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Heating to constant mass
The student determined the concentration of sodium chloride in the salt solution. This is the method used.
- Weigh an empty evaporating dish.
- Add 25.0 cm³ of the salt solution into the evaporating dish.
- Heat the evaporating dish and contents.
- Weigh the evaporating dish and contents.
- Repeat steps 3 to 4 until there is no further change in mass.
- Repeat steps 1 to 5 three more times.
Why did the student heat the evaporating dish and contents until the mass did not change?
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Marking points
- 1 Explains that constant mass shows that all the water has evaporated.
Full-mark answer
This ensured that all the water had evaporated, leaving dry sodium chloride.
Why this answer loses marks
The repeated heating makes the salt hotter and therefore easier to weigh.
The purpose is to show that all water has evaporated, not to change how easily the sample can be weighed.