Water that is safe to drink contains dissolved substances - AQA - GCSE Chemistry Combined Science - Question 1 - 2019 - Paper 2

To test for pure water, we can use the method of boiling. When the water is heated to 100°C, it should boil.

Result: If the water is pure, it will boil at 100°C.

1. Measure out 100 cm³ of river water using a graduated container.
2. Heat the water until it evaporates completely, leaving behind the dissolved solids.
3. Weigh the container before and after evaporation to determine the mass of the dissolved solids remaining.

Given that the concentration of dissolved solids is 125 mg per dm², we first convert the volume:

$250 ext{ cm}^3 = 0.25 ext{ dm}^3$

Now, we can calculate the mass:

$ext{Mass} = 125 ext{ mg/dm}^3 imes 0.25 ext{ dm}^3 = 31.25 ext{ mg}$

To convert mg to grams:

31.25 ext{ mg} = rac{31.25}{1000} ext{ g} = 0.03125 ext{ g} ext{~(or 0.031g to 2 significant figures)}

The maximum allowed mass of sulfate ions is 500 mg/dm², and the sample contains 44 mg/dm².

To find the percentage:

ext{Percentage} = rac{ ext{actual concentration}}{ ext{maximum concentration}} imes 100 = rac{44 ext{ mg}}{500 ext{ mg}} imes 100 = 8.8 ext{ %}