Is 2.5 GHz the optimal frequency for microwaves to cook food?

[ranger]

I'm just curious. Why is it that we use microwave ovens to cook food and not some other wave of the EM spectrum?

--thanks.

 

[D H]

Short answer: The food in the oven absorbs the microwave energy. The oven walls do not.

Long answer: http://www.google.com/search?hl=en&rls=en&q=microwave+oven&btnG=Search".

 

[ShawnD]

1). Low frequency light is inherently safe
Microwaves are huge. They do not fit through tight spaces and they do not have penetrating power. You can see visible light coming through the holes on the door of the microwave when you look at your food, but those holes are small enough to stop microwaves from leaking out and killing you. The walls of the microwave oven do not need to be thick because microwaves just bounce off metallic surfaces. You can even test this by trying to heat water inside of a steel cup. Even after 5 minutes that water will be just as cold as when you put it in; the steel cup reflected all fo the waves.

2). More things are capable of absorbing microwaves
Microwaves cause the bonds in organic molecules to vibrate, which causes heat. This seems to be particularly true for things with lots of oxygen, such as water, sugar, and butter.

Other parts of the EM spectrum do different things. For example, radio waves are very difficult to absorb. Higher up on the energy scale is IR. Most organics will absorb IR, but only in a few specific places; this is why IR is used to identify organic compounds. IR would probably not absorb enough to be effective. Next is visible light. If you had 1000W of visible light in your microwave, it would be like looking at the sun; how can you even tell if your food is done? Next is UV. UV mainly applies to things with pi bonds and lone pairs; that would be things like sugars, fats, and proteins. UV would probably work, but UV is smaller than visible light, so it would easily go through those holes in your microwave door and severely damage your eyes when you look at the microwave oven. Higher energy light will keep doing the same thing as UV - cause cancer and permanent eye damage.

 

[ranger]

Thank you very much ShawnD :)

You mentioned that if the microwaves leaked out, it could kill us. But I wonder, if it could harm us, how is it safe for our food? I see some people sometimes place a sheet of white tissue paper over their food before heating it up. Why do they do this?

 

[triden]

Thank you very much ShawnD :)

You mentioned that if the microwaves leaked out, it could kill us. But I wonder, if it could harm us, how is it safe for our food? I see some people sometimes place a sheet of white tissue paper over their food before heating it up. Why do they do this?

I assume what he means is that the EM waves can cause gene and cell mutations in living creatures. A slab of beef in a microwave isn't going to get cancer if the waves somehow modify it's structure. All the waves do, from my understanding, is heat up the water molecules inside the food item. They do this by finding a microwave frequency that will resonate with a water molecule causing it to vibrate and heat up. As far as I know, most microwaves operate at the 10GHz range.

 

[vivesdn]

Microwaves are at so low frequency that they can just act basically on rotary quantum states. IR will act on vibrational states. UV-VIS carry energy enough to break bonds, for example, those that keep DNA useful. So microwaves are not likely to produce cancer. Ultraviolet and higher can.
The frequency is chosen to fit water rotary states, but of course, there are other molecules that can absorb it. So if you are hit by microwaves, your water will be heated. It depends on the time, intensity and volume involved that it would be dangerous. Just try to put an egg inside a microwave oven and you will understand what I mean.
The fact that microwaves are on the rotary range of energy explains why microwave-heated meal tends to cool quicker than by other methods: rotary energy causes by molecular collisions an increase on kynetic energy quite fast (so temperature), but vibration states tend to catch part of this energy on the mid-term, causing quicker cooling.
At the end, meal is just heated. Of course, if you heat it a lot, it will be damaged just as in a normal oven. The diference is that you need to touch the inside walls of the oven to get burned, and you could be 'burned' by an intense leakage of the microwave.

 

[D H]

The microwave radiation from a microwave won't kill you. Look up how microwaves were first discovered to heat things up. They warmed up the microwave operators, and the candy bars in their pockets. Microwave operators still get a kick out of standing in front of a microwave relay in subzero temperatures.

Microwaves are non-ionizing radiation. Look at it this way. Suppose you got inside a very large conventional oven and someone turned on the heat. You would die. Yet you don't think of questioning whether our food is safe after being cooked in a conventional oven.

As for your last question, people put a paper towel over the food to keep the food from splattering all over the microwave. Things sometimes "explode" in a microwave. For example, a Twinkie explodes in 45 seconds (according to the manufacturer).

 

[ShawnD]

I assume what he means is that the EM waves can cause gene and cell mutations in living creatures.

Microwaves do not have enough energy to cause cell mutations. Even some of the weaker UV light can't do that. It's dangerous more because of heat; your arm is made of the same stuff you intend to put in the microwave oven. Have you ever seen a piece of chicken or steak explode in the microwave because it had a pocket of water in it? That could just as easily be your arm if you're not properly shielded. Without shielding, microwaves can be dangerous simply because people will underestimate what radiation is capable of doing. Wearing an oven mit will protect your hands when you take something out of the oven, but oven mits will not protect you from microwaves. Your hands will heat up just as quickly as if you were not wearing mits, and if I remember correctly, microwaves penetrate several centimeters of the food being cooked. That means your arm won't heat up the same as if you hold a blow dryer to it, it will immediately cook right to the bone and you might not feel this happening.

 

[Mk]

As for your last question, people put a paper towel over the food to keep the food from splattering all over the microwave. Things sometimes "explode" in a microwave. For example, a Twinkie explodes in 45 seconds (according to the manufacturer).

Usually it isn't that violent as anybody that has used a microwave knows, sometimes a few drops of sauces splatter on the walls from escaping air.

UV mainly applies to things with pi bonds and lone pairs; that would be things like sugars, fats, and proteins.

Why pi bonds and lone pairs, as opposed to other kinds.

 

[ShawnD]

Why pi bonds and lone pairs, as opposed to other kinds.

If I remember correctly it had something to do with the n --> pi* transition. n is a lone pair electron and pi* is an excited electron that is part of a double bond. Just as an example, methanol has lone pairs but no pi bonds, so it does not absorb UV. Acetone has a pi bond connected to an oxygen which has lone pairs; acetone strongly absorbs UV.
Back when I was taking chemistry at university, all of the LCs had UV detectors on them, and they only worked for things with double bonds such as sugars, proteins, and aromatics. Things like methanol, hexane, and water do not absorb UV.

 

[Ki Man]

You can even test this by trying to heat water inside of a steel cup.

isnt microwaving a metal object somewhat dangerous

 

[ranger]

isnt microwaving a metal object somewhat dangerous

I stumbled across this while doing a search:
http://www.chemistryquestion.com/English/Questions/ChemistryInDailyLife/23c_microwave_metal.html

 

[ShawnD]

isnt microwaving a metal object somewhat dangerous

No.

5678910 (won't let me post without 10 letters)

 

[quinn]

Microwaves use 2.5 GHz not because it is water's reasonant frequency, but because that was a convenient magnetron frequency to make back in the day. Since that point it has been convention that has held microwaves at 2.5 GHz, and I beilieve that is the frequency allocated to microwaves by the FCC.

There are actually other molecules that absorb much better than water at 2.5 GHz, these are the foods that heat up quickest in microwaves.

Microwaves do generally excite rotational energy levels (I am sure there are exceptions) in polar compounds, which most organic materials (that we eat)happen to be. The rotational energy levels tend to be lower energy and have a large cluster of energy levels close by making the rotational energy levels almost a continum.

The pi and sigma bonds are much higher energy levels and therefore require a much shorter wavelength of light to excite.