When carrying out the test for
reducing sugars using Benedict’s reagent, the mixture should be heated in a water
bath. Why should a water bath be used
rather than a Bunsen burner? (A) It is too dangerous to use a
Bunsen burner near Benedict’s reagent. (B) A Bunsen burner should only be
used to heat up water, not other mixtures. (C) It is easier to maintain a
constant temperature when using a water bath. (D) A Bunsen burner will not reach
the required 75 degrees Celsius to heat the mixture. Or (E) this is incorrect; a Bunsen
burner should be used.
Let’s think about the two heating
options presented in this question. What’s the difference in heating a
mixture with a water bath versus a Bunsen burner?
Bunsen burners provide more direct
heating and raise the heat quickly to high temperatures, which is why they are used
for sterilization and combustion. This quick increase in temperature
can be a problem as it is not easily controlled. Bunsen burners can also cause
pockets of very hot solution and areas where the solution is at a lower
temperature. So the temperature of the solution
being heated is not equally distributed.
When a test tube is placed in a hot
water bath, its contents are completely immersed in the hot liquid, causing uniform
heating. It is also very easy to create a
constant temperature when using a water bath as opposed to other heating
methods. Water baths can be used to heat
solutions up to 100 degrees Celsius, but no higher, seeing as this is the boiling
point of water. The temperature usually used to
drive the reducing reaction in a Benedict’s test is 90 degrees Celsius, so the water
bath can do this. And it would allow the experimenter
to monitor the temperature easily.
Therefore, the correct answer to
this question is (C). A water bath should be used, rather
than a Bunsen burner, to heat the mixture in a Benedict’s test, because it is easier
to maintain a constant temperature when using a water bath.