Why does water heated in a microwave oven boil over when I put in a teabag into the cup?

Question

I make my daily cups of tea in a microwave oven. Heat water for 2 and half minutes. Take out the cup. Dip my teabag. Cool a bit, and drink.

Sometimes, even though the water is just 'bubbling' when I take the cup out of the microwave oven, the minute I dip in my teabag, the water bubbles up furiously, even overflowing the cup last night.

Why does this happen? Is there an agitation of water molecules by the microwave radiation that gets aggravated (or resonates) when tea bags are dipped in?

Or is it because of air pockets within the tea bag itself that are released and heated to steam and released?

Or is there another explanation?

Thanks for your time in helping me explain this curious phenomenon.

Answer 1

Its called superheating and its when a liquid is heated past the boiling point without actually boiling. This can be caused by rapidly heating a homogeneous substance while leaving it undisturbed.

Because a superheated fluid is the result of artificial circumstances, it is metastable, and is disrupted as soon as the circumstances abate, leading to the liquid boiling very suddenly and violently (a steam explosion). Superheating is sometimes a concern with microwave ovens, some of which can quickly heat water without physical disturbance.

Superheating is common when a person puts an undisturbed cup of water into the microwave and heats it. Once finished, the water appears to have not come to a boil. Once the water is disturbed, it violently comes to a boil. This can be simply from contact with the cup, or the addition of substances like instant coffee or sugar, which could result in hot scalding water shooting out.

Answer 2

There is a tendency for water when heated in microwave oven to become "superheated". This means its temperature is above the boiling point, but boiling hasn't happened. The reason is that for boiling to occur there have to be some "nucleus" or disturbance to get it started. When you put the teabag in the cup, you are providing that disturbance; the water will boil rapidly because it is well above boiling point. This is more likely to happen with small quantities of water than large. You might try to put a small insulating object (like a glass bead) in the cup before you heat it to provide a surface for bubbles to form.

Answer 3

I would suggest you turn down your heating time by a few seconds or more.

It is possible with a container that is super smooth on the inside to superheat the water sufficiently to have an almost explosive like conversion of the superheated water to steam when you try to remove it from the microwave. The agitation when you move the container starts the boiling process.

You don't really need boiling water to make good tea. Just very hot.

Answer 4

This is just like the mint Mentos in the Soda bottle trick. The water is super-saturated with H20 gas instead of the CO2 in the depressurized soda bottle after you open it (ever notice how the streams of bubbles in an open soda bottle tend to originate on the sides or bottom, rarely in the middle of the fluid?). As the prior answerers essentially said, something with a lot of surface area can provide nucleation sites for the water vapor bubbles to form on.

Mentos+saturated solution of CO2 gas = Lotsa bubbles
Tea-bag+saturated solution of H20 gas = Lotsa bubbles