a higher melting point than zinc, which is the main chemical issue in salt spray.
Zinc-plated steel should have been selected because the white corrosion products prove it is reacting more, which means it is stronger than stainless steel.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this coastal fastener selection, a complete justification would claim 316 stainless was selected due to its corrosion resistance in chlorides, evidenced by no rust in 30-day salt-spray tests versus failures in others, with reasoning that this prevents oxidation in salty environments, acknowledging higher cost but valuing long-term integrity. Choice B provides complete justification by citing relevant chemical properties like chloride resistance, connecting them to application requirements such as salt spray durability with sound reasoning, and using evidence like test results appropriately. Choice A fails by assuming price equals quality without chemical evidence, so base decisions on properties and tests, not cost alone. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if salts, need chloride resistance). What temperatures? (if outdoor, need stability). What safety needs? (if structural, need no weakening). What durability needed? (if coastal, need oxidation resistance). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs chloride resistance (requirement) → Material resists chlorides (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A hospital considered two materials for reusable surgical instrument trays: anodized aluminum and stainless steel. The trays must withstand repeated cleaning with hot, oxidizing disinfectants and exposure to saline (salt) from biological fluids. After a pilot program, aluminum trays showed pitting and dark spots after 4 months, while stainless steel trays remained unchanged. The hospital chose stainless steel despite higher upfront cost. Which reasoning best justifies the choice?
Aluminum should have been chosen because pitting increases surface area, which makes cleaning chemicals work better.
Stainless steel was chosen because it is magnetic, and magnetic materials are always more sanitary.
Material choice does not matter because disinfectants kill microbes regardless of what the tray is made from.
Stainless steel was chosen because its corrosion resistance (protective oxide layer) helps it resist oxidation and chloride-related pitting during repeated exposure to disinfectants and saline; the pilot evidence of aluminum pitting but stainless stability supports the decision for safety and longevity.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this surgical tray material, a complete justification would claim stainless steel was selected over aluminum due to its corrosion resistance via oxide layer, evidenced by no change after 4 months versus pitting in aluminum, with reasoning that this withstands disinfectants and saline, acknowledging higher cost but ensuring sanitation. Choice B provides complete justification by citing relevant chemical properties like oxide layer resistance, connecting them to application requirements such as pitting prevention with sound reasoning, and using evidence like pilot results appropriately. Choice A fails by linking magnetism to sanitation without chemical basis, so focus on properties like oxidation resistance for cleaning exposures. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if disinfectants and saline, need resistance). What temperatures? (if hot cleaning, need stability). What safety needs? (if medical, need sanitation). What durability needed? (if reusable, need no pitting). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs pitting resistance (requirement) → Material has protective oxide (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A manufacturer redesigned a household drain cleaner bottle. The original cap was made of aluminum, but some customers reported the cap becoming difficult to open after repeated use. The product contains sodium hydroxide (a strong base). The new design uses a polypropylene (PP) cap. In a 3-month shelf test, aluminum caps showed surface dulling and occasional sticking, while PP caps showed no visible change. Which justification best supports the switch to PP?
PP was chosen because it feels smoother to the touch, and smoother materials do not react with chemicals.
Aluminum was replaced because all metals dissolve instantly in any cleaner, so no metal can ever be used in packaging.
PP was chosen because it has a higher density than aluminum, which prevents evaporation of the drain cleaner.
PP was chosen because it is chemically resistant to strong bases like sodium hydroxide and does not corrode or form reaction products that could seize threads; the shelf-test observations of aluminum dulling/sticking versus unchanged PP support the decision.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this drain cleaner bottle cap redesign, a complete justification would claim PP was selected over aluminum due to its resistance to strong bases, evidenced by no change in 3-month tests versus dulling and sticking in aluminum, with reasoning that this prevents corrosion and seizing from NaOH reactions. Choice B provides complete justification by citing relevant chemical properties like base resistance, connecting them to application requirements such as non-reactivity with sound reasoning, and using evidence like shelf-test observations appropriately. Choice A fails by focusing on smoothness, which doesn't address chemical reactions with bases, so tie justifications to specific chemical interactions. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if bases, need base resistance). What temperatures? (if room temp, need stability). What safety needs? (if consumer, need no toxicity). What durability needed? (if repeated use, need no sticking). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs base resistance (requirement) → Material resists bases (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A city replaced old cast-iron pipes in a section of its drinking-water system with copper pipes. The decision was made after routine water tests found elevated iron levels and reddish-brown water complaints, especially after periods of low flow. Which reasoning best justifies choosing copper over cast iron for this application using chemical properties and evidence?
Copper was chosen because it is heavier than iron, so it will stay in place better and prevent leaks caused by vibration.
Copper was chosen because any metal pipe will eventually rust, so the specific metal does not matter as long as pipes are replaced regularly.
Copper was chosen mainly because it is easier to paint and match to other plumbing fixtures, which improves the appearance of the system.
Copper was chosen because it is less chemically reactive with water and dissolved oxygen than iron, so it corrodes less and releases fewer metal ions; the reduced discoloration and lower measured iron after replacement support the decision despite higher material cost.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this drinking-water pipe replacement, a complete justification would claim copper was selected over cast iron due to its lower reactivity with water and oxygen, evidenced by reduced iron levels and discoloration in tests post-replacement, with reasoning that this prevents corrosion and metal ion release into water, acknowledging higher cost but prioritizing water quality. Choice B provides complete justification by citing relevant chemical properties like lower reactivity, connecting them to application requirements such as corrosion resistance with sound reasoning, and using evidence like reduced discoloration appropriately. Choice A fails by focusing on aesthetic properties like paintability, which aren't chemically relevant to corrosion issues, so remember to prioritize functional chemical matches over superficial traits. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if water and oxygen, need low reactivity). What temperatures? (if ambient, need stability). What safety needs? (if drinking water, need low ion release). What durability needed? (if long-term, need corrosion resistance). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs corrosion resistance (requirement) → Material has low reactivity (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A food-processing plant needed new piping to carry a warm ($45^\circ$C) cleaning solution containing dilute sodium hydroxide (a base). Engineers chose stainless steel (304) instead of plain carbon steel. After 18 months, inspections showed no pitting or rust in the stainless steel lines, while a short carbon-steel test section developed rust and rough interior surfaces within 6 weeks. Which justification best supports the stainless-steel choice?
Stainless steel was chosen because it forms a thin, protective chromium oxide layer that resists chemical corrosion in alkaline cleaning solutions; the lack of pitting after 18 months compared with rapid rusting of carbon steel supports the decision even though stainless steel costs more.
Stainless steel was chosen because it conducts electricity better than carbon steel, which is important for moving cleaning solutions through pipes.
Stainless steel is shinier than carbon steel, which makes it easier to see inside the pipe and therefore prevents corrosion.
Stainless steel was chosen because it is always the strongest metal, and strength is the only requirement for chemical piping.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this food-processing piping, a complete justification would claim stainless steel was selected over carbon steel due to its protective chromium oxide layer, evidenced by no pitting after 18 months versus rapid rusting in carbon steel, with reasoning that this resists alkaline corrosion in cleaning solutions, acknowledging higher cost but emphasizing durability in food environments. Choice B provides complete justification by citing relevant chemical properties like the oxide layer, connecting them to application requirements such as corrosion resistance with sound reasoning, and using evidence like inspection results appropriately. Choice A fails by emphasizing shininess, which is irrelevant to chemical corrosion prevention, so focus on functional properties like oxide formation instead of visual traits. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if bases, need alkali resistance). What temperatures? (if 45°C, need thermal stability). What safety needs? (if food contact, need non-toxicity). What durability needed? (if long-term, need pitting resistance). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs alkali resistance (requirement) → Material has protective oxide (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!
A company producing compostable takeout containers chose polylactic acid (PLA) over conventional polyethylene (PE) for a pilot program to reduce long-term plastic waste. In an industrial composting test (high humidity and elevated temperature), PLA containers lost most of their mass within 12 weeks, while PE containers showed no visible change. However, PLA containers softened if hot soup sat in them for 30 minutes. Which option best justifies the decision to use PLA for the pilot, acknowledging trade-offs using chemical properties and evidence?
PLA was chosen because it is always stronger than PE, so it will work better for hot soup without any changes.
PLA was chosen because it biodegrades under industrial composting conditions (chemical breakdown of polymer chains), and the mass-loss test supports the environmental goal; although PLA can soften with hot liquids, the pilot can limit use to cooler foods or add a heat-resistant coating.
PLA was chosen only because it is clear and looks more appealing, and appearance is the main requirement for compostable packaging.
PE should be chosen because it does not change in composting tests, which proves it is chemically safer for the environment.
Explanation
This question tests your ability to justify engineering design decisions by constructing evidence-based arguments that connect chemical properties to application requirements and explain why chosen solutions are appropriate. A complete design justification uses the Claim-Evidence-Reasoning (CER) framework applied to engineering: (1) CLAIM: State the design decision clearly (Material X was chosen for Application Y), (2) EVIDENCE: Cite specific chemical properties of the chosen material (acid-resistant, high melting point, chemically inert, non-toxic, etc.) and any test results or performance data, (3) REASONING: Explain WHY each property matters for the application (acid resistance needed because application exposes material to pH 2 solutions; high melting point needed because operating temperature reaches 150°C). Strong justifications also acknowledge trade-offs: "Though expensive, the chemical inertness is critical for food safety, justifying the cost." This shows you've weighed alternatives and can defend your choice! For this compostable container pilot, a complete justification would claim PLA was selected over PE due to its biodegradability via polymer chain breakdown, evidenced by mass loss in composting tests versus no change in PE, with reasoning that this reduces waste, acknowledging heat softening but suggesting limits or coatings for hot foods. Choice A provides complete justification by citing relevant chemical properties like biodegradability, connecting them to application requirements such as environmental impact with sound reasoning, and using evidence like composting results while acknowledging trade-offs appropriately. Choice B fails by incorrectly claiming PLA is always stronger without evidence, so verify properties and include honest trade-offs for balanced decisions. Building design justifications—the property-requirement matching approach: (1) List the application's chemical requirements: What chemicals will material contact? (if food, need safety). What temperatures? (if variable, need stability or adaptations). What safety needs? (if compostable, need biodegradability). What durability needed? (if takeout, need short-term integrity). (2) List the material's relevant chemical properties: reactivity (inert vs reactive), corrosion resistance (to specific chemicals), thermal stability (temperature range), toxicity (safe vs hazardous), chemical stability (degrades or stable), pH tolerance. (3) Match each requirement to a property: "Application needs biodegradability (requirement) → Material biodegrades (property) → Material is suitable for this requirement (match!)." Do this for each major requirement. (4) Acknowledge any weaknesses or trade-offs: "Material lacks property X but that's acceptable because X is low priority for this application." The matching table makes justification systematic! Comparing justification quality: WEAK justification: "Material X chosen because it's good" or "We picked X because it works" (vague, no properties, no reasoning). MODERATE justification: "Material X chosen because it's corrosion-resistant" (one property mentioned but doesn't explain why that matters or connect to requirements). STRONG justification: "Material X chosen for chemical storage tank because (1) it resists corrosion from both acids and bases (requirement: tank holds various chemicals), (2) it's non-toxic if leakage occurs (requirement: environmental safety), (3) it maintains properties from -10°C to 50°C (requirement: outdoor installation). Though more expensive than alternatives, the superior chemical resistance justifies cost for 20-year service life." This cites multiple properties, connects each to specific needs, acknowledges trade-off, uses evidence (service life). Aim for strong!