PDA

View Full Version : Comments on this please



Robearbam
12-04-2005, 11:50 AM
This is about the third time I heard about this happening. I can understand water not boiling at 212 F under pressure but not when it free stands in a microwave.

Microwaved water and other liquids don't bubble at 212? What changed? Perhaps this was discussed before...maybe I missed it. Forgive me if this is the case.


Here is the article;

Microwaving Water

A 26-year old man decided to have a cup of coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for, but he wanted to bring the water to a boil.. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup, he noted that the water was not boiling, but suddenly the water in the cup "blew up" into his face. The cup remained intact until he threw it out of his hand, but all the water had flown out into his face due to the build up of energy. His whole face is blistered and he has 1st and 2nd degree burns to his face which may leave scarring.

He also may have lost partial sight in his left eye. While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc., (nothing metal).

It is however a much safer choice to boil the water in a tea kettle.

General Electric's Response:

Thanks for contacting us, I will be happy to assist you. The e-mail that you received is correct. Microwaved water and other liquids do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it.

To prevent this from happening and causing injury, do not heat any liquid for more than two minutes per cup. After heating, let the cup stand in the microwave for thirty seconds before moving it or adding anything into it.

Here is what our local science teacher had to say on the matter: "Thanks for the microwave warning. I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new, or when heating a small amount of water (less than half a cup).

What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point.

What then usually happens is that the liquid is bumped or jarred, which is just enough of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken."

If you pass this on you could very well save someone from a lot of pain and suffering

botrous
12-04-2005, 01:50 PM
Risk of Burns from Eruptions of Hot Water Overheated in Microwave Ovens
The FDA has received reports of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil.

This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.

What Can Consumers Do to Avoid Super-Heated Water?

Follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time.
Do not use excessive amounts of time when heating water or liquids in the microwave oven.
Determine the best time setting to heat the water just to the desired temperature and use that time setting regularly.

from : http://www.fda.gov/cdrh/consumer/erupted.html

Safety Alert
Heating Water in Microwave Ovens
January 24, 2000


--------------------------------------------------------------------------------


DOE Lessons Learned recently distributed a warning regarding the common practice of using microwave ovens to heat water in glasses and cups. The caution is based on a report of a person who placed a cup of water in a microwave oven to heat it with the intention of later adding instant coffee. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup he noted that the water was not boiling but instantly the water in the cup "blew up" into his face. His face received 1sst and 2nd degree burns that are expected to leave scarring. He also may have lost partial sight in his left eye. The doctor attending him at the hospital stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven.


--------------------------------------------------------------------------------

What Was Learned?
Several suggestions have been proposed for why the cup of water reacted as it did.


The cup may not have been microwave safe; that is, the cup itself heated, and then caused the water to boil. Significant heat conduction could still have been occurring from the cup into the water causing the "boil over".


Microwave ovens have different patterns of heating than stoves. Heating occurs on and near the surface of the water and, depending on the oven, not very uniformly. Mixing of the various layers of water may have led the water to boil when the cup was moved.


Water is known to superheat (i.e., the temperature of the water exceeds the boiling point). The water could have been superheated and then spontaneously boiled when physically disturbed.


--------------------------------------------------------------------------------

Recommended Action
Some people prefer to use a teakettle or pan to heat the water on a burner or stove. However, this method has its own risks that need to be taken into consideration, such as the potential to burn or cause a fire by the hot burners.

Suggestions for reducing the risk when heating water in a microwave are shown below.


The chance of "bumping" can be reduced by putting the tea bag, hot chocolate, soup, etc., in the cup first, add cold water, then microwave the cup and contents. Irregular and scratched cups may serve the same function.


Another option is placing a wooden stir stick (never metal) into the cup with the water. Hold the container a safe distance away from yourself as you remove it from the microwave until you're sure it will not bubble over.

from : http://www.llnl.gov/es_and_h/lessons/MicrowaveHeating.html

chemi-cool
12-04-2005, 03:38 PM
All I've learnt from this is:

DO NOT BOIL WATER IN A MICROWAVE. ;)

Chemi :)

coolman
13-04-2005, 09:36 PM
The reverse happens when you very slowly without the sligtest movement cool water down wel below freezing point.
It stay liquid until you touch it than suddenly it becomes ice.

Victor

RickVan
15-04-2005, 01:37 PM
It makes sense to me that a nucleation point is needed for liquid water to change phase (either to vapor or solid), and this point can be either an impurity, a scratch on the surface of the vessel, or a "bump" that distrubs the water's quiescence.

I think another factor might be the way microwaves heat the water. It is my understanding that microwaves esentially rotate some individual molecules back and forth, 180-degrees, due to the alternating electro-magnetic waves. This rotational motion is imparted to other molecules, and temperature increases.

Perhaps there is something about this method of imparting heat-energy that results in less convection currents to spread the energy around. I notice food heated in the microwave oven tends to have a local hot-spot. There seems to be a focal point for the microwave energy. If the super-heated region at this focal point gets disturbed, inter-molecular hell breaks loose.

How's that sound?

Robearbam
15-04-2005, 05:28 PM
I understand that the microwaves are attracted to sugar, water and fat which would explain the hotspots in food. Your theory about alternating electromagnetic waves would be a possibility also for creating the hot spots. I would assume, correct me if I'm wrong, that water molecules would be all the same just knitted so closely together that there is no room for compression, but, because of this property would this be an explaination of why water explodes....going into expansion...hidden latent heat of vaporization? It just undermines everything I've learned about water and pressure vessels. It's an interesting subject to me and I do appreciate the comments on this. This forum has been beneficial to me in many ways and I have learned so much in reference to HVAC...Thanks for your input! :)

RogGoetsch
17-04-2005, 07:07 AM
It makes sense to me that a nucleation point is needed for liquid water to change phase (either to vapor or solid), and this point can be either an impurity, a scratch on the surface of the vessel, or a "bump" that distrubs the water's quiescence.

I remember from chem lab using inert "boiling chips" when heating water in a beaker or flask to supply nucleation points to allow small bubbles to form. This prevented boil-over from large bubbles.

Rog

RickVan
18-04-2005, 01:34 PM
I understand that the microwaves are attracted to sugar, water and fat which would explain the hotspots in food.
I don't know if it is quite accurate to say microwaves are attracted. Remember, microwaves are "beams" of electromagnetic radiation, the same as light. Like light, microwaves can be focused. But as electromagnetic waves, they alternate from positive to negative polarity.

Some molecules, like water and perhaps sugars & fats, have a positive and a negative end. As the microwave passes by, it aligns the molecules magnetically. As the wave changes polarity, it flips the molecules 180degrees.

The water molecules are bashing around, not knitted closely together. But their motion may be limited within the beaker, as molecules are bashing into one another and ricocheting back toward their starting point. Hence the rotational energy stays fairly localized to the focal point. I've been out of school for a while and forget how big the molecular mean free path is. I'm assuming that the convection currents needed to actually spread the rotational-heat energy are also relatively small.

Anyway, thanks for giving me a chance to babble.

frank
18-04-2005, 05:20 PM
http://home.howstuffworks.com/framed.htm?parent=micriwave.htm&url=http://www.amasci.com/weird/microexp.html :eek: