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Friday, 6 February 2009

Let's look at the topic of microwaves, more specifically, microwave ovens. 

Most people would say that a microwave oven works by putting the food in the microwave, setting the timer and the power level, and pressing 'start'. That's all very well, but it doesn't explain why the food gets hot inside. Let's see how Peter Barham from BBC Food for Heston Blumenthal's In Search of Perfection explains it.

First of all, microwaves are very good at heating water.  How is that?  Well, water consists of two hydrogen atoms joined to a single oxygen atom. In practice, the oxygen atom is usually more strongly bound to one of the two hydrogen atoms at any particular time, so we tend to think of water as being from a 'hydroxyl' group (the oxygen with one hydrogen joined to it) bound with a hydrogen atom.

When a water molecule absorbs a microwave, the hydroxyl group gets excited and starts to rotate.  Since most of our food contains water molecules, our food "cooks!"  There are many other molecules that contain hydroxyl groups, and most of these will also absorb microwaves and so become hot in the microwave oven, substances like paper, nylon, wood, cheese, and cooking oil.

Now, we've all experienced "cooking" our food in the microwave oven, but what about ice?  What if we wanted a cup of ice cold water and all we have on hand is a bucket of ice?  Try this experiment and see what happens. 

Can Microwaves Heat Ice? 

Materials You Will Need:

Two plastic or paper cups (the sort that you find beside water-coolers that have a conical shape with a flat bottom) and some water.

Your Procedure:

Start by making a beaker out of ice. Fill one of the plastic cups to a depth of about 2cm/¾in and put it (on a flat surface) in the freezer. Leave it until the water is frozen solid. Place the second beaker inside the first one and fill it with sugar (or anything else that comes to hand) to weigh it down, making sure it is centrally located so that there is a gap between the walls of the two cups.

Carefully pour water in the gap so that it comes up to a couple of centimetres (less than an inch) below the lip of the outside cup. Put it back in the freezer and leave until all the water has frozen. When it has frozen you have a beaker made of ice sandwiched between two plastic cups.

Half-fill the ice beaker with water and put it in the microwave on full power. The water will boil and the ice will only just start to melt as it is heated by the hot water. If you add a tea bag you will be able to drink a hot cup of tea from the beaker made of ice. It is best to leave the plastic cups in place so that if the ice should melt, the water won't flow away.

The Result:

The water in the beaker is heated by the microwaves and it is only the heat conducted from the hot water that melts the ice between the walls of the two plastic cups so you can hold a freezing cold ice beaker and drink piping hot tea from it.

Your Conclusion:

Microwaves pass through ice without heating it much but are absorbed by water and heat it rapidly.

Pretty cool, huh? 


Posted by studentsconnectsciencetofood at 11:36 AM EST

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