Chemistry Question Review with Marking Scheme(2025-11-25)

For this question, 50% of marks will be awarded for meaningful participation.

Question: Mary wants to buy a piece of optical eyeglasses, where those of lens made of plastics are sold at HKD $1000 on average and those of lens made of Na2SiO3 are sold at HKD $1500 on average. Which one is more economical? Explain briefly.

Marking Scheme:

This question has 4 marks, the breakdown is as below:

1P: Participation by writing something in prose)

1P: Participation by attempting to deploy expertise in chemistry)

1M: Method by using correct chemical knowldge to make value judgment

1A: Accuracy by choosing the correct option

Suggested answers:

The optical eyeglasses of lens made of Ni2SiO3 is more economical although it is apparently more expensive if only one piece of it is bought. The reason is as below.

Firstly, the optical eyeglasses made of plastic lens will be oxidizied and tinted to yellow for prolonged exposure to air in standard conditions (298K, 1atm). One will have to replace the eyeglasses more frequently.

Secondly, the plastic lens is less likely to be durable because it can deform by heat. For extremely hot weather, the lens will even decompose.

With the above factors considered, the total cost of the choice is as below:

Lens made of Na2SiO3: $1500 x 1

Lens made of plastic: $1000 x n > $1500 where n ≧ 2

So the true economical choice is self-evident.

Annex: Enrichment reading

Further explanation from HKAL Physics:

Think about it: Which one is the safer refrigerant?