||Ascertain the composition of an aqueous
solution that can couple the maximum power from a
750 W microwave oven.
||Teams of up to six.
1) Distilled water at room temperature.
2) Soluble, solid food grade additive(s) (up to a maximum of 5)
that must be a common kitchen item, which will be heated on site
in a 250 mL beaker.
Microwaves heat food because the water molecules rotate due to
their electric dipole moments. The judge will use a microwave oven
with a nominal power of 750 W operating at the 100% power level
to heat 200 g of solution for 1 minute. Prior to the event, teams
must determine the composition of solution (water and solute(s))
that best couples to the microwave energy, and so maximizes the
temperature reached by the food The following conditions must be
met. If not teams are not eligible to participate.
1) The food additive must not precipitate out before or after heating.
2) Teams must provide the recipe for a 200 g solution, which will
be heated in a 250 mL beaker.
3) The composition of the solution must be received by the judge
three days prior to the event.
1) Solutions will be prepared by the judge according to the recipe
provided by the teams.
2) The temperatures of the solution will be measured using a thermometer
before and after heating.
3) Beakers and solutions will be individually heated at the centre
of a 750 W microwave oven at 100% power level for exactly 45 seconds
after which the solution will be stirred for 5 seconds using a straw.
The temperature will then be measured and recorded.
4) The increase in temperature of the solution will be calculated.
5) Teams will be ranked in descending order according to decreasing
change in temperature.
6) If there is a tie for first place, the tests will be repeated
with a heating time of 1 minute.
||There is the potential for injury due to
hot water in this event, if students heat their test samples for more
than 1 minute. In particular, the phenomenon of super heating can
occur. You may have noticed that occasionally, if you heat a cup of
water to boiling and then pour in some instant coffee, the mixture
boils suddenly very vigorously. The water has been heated to more
than its boiling point, but because bubbles did not form in the usual
way, the water heated to over 100 degrees Celcius without boiling.
As soon as some nucleation sites for bubbles are added, i.e. the grains
of coffee, this excess energy is released very suddenly, and hot water
spills out of the cup.
||Technical University of Nova Scotia: Dr
Lino C R. Correia, P.Eng.
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