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Improving an evaporation mass method
A student investigated the mass of dissolved solids in a 100 cm³ sample of sea water. This is the method used.
- Weigh an evaporating basin.
- Measure 100 cm³ of sea water.
- Pour the sea water into the evaporating basin.
- Heat the evaporating basin.
- Weigh the evaporating basin and contents.
- Calculate the mass of dissolved solids in the sea water.
Explain how repeating steps 4 and 5 would improve this method.
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Marking points
- 1 Explains that repeated heating and weighing should continue until constant mass is reached.
- 2 Explains that constant mass shows all the water has evaporated.
Full-mark answer
Repeat the heating and weighing until the mass no longer changes. Reaching a constant mass shows that all the water has evaporated, so only the dissolved solids remain.
Why this answer loses marks
Heat it once and use that mass.
One heating does not show that drying is complete; repeat heating and weighing until the mass stays constant.