[Margaret Feaster]

Margaret Feaster  asked the Naked Scientists:
   
It seems to me that water boiled in the microwave does not get as hot nor stay as hot as water boiled on the stove.
If it is used to make instant coffee (tea for you) then the coffee (tea) cools off really fast.
If it is used to make instant oatmeal then the oatmeal flakes don't puff up and become really pasty the way I really like it!
Is it all in my imagination?
It seems to be the case even if I run the microwave longer and longer to make sure the water boils longer and is really really hot!
LOVE YOUR SHOW!
Thanks

Margaret Feaster
Tallahassee, Florida
USA
What do you think?

[Mike Gale]

I've noticed the same thing. I think it's because the microwaves are monotonic. The frequency of vibration is tuned to match the motion of liquid water molecules at the boiling point so they can't heat the water vapours beyond 100 degC. A spread spectrum microwave might work better.

[Bored chemist]

If there's a real effect, it's more likely to be because the water gets heated by microwaves, but not the container.

[SeanB]

Boiling in the microwave is making it as hot as on the stove, the only difference being the stove heats the water and the kettle as well. The microwave heats just the water and the cup, and you generally stop when the surface starts to steam, before the water is in a roiling boil, so the most part will be slightly under boiling ( perfect for a cup of tea, which does not need boiling water), so it does cool down faster.

[RD]

Margaret Feaster  asked the Naked Scientists:   
It seems to me that water boiled in the microwave does not get as hot ... as water boiled on the stove

Microwaved water can get super-heated, so it appears not to be boiling hot, (like water on the stove), when it is actually above boiling point (>100^oC)C

[SeanB]

True, but you can do that as well in an ordinary glass pot over a slow gas flame as well, if there are no nucleation points for bubbles to form you will get water well past boiling point until a cosmic ray or something provides a nucleation point, which will then cause a mass boil over almost instantly. You just need very pure water, and a vessel with an absolutely smooth interior finish. Thus, typical purified tap water and a glazed ceramic cup, does this cheaply and quite well.

[studentstill]

So which is better and cheaper for making a  cup of tea - the electric powered  microwave or the kettle on the gas stove ?

[chris]

I suspect that the gas heating system is going to have a higher overall efficiency. Electricity is a very "pure" form of energy and in arriving at the plug already embodies considerable losses, such as the 30-50% generating efficiency at the power station, followed by transmission losses in the cables and transformers, followed by inefficiency of your microwave.

But burn gas on the hob and the product of the reaction is heat...

[PmbPhy]

No. Water at a specific temperature will cool at a determined rate which depends only on the environment that its in and not how it arrived at that temperature. I.e. if you have a cup of water at 100C there is no way to determine how it got  to that temperature. And the rate of cooling will not depend on how it was heated.

There is the possibility that the pressure in a microwave oven increases with the heating of the water due to evaporation of the water. The increase in pressure will decrease the boiling point of the water. So what you thought was a cup of water at 100C might have only been a cup of water at 90C. But that's just an educated guess. However it should be easy to demonstrate. Just boil a cup of water in the microwave and measure its temperature as soon as it boils (taking it out of the microwave oven of course).