Microwave chemical changes


gives an update on microwave chemical effects. It includes the comment

  • Everyone agrees microwaves are far too low-energy to break molecular bonds and cause chemical reactions. *

How then do you explain how many labs pipe reagents through modified microwave ovens to get very fast chemical reactions that are also driven to a particular end-state - as compared to leaving the reagents in a beaker and getting little or no reaction and a different end-state - even when heating the reagents in the beaker.

This quote may assist

As mentioned above, each of the energy transfers generated by microwave irradiation occurs very rapidly—every nanosecond (10-9 seconds). The almost constant energy input is achieved at a rate greater than the molecular relaxation rate, which is on the order to 10-5 seconds. Because the energy is added at a rate faster than the molecules are able of fully relaxing, all of the molecules in solution will be in a constant state of disequilibria. This disequilibria situation will provide more than enough energy to overcome the activation energy barrier (Ea) and drive the reaction to completion (Figure 7).


Cecil is (of course) correct to say that microwaves “do not cause chemical reactions”. But chemical reactions, including bond breaking and bond making, do occur in the microwave oven. They are caused not by the microwaves themselves interacting with food, but by the heat that the microwaves generate (unlike the ionizing radiation that Cecil mentions, which directly breaks chemical bonds). These reactions are, as Cecil goes on to say, the same kinds of heat-induced processes that occur in any cooking process.

This is, by the way, the reason why it is extremely unlikely (some would say impossible) that cell phones can cause cancer. The radiation associated with cellphone transmission is too low in energy (too long in wavelength) to break chemical bonds, and, You Tube corn-popping videos to the contrary, not intense enough to cause significant heating of the brain.

Whether “light” is ionizing radiation or not depends on whether you limit the word to the stuff we see. Invisible ultraviolet “light”, especially the short wavelength kind, is indeed ionizing radiation, as anyone who has suffered sunburn can attest. Visible light is not.

None of this is meant to “correct” Cecil (I wouldn’t dare and he got it right anyway), but to clarify the fine points that might lead some readers to foolishly question the Master’s conclusion.

[crumples up paper with list of foolish questions] Yeah, that’s about covers it, there, jonk.

I am inclined to always agree with you, but I will withold judgement on this subject until someone can explain this phenomenon to me. Let me explain.

I enjoy drinking hot tea. I drink lots of hot tea, mostly in the mornings. Of course, in order to make hot tea, one needs to boil some water. I have noticed a big difference in the tea when the water is boiled in the microwave oven vs. the electric kettle or any other method. This is not a subjective difference, there is a physical difference between the two. The tea made with the microwaved water always has a foamy, oily scum floating on top, whereas the tea made with the kettle-boiled water does not.

Every time.

Try this simple experiment and then tell me you don’t agree:
Heat two identical cups of water to boiling, one in the microwave oven and one on the stovetop. Then take two identical tea bags (I use Twinings Earl Grey, but I don’t know if that matters) and dip one in each cup repeatedly for three minutes. You will see that one of the cups has a thin foamy head and the other doesn’t. Hot water is hot water, right? Why does it matter how the water was heated?

e (teeming) pluribus unum,

Why does Sweet 'n Low make microwaved water fizz so strangely?

I propose this explanation:

On a stove, in a kettle or pot, the element or flame heats the bottom of the water. In order to boil, the water must circulate (convection) to distribute its heat – the hot water on the bottom moves up, forcing the less hot water toward the heat, creating a flow cycle. Because of this, and because of the time required to boil, a considerable quantity of dissolved gasses (possibly all) are driven off.

Nuked water acquired heat throughout its body in a fairly consistent distribution. Circulation is not necessary for boiling, and the even heating drives it to a much lesser extent. Hence, a smaller quantity of gasses are driven off, rather, they absorb some heat from the water. Hence, anything that will inter- or react with the gasses will do so vigorously.

For instance, if you reheat this morning’s coffee in the microwave, then add chocolate powder or sweet-n-low, does it behave differently than cold tap water heated in the microwave? (I would think that brewing would flatten the water used for the coffee.)

Well done, For You. I would add that another difference is the container itself. On the stove-top, the kettle heats up with the water, whereas in the µwave the container does NOT heat up. As soon as the µwave stops, the water losses heat to it’s container. “So what” we may ask, and I’m not much of a tea drinker. Those I know who are fanatic tea drinkers tell me it’s critically important for the water to be at boiling as the herbs go in. They never use µwaves for their tea water.

I don’t drink tea … I drink coffee, but I make the coffee in a drip coffee maker and keep the cream in the freezer. When the coffee is ready I microwave the cream in the same plastic container it was purchased in and put the cream in the cup and add the hot coffee, but I don’t see any foam you speak of.

I’ll have to try your experiment and get back with you.

as long as hot liquid touches the container the container is getting hot. the container may not be fully hot by the time the liquid is especially for a small volume container.

I love the fact that after almost 8 years of being a registered member the first post by David the Tea Drinker is indeed about tea.

Just for general interest, I note that there was a postulated mechanism.

Normal body processes are driven by high-energy food goes in, low energy waste goes out. Not needing any radiation at all to break bonds. Some of these reactions are blocked by an energy barriers. Some of these barriers are unlocked by Catalysts called Enzymes.

Given that background, you see that radiation doesn’t need to be high enough in energy to break bounds, only high enough to catalyze an exothermic reation.

This idea was exhaustively analysed and tested. Came up negative though: cell phones don’t cause cancer.

You’re one of them! How much did they pay you?!

(Can you give them my name too? I need some cash.)

it is incorrect to say that cell phones don’t cause cancer.

it may be correct to say that it has not been shown that cell phones cause cancer.

the idea of something (material or energy) may take part in a process, yet not solely responsible for that process is something to be realized. biological systems are complex. the occurrence of disease in biological systems, especially cancer, is complex; genetics injects large variability in that investigation.

Colony collapse disorder of bees is an illustration of the confounding complexity of biological systems.

Then those guys either have the wrong end of the stick, or they want it to be precisely at boiling temp, not HIGHER.

Read this, which did a better job than For You. For You did about as well as I would have, guessing, before reading The Master’s answer. I considered the lack of circulation, but didn’t manage to link that to superheating (heating above boiling temp). The Master points out that heating water in a microwave can cause it to go well above the boiling temperature, and that’s what causes the phenomenon:

I don’t know how sensitive tea is, but a microwave would be terrible for coffee. Good coffee requires water to be at boiling temp, but without all the extra heat content required to get it to actually boil. So, when we use a melitta (or whatever the generic term is for those simple glass funnels that make great coffee but everyone’s too busy to bother to take the extra two minutes to use), we put the water on to boil, but take it off the heat and pour it into the funnel when it starts pinging. Yep, that’s the term, and that’s the sound it makes. You see little bubbles forming at the bottom of the pan and starting to rise but the collapse before they reach the top.

If you use water that’s come to a nice full rolling boil to make coffee, it has too much heat, which makes the coffee bitter. I’ll leave it to the more scientific folks to explain why. My guess is that either it dissolves too many of the heavier oils, or that it actually changes the chemical composition by destroying some of the more fragile chemicals.

As I understand it, coffeemakers are specifically designed to introduce the water to the coffee at 190°F, which is supposed to be the ideal temperature for brewing. That, at least, is the claim I read on my fancy electric vacuum coffee maker. Oddly, I think that is the temperature that water boils in Denver, so they must have to do something different to get a decent cup of coffee there.

Is there any cause for concern over repeatedly standing near microwaves while they’re running? My nuker is one that my parents bought way back in the 80s in case that makes any difference.

As long as they’re not actually burning you (like, heating up your tissue), then no. That’s the only way microwaves are known to interact people, food, or chemicals in reaction vessels is by heating them (and people have looked very hard for other possible effects).

I guess there’s minor worry that they could damage your eye without you realizing it (like how lasers can impart enough power to damage your eyes before you can figure out that they’re being damaged and blink), but even this has never been shown (and again, people have looked: Cecil mentioned a cataract study).