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	50%	8
   light weight	30%	5
   low cost	20%	4

2. 1.

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

   6.3 points
3. 2.

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

   5.1 points
4. 3.

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

   6.8 points
5. 4.

   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?

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

6. 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

7. 5.

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

   • More sulfur makes the rubber stretchier and stretchier, from 0% up to 30% sulfur
   • 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
8. 6.

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

   • Epoxy's chains are shorter than polyethylene's, so heat cannot move them past one another
   • Polyethylene's bonds are stronger than epoxy's, so it flows like a liquid when it is heated
   • Epoxy's chains are bonded into one network; polyethylene's are held by weak attractions that heat loosens
9. 7.

   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
10. 8.

    Alloying copper with zinc lowers its electrical conductivity. Which structural idea explains it, Pia?

    • Zinc atoms of another size disturb the regular rows and scatter the moving electrons
    • Zinc atoms are heavier, which slows each electron by the same amount as it passes
    • Zinc atoms hold on to their electrons, so no electrons are left free to move through the 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. 9.

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

    • Titanium alloy, since it is both strong and light
    • Steel: 6.8, ahead of aluminum alloy's 6.6
    • Aluminum alloy, still the strongest choice overall
13. 10.

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

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