A student uses a flame test to determine
whether the major product of a reaction is a potassium salt. Which of the following would not improve
the quality of the test? A) Closing the inlet of the Bunsen
burner. B) Dissolving the product in hydrochloric
acid. C) Viewing the flame through cobalt
glass. D) Washing the sample loop in
hydrochloric acid before use. Or E) Recrystallizing the product.
A flame test is a way of checking for the
presence of certain ions by the production of a particular colour of flame. The potassium salt will contain potassium
plus ions. The addition of potassium salts to a
roaring blue Bunsen flame will turn it a pale pink.
If when the student does the test the
flame colour is pink, then there’s definitely potassium present. However, if the flame colour is not pink,
it’s not necessarily clear whether potassium is there or not. Our job is to look at the statements and
see which one would not improve the quality of the test. A better quality test is more likely to
produce a positive result.
What about closing the inlet of the
Bunsen burner? If the air inlet of a Bunsen burner is
open, more oxygen gets to the flame. It gets hotter. And it becomes more blue. If you close the inlet of a Bunsen
burner, you’ll produce a safety flame which is a bright yellow. A bright yellow flame is likely to mask
the pink of any potassium. So closing the inlet of the Bunsen burner
would definitely not improve the quality of the test.
Let’s have a look at the other options
just to be safe. Dissolving the product in hydrochloric
acid first will make the solid easier to vaporise. In solution, the ions are already
separated, so when they’re heated up, they’ll all go into the air and start emitting
light. Depending on the concentration, there’s
an opportunity to vastly increase the brightness of the pink colour and improve the quality
of the test. So, this is not a correct answer.
What about viewing the flame through
cobalt glass? Cobalt glass is a blue filter. Viewing the flame through cobalt glass
will remove the orange-yellow bright light from sodium. This will allow the pink colour from
potassium ions to come through more clearly, improving the quality of the test. Sodium ions are quite common
contaminants, so this is likely to be a good idea.
Washing the sample loop in hydrochloric
acid is another way of improving the quality of the test. Washing the loop removes other
contaminants, which might colour the flame differently and lead to an ambiguous result.
Finally, we have recrystallizing the
product. Recrystallization will purify the
product. A more concentrated product means fewer
contaminants that are likely to interfere with the results of the test. So, recrystallizing the product is a good
way of improving the quality of the test, and, therefore, is not our answer. So, of the five options given, a student
doing a flame test to check if potassium is present in the product should probably not close
the inlet of the Bunsen burner in order to improve the quality of the test.