A large amount of aluminium sulfate was accidentally added to the drinking water supply at a water treatment works. Plan an investigation to find the total mass of dissolved solids in a 100 cm3 sample of the drinking water. Your investigation should produce valid results. – 6177

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1. Weigh an empty evaporating basin (or dish) and record its mass.

   Add a measured volume of the water sample to the basin (e.g. 50.0 cm³) and weigh the basin + water. Record the mass so you can check the exact volume used if needed.

   Heat the basin gently to evaporate all the water until only the dissolved solids remain.

   Allow to cool, then reweigh the basin + residue.

   Repeat heating, cooling and weighing until you get the same mass twice (constant mass) — this ensures all water has gone.

   Calculate the mass of dissolved solids by subtracting the mass of the empty basin from the final mass (mass of basin + residue − mass of empty basin).

   Repeat the whole experiment at least once or twice, discard any anomalous result, and calculate a mean mass of solids.

   If asked for mass per 100 cm³

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