ExpertEasy1 markShort Answer
AQA GCSE · Question 01.3 · Quantitative chemistry
The student determined the concentration of sodium chloride in the salt solution using this method:
- 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?
The student determined the concentration of sodium chloride in the salt solution using this method:
- 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?
How to approach this question
Consider the purpose of heating the solution. The goal is to leave only the solid salt behind. How can the student be sure that all the liquid has gone? The process is called heating to a constant mass.
Full Answer
To ensure all the water has evaporated/been removed.
The purpose of heating the salt solution is to evaporate the water, leaving the dissolved sodium chloride behind. By repeatedly heating and weighing the dish, the student can monitor the process. The mass will decrease as water turns to steam and escapes. When all the water has been driven off, the mass will stop changing. This is known as "heating to constant mass" and it ensures that the final mass measured is only that of the evaporating dish and the dry salt, leading to an accurate result.
Common mistakes
✗ Saying "to make sure the reaction is complete" (it's a physical separation, not a reaction).
✗ Saying "to remove impurities".
✗ Giving a vague answer like "to get an accurate result" without explaining why.
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
48 questions · hints · full answers · grading
More questions from this exam
Q01.1A student investigated an aqueous solution of a salt. The student identified that the salt soluti...EasyQ01.2Describe a test to identify chloride ions. Give the result of the test.EasyQ01.4How did the student calculate the mass of solid sodium chloride remaining after steps 1 to 5?EasyQ01.5The student calculated the concentration of sodium chloride in the salt solution. Table 1 shows t...MediumQ02.1This question is about ammonia and nitric acid. In the Haber process ammonia is produced from nit...Easy