Why a material's molecules decide what it can do

Answer Key - Grade 10 Science Worksheet

Circle the best answer, or write your answer on the line.

1. Read this first

   A material's scores and weights (example data)

   criterion	weight	score
   strength	60%	3
   light weight	25%	9
   low cost	15%	7

2. 1.

   Using the table, what is the material's weighted score, to 0.01 points, Bo?

   5.1 points
3. 2.

   Using the table, what is the material's weighted score, to 0.01 points, Esme?

   6.8 points
4. 3.

   A milk jug (HDPE) is stiffer than a grocery bag (LDPE). Both are polyethylene with the same C–C bonds. Which difference explains it, Jun?

   • LDPE has longer chains, which tangle and tear apart more easily than HDPE chains do
   • HDPE has stronger bonds inside each chain than LDPE does, so it resists bending more
   • HDPE's straight chains pack closely and hold on along their length; LDPE's branched chains cannot

5. Rubber with added sulfur (typical behavior)

   sulfur added (% by mass)	behavior
   0	soft and sticky, flows when warm
   2	stretches and snaps back
   30	hard and brittle

6. 4.

   Sulfur links rubber chains together. Which statement does the table support, Kira?

   • Sulfur breaks the long chains into short pieces, so the rubber hardens at 30%
   • A few cross-links let chains snap back, while too many lock them into a hard solid
   • More sulfur makes the rubber stretchier and stretchier, from 0% up to 30% sulfur
7. 5.

   Polyethylene softens when heated and can be remelted; a cured epoxy glue cannot. Which structure explains it, Luz?

   • Polyethylene's bonds are stronger than epoxy's, so it flows like a liquid when it is heated
   • Epoxy's chains are shorter than polyethylene's, so heat cannot move them past one another
   • Epoxy's chains are bonded into one network; polyethylene's are held by weak attractions that heat loosens
8. 6.

   Steel is iron with about 1% carbon and is harder than pure iron. Which structural idea explains it, Milo?

   • Carbon atoms replace iron atoms of the same size, so the rows stay perfect and strong
   • Carbon adds free electrons, which hold the iron atoms together more tightly than before
   • Small carbon atoms sit in gaps between iron atoms and block the layers from sliding

9. Two steel samples (example data)

   sample	chromium (%)	after a month in salt water
   A	8	rust spots
   B	18	no rust

10. 7.

    Stainless steel needs at least 10.5% chromium to form a protective oxide film. Which sample resists rust, and why, Noah?

    • Sample B: its 18% chromium is above 10.5%, enough for a continuous oxide film
    • Both: each contains some chromium, which is what stops rust in any kind of steel
    • Sample A: its 8% chromium leaves more iron, which is the stronger metal

11. Bike frame metals (example scores, 1 to 10)

    material	strength	light weight	low cost
    steel	7	3	9
    aluminum alloy	6	8	6
    titanium alloy	9	8	2

12. 8.

    Weights: strength 40%, light weight 35%, low cost 25%. The cost score must be at least 3. Which material is the best choice, Ren?

    • Steel: 6.1, the material with the top cost score, so it is the safest pick
    • Titanium alloy: 6.9, the highest total of the three materials, so it should be picked
    • Aluminum alloy: 6.7, the highest total among those with a cost score of at least 3
13. 9.

    Weights now: strength 20%, light weight 30%, low cost 50%. Which material has the highest total, Tala?

    • Aluminum alloy, still the strongest choice overall
    • Titanium alloy, since it is both strong and light
    • Steel: 6.8, ahead of aluminum alloy's 6.6
14. 10.

    Compare the first weights (40%, 35%, 25%) with the second (20%, 30%, 50%). Which statement is best supported, Zee?

    • Titanium is the strongest material overall, because it led under the first set of weights
    • The scores must be wrong, since correct scores would give the same winner for any weights
    • The best material depends on the weights, so the job's priorities come before scoring