| Date | November 2017 | Marks available | 1 | Reference code | 17N.3.sl.TZ0.3 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Estimate | Question number | 3 | Adapted from | N/A |
Question
Sodium chloride, NaCl, can be spread on icy roads to lower the freezing point of water.
The diagram shows the effects of temperature and percentage by mass of NaCl on the composition of a mixture of NaCl and H2O.
Estimate the lowest freezing point of water that can be reached by adding sodium chloride.
Estimate the percentage by mass of NaCl dissolved in a saturated sodium chloride solution at +10 ºC.
Calculate the percentage of water by mass in the NaCl•2H2O crystals. Use the data from section 6 of the data booklet and give your answer to two decimal places.
Suggest a concern about spreading sodium chloride on roads.
Markscheme
–21 «ºC»
28 «%»
Accept any specific answer in the range 27 to 29 «%».
Mr = 94.48
«\(2\frac{{\left( {1.01 \times 2 + 16.00} \right)}}{{94.48}} \times 100 = \)» 38.15 «%»
Award M2 only if answer is to 2 decimal places.
Award [2] for correct final answer.
Award [1 max] for 38.10 %.
rust/corrosion «of cars and bridges»
OR
waste of important raw material
OR
soil/water salination/pollution «from run off»
OR
erosion of/damage to the road surface
OR
specific example of damage to the ecosystem
OR
«outdoor» temperatures may go below effective levels for NaCl «to lower freezing point» so NaCl could be wasted
OR
roads can refreeze causing hazards
Do not accept “tyre damage”.
Do not accept “economic issues” OR “environmental issues” unless specified (eg accept “increase in costs for local councils road budgets” but not “cost” alone).
Do not accept “makes roads more slippery”.
