[hamdani yusuf (OP)]

I also tried to use a paper cup as the water container. Unfortunately I couldn't find a new one, thus I took a used paper cup previously used to drink tea. Even after I washed it using demin water, I couldn't produce superheated water in it. The water always boiled normally in the microwave.

I bought new paper cup for the experiment, but I still couldn't produce superheated water in it. To make sure that it wasn't due to contaminating substances from the paper cup, I poured the water into a glass bowl and repeat the experiment. This time the water can be superheated, so I concluded that there are something physical in the paper cup that prevents the demineralized water from being superheated.

Other experiment I've done using microwave oven was placing the glass bowl containing demineralized water inside a metal bowl. When the microwave oven was turned on, the water heated up much slower than before. But after prolonged heating, I can still get superheated water.
The next step was removing the glass bowl from the experiment setup, so the water was in direct contact with the metal bowl. The heating up process was even slower this time. Unfortunately my microwave oven eventually stopped working after prolonged heating. It seems like the water only absorbed a small portion of microwave energy while the majority of it was reflected back to the magnetron.
It would take a while to fix it before I can continue the experiment. I hope it only broke the power supply unit since the control display went completely blank. If I was lucky, it might be just blown out fuse.

[Colin2B]

I bought new paper cup for the experiment, but I still couldn't produce superheated water in it. To make sure that it wasn't due to contaminating substances from the paper cup, I poured the water into a glass bowl and repeat the experiment. This time the water can be superheated, so I concluded that there are something physical in the paper cup that prevents the demineralized water from being superheated.

Have you tried other types of insulating vessels?

[hamdani yusuf (OP)]

Have you tried other types of insulating vessels?

Not yet.
I guess that on the surface of the paper cup there are something that acts as nucleation site that makes the water boiled instead of superheated, just like what boiling chips do.

[hamdani yusuf (OP)]

I have found the

If I was lucky, it might be just blown out fuse.

It was just blown out fuse. The fuse is rated at 8 Amp 250V, although the microwave nameplate says it's 1400 W.
Previously, I can only get 5 Amp fuse, which instantly blown out when the heating is started, although it was set at low power. Apparently, the power setting only affects the resting time for the magnetron. Lower power only means longer resting time, but initial power stays the same. That's why the 5 Amp fuse couldn't stand a chance.

[alancalverd]

You will find the same problem with an induction cooker - they run at full power with a variable mark/space ratio and rely on the thermal inertia of the pot to deliver reasonably constant power to the contents.

There was some mention of paper cups. Unlikely to produce superheating as the surface is never smooth and can trap microscopic air bubbles as you fill it. Essentially, anything that isn't water and a surface that isn't polished, will induce normal boiling. And any solute will increase the normal boiling point.

[hamdani yusuf (OP)]

And any solute will increase the normal boiling point.

What do you mean by that?

[hamdani yusuf (OP)]

I also have tried using a semi transparent plastic cup labeled "safe for microwave oven". The demineralized water always boil normally and shows bubble generated from the surface of the plastic cup. Repeated heating by microwave doesn't change the result, although there supposed to be no longer dissolved air in the water. When the plastic cup is replaced by a smooth and clear glass, I get superheated water. It even exploded inside the microwave when the magnetron is still on. Subsequent trials don't produce explosion, but I still got superheated water since it started boiling when I dip a metal wire into it.

[alancalverd]

The plastic surface will probably have a different contact angle with the water: glass is slightly hydrophilic (water wets it) but most plastics are hydrophobic. So the interface chemistry and physics will be different - apparently sufficiently so that the plastic nucleates boiling.   

"Solute" means anything dissolved in the water. Salt lowers the freezing point and raises the boiling point of water, but if you throw a handful of salt crystals into a hot pan it can nucleate boiling before it dissolves - chefs used this to promote "smooth" boiling rather than dissolve the salt in cold water which can then superheat and "bump".

[hamdani yusuf (OP)]

Is there any solute which can decrease the boiling water in atmospheric pressure? What is the theoretical justification?

[alancalverd]

Silly of me to forget. "Constant boiling solutions" are worth investigating.

A well-known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by mass) boils at 78.2 °C.[7] Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope boils at 78.2 °C, which is lower than either of its constituents.[8] Indeed, 78.2 °C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures or pressure maximum azeotropes.

[Bored chemist]

Is there any solute which can decrease the boiling water in atmospheric pressure? What is the theoretical justification?

Some materials- like HCl can raise or lower the boiling point of the solution, depending on how much you add.

Ammonia reduces the boiling point of water, and forms no constant boiling mixture.