western
Physics and Astronomy

                                   
Some Like It Hot
Objective Ascertain the composition of an aqueous solution that can couple the maximum power from a
750 W microwave oven.
Participants Teams of up to six.
Materials

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.

Rules

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.

Judging

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.

Safety 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.
Source Technical University of Nova Scotia: Dr Lino C R. Correia, P.Eng.

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