The problems are on the synthetic materials worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Synthetic materials are substances made by people through chemical reactions, usually from natural resources such as crude oil. These worked answers correct the 20 problems of the grade 8 science worksheet on synthetic materials, from natural or synthetic? to choosing a bottle plastic: each solution shows the formula, the substitution and the final answer with its unit.
Each answer gives the reasoning before the result, which is in bold. For evaluation problems accept any well-supported opinion that uses the evidence in the problem.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-3
Problem 1 Natural or synthetic?
1. Natural: cotton, wool, latex. Synthetic: nylon, polyester, PVC.2. The usual starting resource is crude oil.
Problem 2 Put the stages in order
1. The order is b, d, a, e, c: crude oil, refinery separation, polymerisation, polythene, bag.2. The stage is (a) polymerisation.
Problem 3 Key words
1. A monomer is a small molecule that can join with others.2. A polymer is a very long molecule made of many monomers joined in a chain.
3. The reaction is polymerisation.
Problem 4 Property to use
1. (a) waterproof and flexible; (b) does not melt when heated; (c) strong and does not rot.2. The pan handle, job (b), needs a thermosetting plastic.
Problem 5 Pros and cons of bottles
1. Any two of: light, unbreakable, cheap, waterproof.2. Any two of: they last for centuries, they litter land and sea, they break into microplastics, they use oil to make.
Problem 6 Float or sink figure
1. Chips A and B float, so their density is less than 1.00 g/cm³.2. Chips C and D sink, so their density is more than 1.00 g/cm³.
Problem 7 Density of a chip
1. ρ = 23 g ÷ 20 cm³ = 1.15 g/cm³.2. 1.15 is more than 1.00, so it sinks.
3. It lies between polystyrene (1.05) and PET (1.38), so it is a denser plastic, closer to PS or PET.
Problem 8 Recycling rate
1. 120 ÷ 480 × 100 = 25 %.2. 480 kg − 120 kg = 360 kg.
Problem 9 Mass in a reaction
1. Mass is conserved, so the polythene has a mass of 56 g.2. The atoms are only rearranged: the same carbon and hydrogen atoms are joined into chains. None are created or destroyed.
Problem 10 Waste in a town
1. The largest share is packaging, 40 %.2. 20 % of 800 t = 0.20 × 800 = 160 t.
3. (25 % + 20 %) of 800 t = 0.45 × 800 = 360 t.
Problem 11 Wool or fleece?
1. Any two of: lighter, dries faster, cheaper, easy to wash.2. Wool is a natural material and does not release plastic fibres in the wash, so it is a better choice for the environment.
3. Wool is renewable, because sheep grow a new fleece every year.
Problem 12 Heating plastics
1. Strip X is the thermoplastic.2. Strip Y suits a kettle handle.
3. A handle must stay rigid and not soften when hot, so a thermosetting plastic is safer.
Problem 13 Spot the errors
1. (a) Polythene is a polymer; ethene is the monomer. (b) No atoms disappear, so the masses are equal. (c) Polythene has a density of about 0.95 g/cm³, so it floats at any size.Problem 14 Bag break-even
1. 1.2 kg ÷ 0.04 kg = 30 bags.2. The cloth bag must be used at least 30 times to break even; any further use is a saving.
Problem 15 Counting chain units
1. There are 8 units.2. 8 × 28 = 224 atomic mass units.
3. 1 000 × 28 = 28 000 atomic mass units. Check 28 = 2 × 12 + 4 × 1.
Problem 16 Plant output
1. 7.0 t × 24 = 168 t.2. 168 t = 168 000 000 g. Bags = 168 000 000 ÷ 8.0 = 21 000 000 bags.
Problem 17 Shed fibres
1. 0.4 g × 3 × 52 = 62.4 g.2. 62.4 g × 50 000 = 3 120 000 g = 3.12 t.
3. Wash less often, use a filter bag or choose natural fibres: capture or avoid the fibres.
Problem 18 Separating a mixture of plastics
1. PP and PE float in water.2. PS (1.05) floats in 1.20 g/cm³ solution.
3. PET (1.38) and PVC (1.40) both sink. Their densities differ by only 0.02 g/cm³, so another method is needed.
Problem 19 Bag ban
1. 124 ÷ 200 × 100 = 62 %.2. 31 ÷ 124 × 100 = 25 %.
3. Bags are one quarter of the plastic litter, so a ban helps but does not solve the problem. Other items such as bottles and packaging also need action, and affordable reusable bags must be available. A partial but useful step.
Problem 20 Choosing a bottle plastic
1. Mass = density × volume: P 0.95 × 40 = 38 g; Q 1.38 × 40 = 55.2 g; R 1.05 × 40 = 42 g.2. Choose P: it is the lightest, it softens only at 120 °C so it survives a hot car, and it is flexible, so it does not shatter if dropped. R is brittle and Q softens at 80 °C.
